return value of `string-set!'

return value of `string-set!'

[Alexandru Cojocaru]

Hi,
currently `string-set!' returns an unspecified value. What about making 
it return `str' instead?

Here is an example of its use:

; make a copy of `str' and replace its first character with `#\a'
(define (make-copy str)
  (string-set! (string-copy str) 0 #\a))

instead of:

(define (make-copy str)
  (let ((s (string-copy str)))
   (begin
     (string-set! s 0 #\a)
     s)))

or perhaps I am doing something wrong?

PS: the documentation of `list-set!' doesn't say what's its return value.

Best regards,
Alexandru Cojocaru

Re: return value of `string-set!'

[William ML Leslie]

On 15 August 2013 08:33, Alexandru Cojocaru <xojoc@gmx.com> wrote:
> Hi,
> currently `string-set!' returns an unspecified value. What about making it
> return `str' instead?

Functions whose specific purpose is to mutate the provided value
usually don't return the value because that is a common source of bugs
- this a common pattern in the string code and generally, too.

> Here is an example of its use:
>
> ; make a copy of `str' and replace its first character with `#\a'
> (define (make-copy str)
>  (string-set! (string-copy str) 0 #\a))
>
> instead of:
>
> (define (make-copy str)
>  (let ((s (string-copy str)))
>   (begin
>     (string-set! s 0 #\a)
>     s)))
>
> or perhaps I am doing something wrong?
>
> PS: the documentation of `list-set!' doesn't say what's its return value.

Thanks for pointing that out!  Curiously, it returns the value that
was set.  I wonder if this was intentional.

> Best regards,
> Alexandru Cojocaru

-- 
William Leslie

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Re: return value of `string-set!'

[Taylan Ulrich B.]

Alexandru Cojocaru <xojoc@gmx.com> writes:

> (define (make-copy str)
>  (let ((s (string-copy str)))
>   (begin
>     (string-set! s 0 #\a)
>     s)))

FYI the `begin' there is redundant.

(define (make-copy str)
  (let ((s (string-copy str)))
    (string-set! s 0 #\a)
    s))

This is idiomatic Scheme code; it makes it obvious that we're 1) getting
a value, 2) doing something to it, 3) returning it.  It is immediately
obvious code, in contrast to the one-liner that relies on a procedure
meant for mutating an object to also return that object (which is one
sensible choice for its return value, but not immediately obvious, or
the only choice, or inherently logical).  Of course this is all
subjective, and there could have been, for example, a convention that
all object-mutating procedures return the object they mutate, but that
is simply not the tradition/idiom; it's best to get used to the
established Scheme style. :)

Re: return value of `string-set!'

[Ludovic Courtès]

William ML Leslie <william.leslie.ttg@gmail.com> skribis:

> On 15 August 2013 08:33, Alexandru Cojocaru <xojoc@gmx.com> wrote:
>> Hi,
>> currently `string-set!' returns an unspecified value. What about making it
>> return `str' instead?
>
> Functions whose specific purpose is to mutate the provided value
> usually don't return the value because that is a common source of bugs
> - this a common pattern in the string code and generally, too.

Plus these functions are standard (RnRS, SRFI-13, etc.)

Ludo’.