Scheme Requests for Implementation

A library of procedures for formatting Scheme objects to text in various ways, and for easily concatenating, composing and extending these formatters efficiently without resorting to capturing and manipulating intermediate strings.

This SRFI defines utility procedures that create, transform, and consume generators. A generator is simply a procedure with no arguments that works as a source of values. Every time it is called, it yields a value. Generators may be finite or infinite; a finite generator returns an end-of-file object to indicate that it is exhausted. For example, read-char, read-line, and read are generators that generate characters, lines, and objects from the current input port. Generators provide lightweight laziness.

This SRFI also defines procedures that return accumulators. An accumulator is the inverse of a generator: it is a procedure of one argument that works as a sink of values.

Continuation marks are a programming language feature that allows one to attach information to and retrieve information from continuations, generalizing stack inspection. Conceptually, a continuation consists of a number of frames where each frame stands for an active procedure call that is not a tail call. A continuation mark is then a key-value pair associated with a frame, with keys compared using eq?. At most one mark for a given key can be attached to a single frame.

Besides stack inspection, continuation marks can be used to implement dynamic scope, delimited continuations, or delayed evaluation that is able to handle iterative lazy algorithms.

This SRFI proposes to add continuation marks to the Scheme programming language. The interface defined here is modelled after Racket's continuation marks. It does not include all forms and procedures provided by Racket but provides a compatible subset.

Recognizing binary predicates as a specific area in which the use of prefix operators is an impediment, we propose a thin layer of "syntactic stevia" for in-fixing such predicates. It can be implemented using regular Scheme macros. We suggest that the code (is x < y) should be transformed to (< x y), and (is x < y <= z) -- to (let ((y* y)) (and (< x y*) (<= y* z))). In addition, we suggest special meaning to the _ symbol: (is _ < y) and (is x < _) should be transformed to (lambda (_) (< _ y)) and (lambda (_) (< x _)), respectively. This SRFI document also describes some other uses of the is macro and its limitations.

Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. This SRFI proposes a coherent and comprehensive set of string-processing procedures. It is a reduced version of SRFI 13 that has been aligned with SRFI 135, Immutable Texts. Unlike SRFI 13, it has been made consistent with the R5RS, R6RS, and R7RS-small string procedures.

This SRFI proposes a coherent and comprehensive set of procedures for performing bitwise logical operations on integers; it is accompanied by a reference implementation of the spec in terms of a set of seven core operators. The sample implementation is portable, as efficient as practical with pure Scheme arithmetic (it is much more efficient to replace the core operators with C or assembly language if possible), and open source.

The precise semantics of these operators is almost never an issue. A consistent, portable set of names and parameter conventions, however, is. Hence this SRFI, which is based mainly on SRFI 33, with some changes and additions from Olin's late revisions to SRFI 33 (which were never consummated). SRFI 60 (based on SLIB) is smaller but has a few procedures of its own; some of its procedures have both native (often Common Lisp) and SRFI 33 names. They have been incorporated into this SRFI. R6RS is a subset of SRFI 60, except that all procedure names begin with a bitwise- prefix. A few procedures have been added from the general vector SRFI 133.

Among the applications of bitwise operations are: hashing, Galois-field calculations of error-detecting and error-correcting codes, cryptography and ciphers, pseudo-random number generation, register-transfer-level modeling of digital logic designs, Fast-Fourier transforms, packing and unpacking numbers in persistent data structures, space-filling curves with applications to dimension reduction and sparse multi-dimensional database indexes, and generating approximate seed values for root-finders and transcendental function algorithms.

This SRFI differs from SRFI 142 in only two ways:

1. The bitwise-if function has the argument ordering of SLIB, SRFI 60, and R6RS rather than the ordering of SRFI 33.

2. The order in which bits are processed by the procedures listed in the "Bits conversion" section has been clarified and some of the procedures' names have been changed. See "Bit processing order" for details.

This SRFI provides a specification and portable implementation of an extension of the ERR5RS record syntax of SRFI 131, where field names inserted by macro transformers are effectively renamed as if the macro transformer inserted a binding. This makes this SRFI compatible with the semantics of the record-type definitions of the R7RS as intended by its authors. In addition, field names may also be other types of Scheme datums, like numbers and strings, or SRFI 88 keyword objects.

The rules for valid <template>s of <syntax rules> are slightly softened to allow for more than one consecutive <ellipsis> in subtemplates, and to allow pattern variables in subtemplates to be followed by more instances of the identifier <ellipsis> than they are followed in the subpattern in which they occur.

Writing powerful syntax-rules macros is hard because they do not compose well: The arguments of a macro expansion are not expanded. This SRFI defines an easy to comprehend high-level system for writing powerful, composable (or eager) macros, two of whose defining features are that its macro arguments are (in general) eagerly expanded and that it can be portably implemented in any Scheme implementation conforming to the R7RS.

Each syntax definition assigns a macro transformer to a keyword. The macro transformer is specified by a transformer spec, which is either an instance of syntax-rules, an existing syntactic keyword (including macro keywords and the syntactic keywords that introduce the core forms, like lambda, if, or define), or a use of a macro that eventually expands into an instance of syntax-rules. In the latter case, the keyword of macro use is called a custom macro transformer.

Mappings are finite sets of associations, where each association is a pair consisting of a key and an arbitrary Scheme value. The keys are elements of a suitable domain. Each mapping holds no more than one association with the same key. The fundamental mapping operation is retrieving the value of an association stored in the mapping when the key is given.

A means to denote the invalidity of certain code paths in a Scheme program is proposed. It allows Scheme code to turn the evaluation into a user-defined error that need not be signalled by the implementation. Optimizing compilers may use these denotations to produce better code and to issue better warnings about dead code.

This SRFI describes numeric procedures applicable to flonums, a subset of the inexact real numbers provided by a Scheme implementation. In most Schemes, the flonums and the inexact reals are the same. These procedures are semantically equivalent to the corresponding generic procedures, but allow more efficient implementations.

This SRFI describes arithmetic procedures applicable to a limited range of exact integers only. These procedures are semantically similar to the corresponding generic-arithmetic procedures, but allow more efficient implementations.

This SRFI provides a fairly complete set of integral division and remainder operators.

This attempts to solve the same issues with R7RS strings raised by SRFI-135, but with better integration with the Scheme language.

We propose to retain the name string as the type of sequences of Unicode characters (scalar values). There are two standard subtypes of string:

• Immutable strings, also called istrings, cannot be modified after they have been created. Calling string-set! on an istring throws an error. On the other hand, the core operations string-ref and string-length are guaranteed to be O(1).
• Mutable strings can be modified in-place using string-set! and other operations. However, string-ref, string-set!, or string-length have no performance guarantees. On many implementation they may take time proportional to the length of the string.

