SRFI Libraries


David Van Horn


This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-97@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

Table of contents


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.


With the advent of R6RS, it has become possible to write and distribute portable Scheme code as libraries. However, there is no agreed upon way to refer to SRFI libraries, leaving implementors to create their own conventions thus thwarting portability of code which relies on SRFIs. So there is an immediate and obvious need for standardized SRFI library names.

The naming convention proposed in this document enjoys the following properties:

  1. it fully conforms with the R6RS library reference syntax,
  2. it is consistent with the naming convention employed by R6RS Standard Libraries,
  3. it is based on SRFI numbers so that names for libraries can be chosen simply and without conflict,
  4. it allows (optional) mnemonic based references for readability and sub-libraries,
  5. it ensures uniqueness of names over time,
  6. it avoids the need for a naming authority,
  7. and it represents a consensus among several Scheme implementors at the time.


This specification includes the following: a library referencing scheme for all past and future SRFI libraries, a naming convention for selecting library names, and enumeration of library names selected for all currently finalized SRFIs that specify libraries.

Although R6RS's notion of a library is the one used here, this SRFI attempts to minimize assumptions made about a Scheme system's conformance to R6RS so that systems may support SRFI Libraries without needing to support all of the R6RS library system. In particular, the for clause is not needed for any existing SRFI Library, and versioning information is not used. This is important for systems such as ERR5RS, which do not support these features.

Referencing SRFI Libraries

The purpose of this SRFI is to name SRFI libraries in a consistent and uniform manner, both in the case of existing SRFIs and in the case of SRFIs yet to be written.

A SRFI library reference consists of two required components:

Discussion: Several current Scheme implementations supporting SRFIs use names consisting of srfi and the relevant SRFI number. For example, Larceny, PLT Scheme, Scheme 48 all do. Use of these components also has precedents in SRFIs 0 and 7.

Discussion: Since this SRFI is only concerned with the naming of SRFI libraries, the srfi prefix is included to clearly distinguish it as a SRFI reference, and the number component is included to disambiguate among SRFIs, while allowing for simple naming without clashes or authorities. During the discussion period, it was made clear some would prefer libraries not be distinguished as SRFIs and that names be strictly mnemonic. Such a requirement broadens the scope of this proposal from a convention for SRFI libraries to the more general case of Scheme libraries and poses a much more difficult problem to solve. Instead, the particular problem is solved in such a way as to hopefully not interfere with future more general naming conventions.

Additionally, a SRFI library reference consists of one or more optional mnemonic components following the naming convention described below.

A SRFI library reference has the following form,

(srfi :<uinteger 10> <identifier> ...),

where <identifier> is one of the included SRFI library names defined below, and <uinteger 10> is a SRFI number. The non-terminals <uinteger 10> and <identifier> are defined as given in R6RS Chapter 4, Lexical syntax.

Discussion: The colon (:) prefix is used in order to make the SRFI number component an identifier, as required by R6RS. Several Scheme implementors expressed the importance of R6RS conformance and supported the colon prefix as a reasonable, if somewhat arbitrary, way to render the number as an identifier. The issue of mapping library references to file paths is left completely unspecified by this SRFI and R6RS.

A SRFI Library can be referenced by number, as in

(srfi :1),

or equivalently using the library's name, as in

(srfi :1 lists).

Discussion: Even though the latter uses a library name, the SRFI number must still be supplied. This is done to ensure fairness and uniqueness—a SRFI should not be able to prevent other SRFIs from using the same name, but at the same time, SRFI library references should remain unambiguous.

Discussion: Since the number-only naming scheme ensures both uniqueness and fairness, the named-library scheme is redundant, but it is included to make library imports more readable. Although the same could be achieved via a comment, comments are not checked. The named-library scheme can thus be used to increase the likelihood that the correct library is imported.

The remaining identifier subforms are used to name sublibraries within a SRFI Library. For example, SRFI 41, Streams, defines primitive and derived libraries within the streams library. These can be referenced as

(srfi :41 streams primitive),
(srfi :41 streams derived).

Naming convention

Mnemonic library names are chosen according to the following conventions: Libraries that define datatypes are typically pluralized (e.g., arrays, basic-string-ports). Libraries that provide new expression forms are named by the name of their new form (e.g., let-values, rec, and-let*). Libraries that define a single procedure (or that have a primary procedure) are given the name of the procedure (e.g., error, cat). Libraries that provide a large API of syntaxes and values are named in a descriptive manner derived from their title (e.g., lightweight-testing, real-time-multithreading). This naming convention is consistent with the names chosen in the R6RS Standard Libraries.

