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set!. For example:
(set! (car x) (car y))becomes equivalent to
(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
so that it provides similar functionality as Common Lisp's
except that the updater is associated with a procedure value,
rather than a name.
For most languages, the set of lvalue-producing operators is limited
(typically array indexing and field selection). Some languages have
general lvalues as first class values. For example Algol 68 has
expressions that have reference type. However, this is made convenient
by using automatic dereferencing coercions, which would not work for
a dynamically typed language like Scheme. ML goes further: All
mutable variables are first-class "cells", and accessing the
contents of a cell requires an explicit operator. This is also not
compatible with Scheme. Instead we need to stick to the model
where using a variable in most contexts means using its value,
but refering to a variable in certain lvalue contexts (lhs of
assignment) refers to its actual location. Sticking to this model
for general "lvalue expressions" in
that "evaluation" must be done differently from normal
evaluation when in an "lvalue context". That is what this proposal does.
This is a controversial issue. This srfi does not wish to imply that
all Scheme implementations should support this feature; it only
requests that implementations that do implement
set! should be compatible with this srfi.
set!is extended so the first operand can be a procedure application, and not just a variable. The procedure is typically one that extracts a component from some data structure. Informally, when the procedure is called in the first operand of
set!, it causes the corresponding component to be replaced by the second operand. For example,
(set (vector-ref x i) v)would be equivalent to:
(vector-set! x i v)
Each procedure that may be used as the first operand to
must have a corresponding "setter" procedure.
The builtin procedure
setter takes a procedure and returns the
corresponding setter procedure.
(set! (proc arg ...) value)as:
((setter proc) arg ... value)
This definition matches
the existing Scheme convention for setter procedures, where
the new value is given last. For example we can define
(setter car) to be
An alternative definition would be:
((setter proc) value arg ...) ;; Not the actual definition.This definition would work better when you consider procedures that take a variable number of arguments. This is because it is straight-forward to add one extra initial fixed argument, but if you add an extra fixed argument to the end of an argument list that has a "rest" parameter, then things get more messy. However, consistency with the existing new-value-last convention seems more valuable.
(set! (car x) v) == (set-car! x v) (set! (cdr x) v) == (set-cdr! x v) (set! (caar x) v) == (set-car! (car x) v) (set! (cadr x) v) == (set-car! (cdr x) v) .... (set! (caXXr x) v) == (set-car! (cXXr x) v) (set! (cdXXr x) v) == (set-cdr! (cXXr x) v) (set! (string-ref x i) v) == (string-set! x i v) (set! (vector-ref x i) v) == (vector-set! x i v)
documentationfunction, where for example:
(documentation sqrt)returns the "documentation string" (if defined) for
sqrt. Such properties should also be settable using generalized
set!. For example:
(set! (documentation sqrt) "Calculates the square root.")
This SRFI does
not propose a general "procedure properties" feature, but it
should be compatible with it. It does specify the special case
setter property. This is defined such that:
(set! (setter proc) setter)sets the setter procedure associated with proc to setter. For example, we can assume
(set! (setter car) set-car!)has been executed by the Scheme prologue.
(set! (foo ..) ...)is to be the preferred idiom, we want to make
((setter foo) ...)as efficient as
(set-foo! ...). This is only possible when the compiler knows both what function the symbol
foois bound to, and the setter associated with that function. Scheme (as opposed to Common Lisp) does not say anything about a compiler or what it can inline, so we cannot say much here. A compiler that does whole-program analysis can safely inline calls using a variable bound unchangably to a known procedure; it can make the same deduction if the procedure's setter is set. If separate compilation is supported, then a compiler cannot safely make such deductions for either plain calls or setter calls, without extra information, such as a module system, compiler switches, or other non-standard declarations. Thus my belief is that this srfi does not inherently make efficient compilation more difficult. However, the next section does define a way to inherently tie a setter to a procedure, which does reduce the problem of inlining generalized
set!to the standard inlining problem.
getter-with-setter can be used
to define a procedure with associated properties.
(getter-with-setter getter setter)This evaluates to a new anonymous procedure which when applied invokes getter, and whose setter is setter. It is an error for a program to subsequently try to modify the setter of the resulting compound.
For example, we could define:
(define car (getter-with-setter %primitive-car %primitive-set-car!)) (define set-car! %primitive-set-car!)The advantage of this approach that whenever a compiler can inline
car, it can also inline
Here is a sample definition of
(define (getter-with-setter get set) (let ((proc (lambda args (apply get args)))) (set! (setter proc) set) proc))
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