An implementation may support other kinds of strings. For example on the Java platform it may be reasonable to consider any instance of java.lang.CharSequence to be a string.

The main part of the proposal specifies the default bindings of various procedure names, as might be pre-defined in a REPL. Specifically, some procedures that traditionally return mutable strings are changed to return istrings. We later discuss compatibility and other library issues.

This combines SRFI-13, SRFI-135, and SRFI-118.

Syntax parameters are to the expansion process of a Scheme program what parameters are to the evaluation process of a Scheme program. They allow hygienic implementation of syntactic forms that would otherwise introduce implicit identifiers unhygienically.

This SRFI describes, for sufficiently POSIX-compatible systems, a portable interface for compiling Scheme programs conforming to the R7RS to binaries that can be directly executed on the host system.

This SRFI is intended to standardize a primitive run-time mechanism to create disjoint types.

SRFI 9 and the compatible R7RS-small provide Scheme with record types. The basic problem that is solved by these record types is that they allow the user to introduce new types, disjoint from all existing types. The record type system described in this document is a conservative extension to SRFI 9 and R7RS record types (in other words, the keyword define-record-type defined in this specification can serve as the equally named keyword from SRFI 9 and R7RS and can thus be safely exported from (srfi 9) and (scheme base)) that is intended to solve another fundamental problem, namely the introduction of subtypes.

In Scheme, strings are a mutable data type. Although it "is an error" (R5RS and R7RS) to use string-set! on literal strings or on strings returned by symbol->string, and any attempt to do so "should raise an exception" (R6RS), all other strings are mutable.

Although many mutable strings are never actually mutated, the mere possibility of mutation complicates specifications of libraries that use strings, encourages precautionary copying of strings, and precludes structure sharing that could otherwise be used to make procedures such as substring and string-append faster and more space-efficient.

This SRFI specifies a new data type of immutable texts. It comes with efficient and portable sample implementations that guarantee O(1) indexing for both sequential and random access, even in systems whose string-ref procedure takes linear time.

The operations of this new data type include analogues for all of the non-mutating operations on strings specified by the R7RS and most of those specified by SRFI 130, but the immutability of texts and uniformity of character-based indexing simplify the specification of those operations while avoiding several inefficiencies associated with the mutability of Scheme's strings.

This SRFI defines immutable deques. A deque is a double-ended queue, a sequence which allows elements to be added or removed efficiently from either end. A structure is immutable when all its operations leave the structure unchanged. Note that none of the procedures specified here ends with an exclamation point.

This SRFI proposes a comprehensive library of vector operations accompanied by a freely available and complete reference implementation. The reference implementation is unencumbered by copyright, and useable with no modifications on any Scheme system that is R5RS-compliant. It also provides several hooks for implementation-specific optimization as well.

This SRFI describes the API for a full-featured sort toolkit.

This SRFI is a reduced version of the SRFI 99 syntactic layer that can be implemented with syntax-rules without requiring low-level macros. Like SRFI-99's syntax layer, it is backward compatible with the define-record-type macro from SRFI 9 or R7RS-small. It is forward compatible with SRFI 99.

R5RS Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. Although R7RS provides some extensions and improvements, it is still very incomplete. This SRFI proposes a coherent and comprehensive set of string-processing procedures; it is accompanied by a portable sample implementation of the spec.

This SRFI is derived from SRFI 13. The biggest difference is that it allows subsequences of strings to be specified by cursors as well as the traditional string indexes. In addition, it omits the comparison, case-mapping, and mutation operations of SRFI 13, as well as all procedures already present in R7RS.

This SRFI defines R7RS-style char-title-case?, char-titlecase, and string-titlecase procedures.

This SRFI provides comparators, which bundle a type test predicate, an equality predicate, an ordering predicate, and a hash function (the last two are optional) into a single Scheme object. By packaging these procedures together, they can be treated as a single item for use in the implementation of data structures.

Lazy sequences (or lseqs, pronounced "ell-seeks") are a generalization of lists. In particular, an lseq is either a proper list or a dotted list whose last cdr is a SRFI 121 generator. A generator is a procedure that can be invoked with no arguments in order to lazily supply additional elements of the lseq. When a generator has no more elements to return, it returns an end-of-file object. Consequently, lazy sequences cannot reliably contain end-of-file objects.

This SRFI provides a set of procedures suitable for operating on lazy sequences based on SRFI 1.

We provide a hashtable API that takes the R6RS hashtables API as a basis and makes backwards compatible additions such as support for weak hashtables, external representation, API support for double hashing implementations, and utility procedures.

This SRFI defines an interface to hash tables, which are widely recognized as a fundamental data structure for a wide variety of applications. A hash table is a data structure that:

• Is disjoint from all other types.
• Provides a mapping from objects known as keys to corresponding objects known as values.
  • Keys may be any Scheme objects in some kinds of hash tables, but are restricted in other kinds.
  • Values may be any Scheme objects.
• Has no intrinsic order for the key-value associations it contains.
• Provides an equality predicate which defines when a proposed key is the same as an existing key. No table may contain more than one value for a given key.
• Provides a hash function which maps a candidate key into a non-negative exact integer.
• Supports mutation as the primary means of setting the contents of a table.
• Provides key lookup and destructive update in (expected) amortized constant time, provided a satisfactory hash function is available.
• Does not guarantee that whole-table operations work in the presence of concurrent mutation of the whole hash table (values may be safely mutated).

An ephemeron is an object with two components called its key and its datum. It differs from an ordinary pair as follows: if the garbage collector (GC) can prove that there are no references to the key except from the ephemeron itself and possibly from the datum, then it is free to break the ephemeron, dropping its reference to both key and datum. In other words, an ephemeron can be broken when nobody else cares about its key. Ephemerons can be used to construct weak vectors or lists and (possibly in combination with finalizers) weak hash tables.

Much of this specification is derived with thanks from the MIT Scheme Reference Manual.

Lisp dialects including Scheme have traditionally lacked short, simple, generic syntax for accessing and modifying the fields of arbitrary "collection" objects. We fill this gap for Scheme by defining generalized accessors, and an associated SRFI-17 setter.