Discussion: Some authors specified names that should be used for libraries. These names are not used here. Instead, a consistent naming scheme is used that happens to differ from the authors' originally chosen names.

Discussion: There is opportunity for grouping related SRFIs together. For example, SRFI 28, Basic Format Strings, and SRFI 48, Intermediate Format Strings, could be grouped into a common format library with simple and intermediate components. This has been avoided, preferring instead to follow a one SRFI, one (top-level) library convention. Although within a single SRFI, several sub-libraries can be defined, thus achieving grouping. This simplifies the naming convention and prevents the need for a naming authority which decides what shall and shall not be grouped together. Instead, grouping must be achieved by the SRFI process, as it should.

Included SRFIs

The criterion for inclusion in the set of SRFI Libraries is that the finalized SRFI only specify a set of bindings which can be provided as a library. SRFI Libraries need not be implementable as an R6RS library, but they must specify an interface which could be used as if it were. This means, for example, that lexical syntax extensions cannot be a part of a SRFI Library. SRFIs that specify global semantic changes to Scheme also cannot be SRFI Libraries. For example, a Lazy Scheme SRFI or a SRFI that fixed order of evaluation in procedure application could not be a SRFI Library because these features cannot be provided as a set of bindings.

Some SRFIs specify alternative semantics to standard Scheme bindings. In some cases, such as SRFI 5's let, it's clear that making the alternative binding for let available as a library binding is desirable. Authors that wish to use SRFI 5's let, rather than R6RS's let, simply import the SRFI 5 binding and exclude the R6RS binding for let (or rename to avoid the clash).

Other SRFIs, however, define alternative bindings that are intended to be used throughout a Scheme system. For example, SRFI 63's equal? and most of SRFI 70, Numbers, are defined like this. But if the author intends for these bindings to replace the standard bindings, then there is no way of making this feature available as a library, so the SRFI cannot be a SRFI Library.

The approach taken in this SRFI is that a SRFI is excluded as a SRFI Library if it explicitly states the binding is to replace the standard one. For example, SRFI 70, Numbers, is excluded because it explicitly revises the text of R5RS, thus intending to make a global semantic change to the language which cannot be provided as a library. On the other hand, SRFI 63, Homogeneous and Heterogeneous Arrays, is included and provides a binding for equal? because the SRFI 63 document does not explicitly state this should replace the standard equal? binding.

The following SRFI library names are defined:

SRFILibrary namesTitle
1listsList Library
2and-let*AND-LET*: an AND with local bindings, a guarded LET* special form
5letA compatible let form with signatures and rest arguments
6basic-string-portsBasic String Ports
8receivereceive: Binding to multiple values
9recordsDefining Record Types
11let-valuesSyntax for receiving multiple values
13stringsString Libraries
14char-setsCharacter-Set Library
16case-lambdaSyntax for procedures of variable arity
17generalized-set!Generalized set!
18multithreadingMultithreading support
19timeTime Data Types and Procedures
21real-time-multithreadingReal-time multithreading support
23errorError reporting mechanism
25multi-dimensional-arraysMulti-dimensional Array Primitives
26cutNotation for Specializing Parameters without Currying
27random-bitsSources of Random Bits
28basic-format-stringsBasic Format Strings
31recA special form for recursive evaluation
38with-shared-structureExternal Representation for Data With Shared Structure
39parametersParameter objects
41streams, streams primitive, streams derivedStreams
42eager-comprehensionsEager Comprehensions
43vectorsVector Library
45lazyPrimitives for expressing iterative lazy algorithms
46syntax-rulesBasic Syntax-rules Extensions
48intermediate-format-stringsIntermediate Format Strings
51rest-valuesHandling rest list
60integer-bitsIntegers as Bits
61condA more general cond clause
63arraysHomogeneous and Heterogeneous Arrays
64testingA Scheme API for test suites
66octet-vectorsOctet Vectors
67compare-proceduresCompare Procedures
69basic-hash-tablesBasic hash tables
71letLET-syntax for multiple values
74blobsOctet-Addressed Binary Blocks
78lightweight-testingLightweight testing
86mu-and-nuMU and NU simulating VALUES & CALL-WITH-VALUES, and their related LET-syntax
87case=> in case clauses
95sorting-and-mergingSorting and Merging
For the purposes of R6RS conforming systems, export levels are assumed to be 0 for all exported bindings of all SRFIs, except SRFI-46: Basic syntax-rules extensions, which should export syntax-rules for level 1. This allows all existing SRFI Libraries to be portably imported and used without use of the R6RS for clause.