This SRFI specifies an array mechanism for Scheme. Arrays as defined here are quite general; at their most basic, an array is simply a mapping, or function, from multi-indices of exact integers $i_0,\ldots,i_{d-1}$ to Scheme values. The set of multi-indices $i_0,\ldots,i_{d-1}$ that are valid for a given array form the domain of the array. In this SRFI, each array's domain consists of a rectangular interval $[l_0,u_0)\times[l_1,u_1)\times\cdots\times[l_{d-1},u_{d-1})$, a subset of $\mathbb Z^d$, $d$-tuples of integers. Thus, we introduce a data type called intervals, which encapsulate the cross product of nonempty intervals of exact integers. Specialized variants of arrays are specified to provide portable programs with efficient representations for common use cases.

This SRFI defines utility procedures that create, transform, and consume generators. A generator is simply a procedure with no arguments that works as a source of a series of values. Every time it is called, it yields a value. Generators may be finite or infinite; a finite generator returns an end-of-file object to indicate that it is exhausted. For example, read-char, read-line, and read are generators that generate characters, lines, and objects from the current input port. Generators provide lightweight laziness.

This SRFI defines interfaces to handle timer processes.

This SRFI describes a simple syntax which allows making scheme easier to read for newcomers while keeping the simplicity, generality and elegance of s-expressions. Similar to SRFI 110, SRFI 49 and Python it uses indentation to group expressions. Like SRFI 110 wisp is general and homoiconic.

Different from its predecessors, wisp only uses the absolute minimum of additional syntax-elements which are required for writing and exchanging arbitrary code-structures. As syntax elements it only uses a colon surrounded by whitespace, the period followed by whitespace as first code-character on the line and optional underscores followed by whitespace at the beginning of the line.

It resolves a limitation of SRFI 110 and SRFI 49, both of which force the programmer to use a single argument per line if the arguments to a procedure need to be continued after a procedure-call.

Wisp expressions can include arbitrary s-expressions and as such provide backwards compatibility.

wisp	s-exp
define : factorial n __ if : zero? n ____ . 1 ____ * n : factorial (- n 1) display : factorial 5 newline	(define (factorial n) (if (zero? n) 1 (* n (factorial (- n 1))))) (display (factorial 5)) (newline)

Scheme specifies mutable fixed-length strings. We add two procedures string-append! and string-replace! which allow the size of the string to change. We also require that the standard Scheme procedures make-string and string-copy return variable-size strings.

List queues are mutable ordered collections that can contain any Scheme object. Each list queue is based on an ordinary Scheme list containing the elements of the list queue by maintaining pointers to the first and last pairs of the list. It's cheap to add or remove elements from the front of the list or to add elements to the back, but not to remove elements from the back. List queues are disjoint from other types of Scheme objects.

Scheme currently does not provide immutable pairs corresponding to its existing mutable pairs, although most uses of pairs do not exploit their mutability. The Racket system takes the radical approach of making Scheme's pairs immutable, and providing a minimal library of mutable pairs with procedures named mpair?, mcons, mcar, mcdr, set-mcar!, set-mcdr!. This SRFI takes the opposite approach of leaving Scheme's pairs unchanged and providing a full set of routines for creating and dealing with immutable pairs. The sample implementation is portable (to systems with SRFI 9) and efficient.

This SRFI provides a library for matching strings with regular expressions described using the SRE "Scheme Regular Expression" notation first introduced by SCSH, and extended heavily by IrRegex.

Sets and bags (also known as multisets) are unordered collections that can contain any Scheme object. Sets enforce the constraint that no two elements can be the same in the sense of the set's associated equality predicate; bags do not.

This is a proposal for environment inquiry, providing human-readable information at run time about the hardware and software configuration on which a Scheme program is being executed. They are mostly based on Common Lisp, with additions from the Posix uname() system call.

Boxes are objects with a single mutable state. Several Schemes have them, sometimes called cells. A constructor, predicate, accessor, and mutator are provided.

This SRFI describes a set of syntax extensions for Scheme, called sweet-expressions (t-expressions), that has the same descriptive power as s-expressions but is designed to be easier for humans to read. The sweet-expression syntax enables the use of syntactically-meaningful indentation to group expressions (similar to Python), and it builds on the infix and traditional function notation defined in SRFI-105 (curly-infix-expressions). Unlike nearly all past efforts to improve s-expression readability, sweet-expressions are general (the notation is independent from any underlying semantic) and homoiconic (the underlying data structure is clear from the syntax). This notation was developed by the “Readable Lisp S-expressions Project” and can be used for both programs and data.

Sweet-expressions can be considered a set of additional abbreviations, just as 'x already abbreviates (quote x). Sweet-expressions and traditionally formatted s-expressions can be freely mixed; this provides backwards compatibility, simplifies transition, and enables developers to maximize readability. Here is an example of a sweet-expression and its equivalent s-expression (note that a sweet-expression reader would accept either format):

define fibfast(n)   ; Typical function notation
  if {n < 2}        ; Indentation, infix {...}
     n              ; Single expr = no new list
     fibup n 2 1 0  ; Simple function calls

(define (fibfast n)
  (if (< n 2)
      n
      (fibup n 2 1 0)))

This specifies a reader extension for extended string quasi-literals, including nicer multi-line strings, and enclosed unquoted expressions.

This proposal is related to SRFI-108 (extended string quasi-literals) and SRFI-107 (XML reader syntax), as they share quite a bit of syntax.

This specifies an extensible reader syntax for named value constructors. A reader prefix is followed by a tag (an identifier), and then expressions and literal text parameters. The tag can be though of as a class name, and the expression and literal text are arguments to an object constructor call. The reader translates &tag{...} to a list ($construct$:tag ...), where $construct$:tag is normally bound to a predefined macro.

This propsal depends on SRFI-109 (extended string quasi-literals) (in spite of having a lower number). It also shares quite of bit of syntax with SRFI-107 (XML reader syntax).

We specify a reader extension that reads data in a superset of XML/HTML format, and produces conventional S-expressions. We also suggest a possible semantics interpretation of how these forms may be evaluated to produce XML-node values, but this is non-normative.

This document specifies basic socket interfaces.

Lisp-based languages, like Scheme, are almost the only programming languages in modern use that do not support infix notation. In addition, most languages allow infix expressions to be combined with function call notation of the form f(x). This SRFI provides these capabilities, both for developers who already use Scheme and want these conveniences, and also for other developers who may choose to use other languages in part because they miss these conveniences. Scheme currently reserves {...} “for possible future extensions to the language”. We propose that {...} be used to support “curly-infix-expression” notation as a homoiconic infix abbreviation, as a modification of the Scheme reader. It is an abbreviation in much the same way that 'x is an abbreviation for (quote x).

A curly-infix list introduces a list whose visual presentation can be in infix order instead of prefix order. For example, {n > 5} ⇒ (> n 5), and {a + b + c} ⇒ (+ a b c). By intent, there is no precedence, but e.g., {x + {y * z}} maps cleanly to (+ x (* y z)). Forms with mixed infix operators and other complications have “$nfx$” prepended to enable later processing, e.g., {4 + 5 * 6} ⇒ ($nfx$ 4 + 5 * 6). Also, inside a curly-infix list (recursively), expressions of the form f(...) are simply an abbreviation for (f ...).

Note that this is derived from the “readable” project. We intend to later submit at least one additional SRFI that will build on top of this SRFI, but curly-infix-expressions are useful on their own.

Random-access lists [1] are a purely functional data structure for representing lists of values. A random-access list may act as a drop in replacement for the usual linear-access pair and list data structures (pair?, cons, car, cdr), which additionally supports fast index-based addressing and updating (list-ref, list-set). The impact is a whole class of purely-functional algorithms expressed in terms of index-based list addressing become feasible compared with their linear-access list counterparts.

This document proposes a library API for purely functional random-access lists consistent with the R⁶RS [2] base library and list utility standard library [3].

This SRFI introduces a macro, DEFINE-LAMBDA-OBJECT which defines a set of procedures, that is, a group, two constructors, and a predicate. The constructors also make a group of procedures, namely lambda objects. The macro extends DEFINE-RECORD-TYPE (SRFI 9) in being more general but much less general than DEFCLASS (CLOS). The macro has no explicit field accessors and mutators but parent groups, required fields, optional fields, automatic fields, read-write fields, read-only fields, inaccessible hidden fields, immutable virtual fields, and common sharing fields.

Many Scheme programmers have considered records to be one of the most important features missing from the R5RS. The R6RS proposed a record system, but its design has been widely criticized and it was not intended for use in R5RS programs anyway.

This SRFI proposes a better record system for use in R5RS, ERR5RS, and R6RS programs. The syntactic layer of this SRFI's record system is an extension of SRFI 9. The procedural and inspection layers of this SRFI's record system are perfectly compatible with its syntactic layer. This entire SRFI is compatible with the procedural and inspection layers of the R6RS record system, but offers several worthwhile improvements over the R6RS system.

This SRFI specifies the procedure get-environment-variable, which gets the value of the specified environment variable, and the procedure get-environment-variables, which gets an association list of all environment variables.

Over the past ten years, numerous libraries have been specified via the Scheme Requests for Implementation process. Yet until the recent ratification of the Revised⁶ Report on the Algorithmic Language Scheme, there has been no standardized way of distributing or relying upon library code. Now that such a library system exists, there is a real need to organize these existing SRFI libraries so that they can be portably referenced.

This SRFI is designed to facilitate the writing and distribution of code that relies on SRFI libraries. It identifies a subset of existing SRFIs that specify features amenable to provision (and possibly implementation) as libraries (SRFI Libraries) and proposes a naming convention for this subset so that these libraries may be referred to by name or by number.

This SRFI specifies a set of procedures and macros presenting a uniform interface sufficient to host the SLIB Scheme Library system.

Sorting and Merging are useful operations deserving a common API.

In the coding of numerial calculations in latent-typed languages it is good practice to assure that those calculations are using the intended number system. The most common number systems for programmatic calculations are the integers, reals, and complexes. This SRFI introduces 14 real-only and 3 integer-only variants of R5RS procedures to facilitate numerical type checking and declaration.

This SRFI specifies the procedure make-table, a hash table constructor compatible with SRFI 69 (Basic hash tables). The procedure make-table allows various parameters of the hash table to be specified with optional named parameters when it is constructed. These parameters are: the initial size, the minimum and maximum load factor, the key equivalence function, the key hashing function, whether the references to the keys are weak, and similarly for the values. By using optional named parameters, as specified in SRFI 89 (Optional positional and named parameters), the constructor's API can be easily extended in a backward compatible way by other SRFIs and Scheme implementations.

This SRFI specifies the define* and lambda* special forms. These forms extend the R5RS define and lambda special forms to simplify the use of optional positional and named parameters. Optional positional parameters, optional named parameters and required named parameters are covered by this SRFI. The formal parameter list syntax specified in this SRFI is different from the syntax used by Common Lisp and the DSSSL languages but nevertheless offers similar functionality and a nicer syntax. Formal parameter lists which conform to the R5RS syntax have the same meaning as in R5RS.

This SRFI defines keyword objects, a data type similar to Scheme symbols. Keyword objects have the same lexical syntax as symbols but they must end in a colon. Moreover keyword objects are self-evaluating. Procedures for converting between strings and keyword objects (string->keyword and keyword->string) and a type predicate (keyword?) are defined. Finally this SRFI specifies the changes to the Scheme lexical syntax required to accomodate keywords.

This SRFI proposes an extension to the case syntax to allow the => clauses as in cond.

Unlike the values/call-with-values mechanism of R5RS, this SRFI uses an explicit representation for multiple return values as a single value, namely a procedure. Decomposition of multiple values is done by simple application. Each of the two macros, mu and nu, evaluates to a procedure that takes one procedure argument. The mu and nu can be compared with lambda. While lambda expression that consists of <formals> and <body> requires some actual arguments later when the evaluated lambda expression is called, mu and nu expressions that consist of <expression>s corresponding to actual arguments of lambda require <formals> and <body>, that is, an evaluated lambda expression, later when the evaluated mu and nu expressions are called.

This SRFI also introduces new let-syntax depending on mu and nu to manipulate multiple values, alet and alet* that are compatible with let and let* of R5RS in single value bindings. They also have a binding form making use of values and call-with-values to handle multiple values. In addition, they have several new binding forms for useful functions such as escape, recursion, etc.

A simple mechanism is defined for testing Scheme programs. As a most primitive example, the expression

   (check (+ 1 1) => 3)

   (+ 1 1) => 2 ; *** failed ***
   ; expected result: 3

In addition to the simple test above, it is also possible to execute a parametric sequence of checks. Syntactically, this takes the form of an eager comprehension in the sense of SRFI 42 [5]. For example,

   (check-ec (:range e 100)
             (:let x (expt 2.0 e))
             (= (+ x 1) x) => #f (e x))

   (let ((e 53) (x 9007199254740992.0)) (= (+ x 1) x)) => #t ; *** failed ***
    ; expected result: #f

• A way to specify a different equality predicate (default is equal?).
• Controlling the amount of reporting being printed.
• Switching off the execution and reporting of checks entriely.
• Retrieving a boolean if all checks have been executed and passed.

This SRFI defines a set of procedures for creating, accessing, and manipulating octet-addressed blocks of binary data, in short, blobs. The SRFI provides access primitives for fixed-length integers of arbitrary size, with specified endianness, and a choice of unsigned and two's complement representations.