Omitted SRFIs

SRFIs are not libraries, they are requests for features. Thus, several SRFIs specify features which cannot be provided as a library and those SRFIs are intentionally omitted here. For example, SRFIs which specify extensions to concrete syntax or extensions to the semantics of top-level programs cannot be provided as libraries and are therefore omitted from this SRFI. The following table lists omitted (finalized) SRFIs and the reason for their omission.

SRFITitleReason for omission
0Feature-based conditional expansion constructModifies semantics of top-level programs.
4Homogeneous numeric vector datatypesModifies lexical syntax.
7Feature-based program configuration languageDefines a configuration language distinct from Scheme.
10#, external formModifies lexical syntax.
22Running Scheme Scripts on UnixDoes not specify a library.
30Nested Multi-line CommentsModifies lexical syntax (subsumed by R6RS).
34Exception Handling for ProgramsSubsumed by R6RS.
36I/O ConditionsSubsumed by R6RS.
37args-fold: a program argument processorDoes not specify a library.
40A Library of StreamsDeprecated by SRFI 41, Streams.
49Indentation-sensitive syntaxModifies lexical syntax.
55require-extensionModifies semantics of top-level programs.
58Array NotationModifies lexical syntax.
62S-expression commentsModifies lexical syntax (subsumed by R6RS).
70NumbersModifies standard semantics for number system.
72Hygienic macrosModifies macro expansion semantics.
88Keyword objectsModifies lexical syntax.
89Optional positional and named parametersModifies syntax and semantics of application, which cannot be provided by a library. Dependent on lexical syntax modification (SRFI 88).
90Extensible hash table constructorDependent on lexical syntax modification (SRFI 88).
94Type-Restricted Numerical FunctionsDependent on number system modification (SRFI 70).

Withdrawn SRFIs

Withdrawn SRFIs are considered outside the scope of this proposal, so their names are not considered.

Future SRFIs

Authors of future SRFIs that specify libraries may choose names as they please, but they are encouraged to use library names following the naming convention of this SRFI. If names are chosen that are not compatible with the referencing scheme given here, authors should specify alternative names that can be used to refer to the library consistent with this specification.

SRFI 41, Streams, for example, specifies that its libraries are named by (streams), (streams primitive), and (streams derived), but according to this SRFI, they must also be named with the prefix (srfi :41 *).

Issues with existing SRFIs and R6RS

There are many issues involved in the use and implementation of some of the existing SRFIs in an R6RS or related system. This SRFI resolves none of them.

See the discussion archive and previous revisions of this document for a list of some of these issues.

Relation to SRFIs 0 and 7

Both SRFI 0, Feature-based conditional expansion construct, and SRFI 7, Feature-based program configuration language, use feature identifiers of the form srfi-N, with the expectation "that features will eventually be assigned meaningful names (aliases) by the SRFI editors to make reading and writing code less tedious than when using srfi-N feature identifiers."

It is recommended that implementations which support SRFIs 0 or 7 use the SRFI names defined here as aliases. SRFI feature identifiers are constructed by

  1. interpolating library references with a hyphen (-),
  2. interpreting the SRFI number component of a library reference ":n" as "n".

For example, the feature identifier for SRFI 1, List Library, is srfi-1, or equivalently, srfi-1-lists. Feature identifiers for sub-libraries are constructed in the same way, so for example the feature identifier for the streams primitive library is srfi-41-streams-primitive.


There is no reference implementation as this SRFI only specifies a naming convention.


For discussions, I thank Will Clinger, Aziz Ghuloum, Dave Herman, Olin Shivers, Andre van Tonder, and my fellow SRFI editors. I also thank all those who participated during the draft period of this SRFI.


Revised6 Report on the Algorithmic Language Scheme
Michael Sperber, et al. (Editors)
A proposal for an Extended R5RS Scheme
William Clinger, et al.
Ikarus Scheme
PLT Scheme
Scheme 48


Copyright (C) David Van Horn 2008. All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


Editor: Donovan Kolbly