This SRFI describes a procedural macro proposal for Scheme with the following features:

• Improved hygiene:

  We argue that conventional hygiene algorithms may lead to accidental variable capture errors in procedural macros. We propose an improved algorithm that avoids these problems.

• Reflective tower:

  We specify a reflective tower of arbitrary height, and propose a refinement of lexical scoping that takes into account the phase of use of an identifier in determining its meaning.

• Syntax-case:

  In the current proposal, the syntax-case form is expressible as a macro in terms of a simpler set of primitives and is specified as library syntax.

• Procedural interface:

  The primitive interface for manipulating compound syntax objects consists of procedures rather than special forms. In particular, the traditional abstractions car, cdr, cons , ... can be used on syntactic data.

• Fast hygiene algorithm:

  The reference implementation documents a fast imperative hygiene algorithm that is eager and linear in expression size.

• Capturing identifiers:

  A primitive make-capturing-identifier is provided for intentional variable capture and for building expansion-time fluid binding constructs.

This SRFI is a proposal for extending let, let*, and letrec for receiving multiple values. The syntactic extension is fully compatible with the existing syntax. It is the intention that single-value bindings, i.e. (let ((var expr)) ...), and multiple-value binding can be mixed freely and conveniently.

The most simple form of the new syntax is best explained by an example:

(define (quo-rem x y)
  (values (quotient x y) (remainder x y)))

(define (quo x y)
  (let ((q r (quo-rem x y)))
    q))

The procedure quo-rem delivers two values to its continuation. These values are received as q and r in the let-expression of the procedure quo. In other words, the syntax of let is extended such that several variables can be specified---and these variables receive the values delivered by the expression (quo-rem x y).

The syntax of let is further extended to cases in which a rest argument receives the list of all residual values. Again by example,

(let (((values y1 y2 . y3+) (foo x)))
   body)

A common application of binding multiple values is decomposing data structures into their components. This mechanism is illustrated in its most primitive form as follows: The procedure uncons (defined below) decomposes a pair x into its car and its cdr and delivers them as two values to its continuation. Then an extended let can receive these values:

(let ((car-x cdr-x (uncons x)))
  (foo car-x cdr-x))

Of course, for pairs this method is probably neither faster nor clearer than using the procedures car and cdr. However, for data structures doing substantial work upon decomposition this is different: Extracting the element of highest priority from a priority queue, while at the same time constructing the residual queue, can both be more efficient and more convenient than doing both operations independently. In fact, the quo-rem example illustrates this point already as both quotient and remainder are probably computed by a common exact division algorithm. (And often caching is used to avoid executing this algorithm twice as often as needed.)

As the last feature of this SRFI, a mechanism is specified to store multiple values in heap-allocated data structures. For this purpose, values->list and values->vector construct a list (a vector, resp.) storing all values delivered by evaluating their argument expression. Note that these operations cannot be procedures.

This SRFI proposes text to replace section 6.2 "Numbers" of R5RS in order to extend its capabilities, correct errors in its specification, make it more explicit about limitations of precision and magnitude, and improve portability between implementations. More specifically, this new text:

• incorporates an inexact real positive infinity and an inexact real negative infinity,
• extends number syntax to incorporate inexact real infinities,
• adapts Common-Lisp semantics for `expt' and extends them to include inexact real infinities,
• corrects the description of `sqrt',
• sharpens the distinction between exact and inexact numbers,
• removes a contradiction related to exactness,
• extends `gcd' and `lcm' to exact rational numbers,
• extends `quotient', `modulo', and `remainder' to finite real numbers,
• clarifies the behavior of `inexact->exact' applied to an exact argument,
• clarifies the behavior of `exact->inexact' applied to an inexact argument,
• adds convenience procedures `exact-round', `exact-ceiling', `exact-floor', and `exact-truncate',
• and adds examples.

This SRFI defines basic hash tables. Hash tables are widely recognised as a fundamental data structure for a wide variety of applications. A hash table is a data structure that:

1. provides a mapping from some set of keys to some set of values associated to those keys
2. has no intrinsic order for the (key, value) associations it contains
3. supports in-place modification as the primary means of setting the contents of a hash table
4. provides key lookup and destructive update in amortised constant time, provided that a good hash function is used.

This SRFI aims to accomplish these goals:

1. to provide a consistent, generic and widely applicable API for hash tables
2. to improve code portability by providing a standard hash table facility with guaranteed behaviour
3. to help the programmer by defining utility routines that account for the most common situations of using hash tables.

This SRFI can be seen as an extension of the standard procedures =, <, char<? etc. of R⁵RS -- or even as a replacement. The primary design aspect in this SRFI is the separation of representing a total order and using it. For representing the order, we have chosen for truly 3-way comparisons. For using it we provide an extensive set of operations, each of which accepts a procedure used for comparison. Since these compare procedures are often optional, comparing built-in types is as convenient as R⁵RS , sometimes more convenient: For example, testing if the integer index i lies in the integer range {0, ..., n - 1} can be written as (<=/<? 0 i n), implicitly invoking default-compare.

As soon as new total orders are required, the infrastructure provided by this SRFI is far more convenient and often even more efficient than building each total order from scratch.

Moreover, in case Scheme users and implementors find this mechanism useful and adopt it, the benefit of having a uniform interface to total orders to be used in data structures will manifest itself. Most concretely, a new sorting procedure in the spirit of this SRFI would have the interface (my-sort [ compare ] xs), using default-compare if the optional compare was not provided. Then my-sort could be defined using the entire infrastructure of this SRFI: Efficient 2- and 3-way branching, testing for chains and pairwise inequality, min/max, and general order statistics.

This SRFI defines a set of procedures for creating, accessing, and manipulating uniform vectors of octets.

This defines an API for writing test suites, to make it easy to portably test Scheme APIs, libraries, applications, and implementations. A test suite is a collection of test cases that execute in the context of a test-runner. This specifications also supports writing new test-runners, to allow customization of reporting and processing the result of running test suites.

The SRFI, which is to supersede SRFI-47, "Array",

• synthesizes array concepts from Common-Lisp and Alan Bawden's "array.scm";
• incorporates all the uniform vector types from SFRI-4 "Homogeneous numeric vector datatypes";
• adds a boolean uniform array type;
• adds 16.bit and 128.bit floating-point uniform-array types;
• adds decimal floating-point uniform-array types; and
• adds array types of (dual) floating-point complex numbers.

SRFI-58 gives a read/write invariant syntax for the homogeneous and heterogeneous arrays described here.

This SRFI proposes a simple extension to Scheme's lexical syntax that allows individual S-expressions to be made into comments, ignored by the reader. This contrasts with the standard Lisp semicolon comments, which make the reader ignore the remainder of the line, and the slightly less common block comments, as SRFI 30 defines: both of these mechanisms comment out slices of text, not S-expressions.

This SRFI proposes an extension to the cond syntax to allow a more general clause, one that allows binding the results of tests as in the => clauses and user-defined meaning of the success & failure of tests.

Treating integers as two's-complement strings of bits is an arcane but important domain of computer science. It is used for:

• hashing;
• Galois-field[2] calculations of error-detecting and error-correcting codes;
• cryptography and ciphers;
• pseudo-random number generation;
• register-transfer-level modeling of digital logic designs;
• Fast-Fourier transforms;
• packing and unpacking numbers in persistant data structures;
• space-filling curves with applications to dimension reduction and sparse multi-dimensional database indexes; and
• generating approximate seed values for root-finders and transcendental function algorithms.

A vicinity is a descriptor for a place in the file system. Vicinities hide from the programmer the concepts of host, volume, directory, and version. Vicinities express only the concept of a file environment where a file name can be resolved to a file in a system independent manner.

All of these procedures are file-system dependent. Use of these vicinity procedures can make programs file-system independent.

SRFI-47 and its successor SRFI-63 provide both homogeneous numeric and heterogeneous multidimensional arrays which subsume Scheme vectors. The notation presented here builds upon Common-Lisp array syntax to represent heterogeneous arrays; and introduces a new Scheme-based notation for denoting homogeneous numeric arrays.

We describe a syntax for defining record types. A predicate, constructor, and field accessors and modifiers may be specified for each record type. We also introduce a syntax for declaring record type schemes, representing families of record types that share a set of field labels. A polymorphic predicate and polymorphic field accessors and modifiers may be specified for each record type scheme. A syntax is provided for constructing records by field label, for in-place and for functional record update, and for composing records.

This SRFI specifies an extremely simple facility for making an extension or library available to a Scheme toplevel environment.

This SRFI introduces the CAT procedure that converts any object to a string. It takes one object as the first argument and accepts a variable number of optional arguments, unlike the procedure called FORMAT.

This SRFI introduces the rest-values procedure which has three modes of operation:

1. it processes a rest list after checking its elements with default values or predicate procedures,
2. it processes a rest list with default values without checking its elements,
3. it processes a default list whose elements are lists or pairs, after checking their elements that are default values or predicate procedures with the elements of a rest list,

and eight macros which additionally check the rest arguments that are returned by rest-values.

This SRFI descibes a new syntax for Scheme, called I-expressions, whith equal descriptive power as S-expressions. The syntax uses indentation to group expressions, and has no special cases for semantic constructs of the language. It can be used both for program and data input.

It also allows mixing S-expressions and I-expressions freely, giving the programmer the ability to layout the code as to maximize readability.

This document specifies Format Strings, a method of interpreting a Scheme string which contains a number of format directives that are replaced with other string data according to the semantics of each directive. This SRFI extends SRFI-28 in being more generally useful but is less general than advanced format strings in that it does not allow, aside from ~F, for controlled positioning of text within fields.

"slib/array.scm" synthesizes array ideas from Common-Lisp and Alan Bawden with homogeneous vector ideas from SRFI-4 and SCM. The result portably integrates homogeneous and heterogeneous arrays into Scheme.

This SRFI proposes two extensions to the R5RS¹ syntax-rules pattern language: the first allows syntax-rules macros to generate macros, where the macro-generated macros use ellipsis that is not used by the macro-generating macros; the second allows for 'tail patterns.'

Lazy evaluation is traditionally simulated in Scheme using delay and force. However, these primitives are not powerful enough to express a large class of lazy algorithms that are iterative. Indeed, it is folklore in the Scheme community that typical iterative lazy algorithms written using delay and force will often require unbounded memory.

Although varous modifications of delay and force had been proposed to resolve this problem (see e.g., the SRFI-40 discussion list) they all fail some of the benchmarks provided below. To our knowledge, the current SRFI provides the first exhaustive solution to this problem.

As motivation, we first explain how the usual laziness encoding using only delay and force will break the iterative behavior of typical algorithms that would have been properly tail-recursive in a true lazy language, causing the computation to require unbounded memory.

The problem is then resolved by introducing a set of three operations:

    {lazy, delay, force}

Although this SRFI redefines delay and force, the extension is conservative in the sense that the semantics of the subset {delay, force} in isolation (i.e., as long as the program does not use lazy) agrees with that in R5RS. In other words, no program that uses the R5RS definitions of delay and force will break if those definition are replaced by the SRFI-45 definitions of delay and force.

A Collections API which defines a common naming scheme and set of operations for creating, accessing, and manipulating common datastructures in Scheme. The API defines accessors, a common protocol for value access via generic and specific enumeration, and functions for inter-datastructure cooperation. Finally, a concrete specification of a compliant set of operators for the standard Scheme heterogenous datastructures (lists and vectors) and for the homogeneous Scheme string is provided.

This SRFI proposes a comprehensive and complete library of vector operations accompanied by a freely available and complete reference implementation. The reference implementation is unencumbered by copyright, and useable with no modifications on any Scheme system that is R5RS-compliant. It also provides several hooks for implementation-specific optimization as well.

Because this SRFI is more of a library or module specification than a request for additions to readers or any other internal implementation detail, in an implementation that supports a module or structure or package or library or unit (et cetera) systems, these procedures should be contained in a module / structure / package / library / unit called vector-lib.

This SRFI defines a modular and portable mechanism for eager comprehensions extending the algorithmic language Scheme [R5RS]. An eager comprehension is a convenient notation for one or more nested or parallel loops generating a sequence of values, and accumulating this sequence into a result.

Streams, sometimes called lazy lists, are a sequential data structure containing elements computed only on demand. A stream is either null or is a pair with a stream in its cdr. Since elements of a stream are computed only when accessed, streams can be infinite. Once computed, the value of a stream element is cached in case it is needed again.

Streams without memoization were first described by Peter Landin in 1965. Memoization became accepted as an essential feature of streams about a decade later. Today, streams are the signature data type of functional programming languages such as Haskell.

This Scheme Request for Implementation describes two libraries for operating on streams: a canonical set of stream primitives and a set of procedures and syntax derived from those primitives that permits convenient expression of stream operations. They rely on facilities provided by R6RS, including libraries, records, and error reporting. To load both stream libraries, say:

(import (streams))

This SRFI defines parameter objects, the procedure make-parameter to create parameter objects and the parameterize special form to dynamically bind parameter objects. In the dynamic environment, each parameter object is bound to a cell containing the value of the parameter. When a procedure is called the called procedure inherits the dynamic environment from the caller. The parameterize special form allows the binding of a parameter object to be changed for the dynamic extent of its body.

This SRFI proposes (write-with-shared-structure) and (read-with-shared-structure), procedures for writing and reading external representations of data containing shared structure. These procedures implement a proposed standard external notation for data containing shared structure.

This SRFI permits but does not require replacing the standard (write) and (read) functions. These functions may be implemented without the overhead in time and space required to detect and specify shared structure.

An implementation conforms to this SRFI if it provides procedures named (write-with-shared-structure) and (read-with-shared-structure), which produce and read the same notation as produced by the reference implementation. It may also provide (read/ss) and (write/ss), equivalent functions with shorter names.

Many operating systems make the set of argument strings used to invoke a program available (often following the program name string in an array called argv). Most programs need to parse and process these argument strings in one way or another. This SRFI describes a set of procedures that support processing program arguments according to POSIX and GNU C Library Reference Manual guidelines.

This SRFI specifies a set of condition types for I/O errors. The condition types are defined in terms of SRFI 35. Moreover, this SRFI requires a Scheme system implementing it to raise exceptions in the sense of SRFI 34 for errors occurring during the execution of the R5RS I/O operations.

The SRFI defines constructs for creating and inspecting condition types and values. A condition value encapsulates information about an exceptional situation, or exception. This SRFI also defines a few basic condition types.

This SRFI defines exception-handling and exception-raising constructs for Scheme, including

• a with-exception-handler procedure and a guard form for installing exception-handling procedures,
• a raise procedure for invoking the current exception handler.

This SRFI is based on (withdrawn) SRFI 12: Exception Handling by William Clinger, R. Kent Dybvig, Matthew Flatt, and Marc Feeley.

We propose the implementation of a special form called rec. This form is a generalization and combination of the forms rec and named-lambda of [Clinger1985]. It allows the simple and non-imperative construction of self-referential expressions. As an important special case, it extends the A. Church form lambda such that it allows the direct definition of recursive procedures without using further special forms like let or letrec, without using advanced constructions like the H. B. Curry combinator and, unlike define, without introducing variable bindings into the external environment.

This SRFI extends R5RS by possibly nested, multi-line comments. Multi-line comments start with #| and end with |#.

This document specifies an interface to retrieving and displaying locale sensitive messages. A Scheme program can register one or more translations of templated messages, and then write Scheme code that can transparently retrieve the appropriate message for the locale under which the Scheme system is running.

This document specifies Format Strings, a method of interpreting a Scheme string which contains a number of escape sequences that are replaced with other string data according to the semantics of each sequence.

This document specifies an interface to sources of random bits, or "random sources" for brevity. In particular, there are three different ways to use the interface, with varying demands on the quality of the source and the amount of control over the production process:

• The "no fuss" interface specifies that (random-integer n) produces the next random integer in {0, ..., n-1} and (random-real) produces the next random real number between zero and one. The details of how these random values are produced may not be very relevant, as long as they appear to be sufficiently random.
• For simulation purposes, on the contrary, it is usually necessary to know that the numbers are produced deterministically by a pseudo random number generator of high quality and to have explicit access to its state. In addition, one might want to use several independent sources of random numbers at the same time and it can be useful to have some simple form of randomization.
• For security applications a serious form of true randomization is essential, in the sense that it is difficult for an adversary to exploit or introduce imperfections into the distribution of random bits. Moreover, the linear complexity of the stream of random bits is more important than its statistical properties. In these applications, an entropy source (producing truely random bits at a low rate) is used to randomize a pseudo random number generator to increase the rate of available bits.

Once random sources provide the infrastructure to obtain random bits, these can be used to construct other random deviates. Most important are floating point numbers of various distributions and random discrete structures, such as permutations or graphs. As there is an essentially unlimited number of such objects (with limited use elsewhere), we do not include them in this SRFI. In other words, this SRFI is not about making all sorts of random objects---it is about obtaining random bits in a portable, flexible, reliable, and efficient way.

When programming in functional style, it is frequently necessary to specialize some of the parameters of a multi-parameter procedure. For example, from the binary operation cons one might want to obtain the unary operation (lambda (x) (cons 1 x)). This specialization of parameters is also known as "partial application", "operator section" or "projection".

The mechanism proposed here allows to write this sort of specialization in a simple and compact way. The mechanism is best explained by a few examples:

As you see, the macro cut specializes some of the parameters of its first argument. The parameters that are to show up as formal variables of the result are indicated by the symbol <>, pronouced as "slot". In addition, the symbol <...>, pronounced as "rest-slot", matches all residual arguments of a variable argument procedure. As you can see from the last example above, the first argument can also be a slot, as one should expect in Scheme.

In addition to cut, there is a variant called cute (a mnemonic for "cut with evaluated non-slots") which evaluates the non-slot expressions at the time the procedure is specialized, not at the time the specialized procedure is called. For example,

As you see from comparing this example with the first example above, the cute-variant will evaluate (+ a 1) once, while the cut-variant will evaluate it during every invokation of the resulting procedure.

The mechanism proposed in this SRFI allows specializing any subset of the variables of a procedure. The result can be of fixed arity or of variable arity. The mechanism does not allow permutation, omission, duplication or any other processing of the arguments; for this it is necessary to write to use a different mechanism such as lambda.

A core set of procedures for creating and manipulating heterogeneous multidimensional arrays is proposed. The design is consistent with the rest of Scheme and independent of other container data types. It provides easy sharing of parts of an array as other arrays without copying, encouraging a declarative style of programming.

The specification is based on an original contribution by Alan Bawden in 1993.

A mechanism is proposed to allow Scheme code to report errors and abort execution. The proposed mechanism is already implemented in several Scheme systems and can be implemented, albeit imperfectly, in any R5RS conforming Scheme.

This SRFI describes basic prerequisites for running Scheme programs as Unix scripts in a uniform way. Specifically, it describes:

• the syntax of Unix scripts written in Scheme,
• a uniform convention for calling the Scheme script interpreter, and
• a method for accessing the Unix command line arguments from within the Scheme script.

This SRFI defines the following multithreading datatypes for Scheme

• Thread
• Mutex
• Condition variable
• Time

It also defines a mechanism to handle exceptions and some multithreading exception datatypes.

Points in time are represented a the number of seconds (with nanosecond precision) since "the epoch," a zero point in time. Several standard variants are defined, including UTC (universal coordinated time), TAI (international atomic time), and monotonic time. A point in time can also be represented as a Julian Day or Modified Julian Day number. Time durations, including time spent in a process or thread, are defined. Conversion routines are provided. The procedure CURRENT-TIME queries the current time in a specified variant, with a system-dependent resolution. Procedures for time arithmetic and time comparisons are also provided.

A date is a representation of a point in time in the Gregorian calendar, a 24 hour clock (with nanosecond precision) and a time zone offset from UTC. Procedures for converting between time and dates are provided, as well as for reading and writing string representations of dates.

This SRFI defines the following multithreading datatypes for Scheme

• Thread
• Mutex
• Condition variable
• Time

It also defines a mechanism to handle exceptions and some multithreading exception datatypes.

This is a proposal to allow procedure calls that evaluate to the "value of a location" to be used to set the value of the location, when used as the first operand of set!.For example:

(set! (car x) (car y))

(set-car! x (car y))

Many programming languages have the concept of an lvalue. that is an "expression" that "evaluates" to a location, and which can appear on the left-hand-side of an assignment. Common Lisp has a related concept of "generalized variables" which can be used in setf and some other special forms. However, the Common Lisp concept is based on the idea of compile-time recognition of special "location-producing" functions; this does not seem to be in the "spirit of Scheme".

This SRFI proposes an extension of set! so that it provides similar functionality as Common Lisp's setf, except that the updater is associated with a procedure value, rather than a name.

CASE-LAMBDA, a syntax for procedures with a variable number of arguments, is introduced.

The ability to efficiently represent and manipulate sets of characters is an unglamorous but very useful capability for text-processing code -- one that tends to pop up in the definitions of other libraries. Hence it is useful to specify a general substrate for this functionality early. This SRFI defines a general library that provides this functionality.

It is accompanied by a reference implementation for the spec. The reference implementation is fairly efficient, straightforwardly portable, and has a "free software" copyright. The implementation is tuned for "small" 7 or 8 bit character types, such as ASCII or Latin-1; the data structures and algorithms would have to be altered for larger 16 or 32 bit character types such as Unicode -- however, the specs have been carefully designed with these larger character types in mind.

Several forthcoming SRFIs can be defined in terms of this one:

• string library
• delimited input procedures (e.g., read-line)
• regular expressions

R5RS Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. This SRFI proposes a coherent and comprehensive set of string-processing procedures; it is accompanied by a reference implementation of the spec. The reference implementation is

• portable
• efficient
• open source

The routines in this SRFI are backwards-compatible with the string-processing routines of R5RS.

The SRFI introduces syntactic forms LET-VALUES and LET*-VALUES that bind the values of expressions that return multiple values.

The present SRFI proposes an extensible external representation of Scheme values, a notational convention for future SRFIs. This SRFI adds #,( as a new token and extends production rules of the grammar for a Scheme reader. The #,() form can be used for example to denote values that do not have a convenient printed representation, as well for conditional code compilation. It is proposed that future SRFIs that contain new read syntax for values use the #,() notation with an appropriate tag symbol.

As a particular example and the reference implementation for the #,() convention, this SRFI describes an interpretation of the #,() external form as a read-time application.

This SRFI describes syntax for creating new data types, called record types. A predicate, constructor, and field accessors and modifiers are defined for each record type. Each new record type is distinct from all existing types, including other record types and Scheme's predefined types.

The only mechanism that R⁵RS provides for binding identifiers to the values of a multiple-valued expression is the primitive call-with-values. This SRFI proposes a more concise, more readable syntax for creating such bindings.

This SRFI describes a configuration language to be used for specifing the set of Scheme features or extensions required to run a program. In addition to a list of required features, a program may also contain Scheme code to be used only when a particular feature or combination of features is available.

The configuration language is entirely distinct from Scheme; it is neither embedded in Scheme nor includes Scheme as a subset.

Scheme's i/o primitives are extended by adding three new procedures that

• create an input port from a string,
• create an output port whose contents are accumulated in Scheme's working memory instead of an external file, and
• extract the accumulated contents of an in-memory output port and return them in the form of a string.

The named-let incarnation of the let form has two slight inconsistencies with the define form. As defined, the let form makes no accommodation for rest arguments, an issue of functionality and consistency. As defined, the let form does not accommodate signature-style syntax, an issue of aesthetics and consistency. Both issues are addressed here in a manner which is compatible with the traditional let form but for minor extensions.

This SRFI describes a set of datatypes for vectors whose elements are of the same numeric type (signed or unsigned exact integer or inexact real of a given precision). These datatypes support operations analogous to the Scheme vector type, but they are distinct datatypes. An external representation is specified which must be supported by the read and write procedures and by the program parser (i.e. programs can contain references to literal homogeneous vectors).

Like an ordinary AND, an AND-LET* special form evaluates its arguments -- expressions -- one after another in order, till the first one that yields #f. Unlike AND, however, a non-#f result of one expression can be bound to a fresh variable and used in the subsequent expressions. AND-LET* is a cross-breed between LET* and AND.

R5RS Scheme has an impoverished set of list-processing utilities, which is a problem for authors of portable code. This SRFI proposes a coherent and comprehensive set of list-processing procedures; it is accompanied by a reference implementation of the spec. The reference implementation is

• portable
• efficient
• completely open, public-domain source

It is desirable that programs which depend on additions to standard Scheme name those additions. SRFIs provide the specifications of these additions ("features"), and SRFI 0 provides the means to actually check that these features are present in the Scheme system by means of the cond-expand construct. It is anticipated that there will be two main classes of features:

• sets of value and syntax bindings
• reader syntax extensions

("Reader syntax" refers to aspects of the syntax described by the grammars in the Scheme reports.)

The former class of features will probably include most SRFIs, exemplified by the list library specified in SRFI 1. The latter class includes Unicode source code support and different kinds of parentheses.

Control over the presence of individual features will vary over different Scheme systems. A given feature may be absent or provided by default in some Scheme systems and in others some mechanism (such as an "import" clause in the code or a program configuration file, a command line option, a dependency declaration in a module definition, etc.) will be required for the feature to be present in the system.

Moreover, in some systems a given feature may be in effect throughout the entire program if it is in effect anywhere at all. Other systems may have more precise mechanisms to control the scope of a feature (this might be the case for example when a module system is supported). In general it is thus possible that a feature is in effect in some parts of the program and not in others. This allows conflicting SRFIs to be present in a given program as long as their scope do not intersect.

SRFI 0 does not prescribe a particular mechanism for controlling the presence of a feature as it is our opinion that this should be the role of a module system. We expect that future module system SRFIs will need to extend the semantics of SRFI 0 for their purposes, for example by defining feature scoping rules or by generalizing the feature testing construct.