Documentation, assignment in expression.

Jussi Piitulainen jpiitula at ling.helsinki.fi
Mon Mar 26 13:13:48 CEST 2012


Kiuhnm writes:
> On 3/26/2012 10:52, Devin Jeanpierre wrote:
> > On Sun, Mar 25, 2012 at 11:16 AM, Kiuhnm
> > <kiuhnm03.4t.yahoo.it at mail.python.org>  wrote:
> >> On 3/25/2012 15:48, Tim Chase wrote:
> >>>
> >>> The old curmudgeon in me likes the Pascal method of using "=" for
> >>> equality-testing, and ":=" for assignment which feels a little closer to
> >>> mathematical use of "=".
> >>
> >>
> >> Unfortunately, ":=" means "is defined as" in mathematics. The "right"
> >> operator would have been "<-".
> >
> >
> > "Is defined as" is actually pretty reasonable. "Define this to be
> > that" is a common way to speak about assignment. Its only difference
> > is the present tense. For example, in Python, "def" stands for
> > "define", but we can overwrite previous definitions::
> >
> >      def f(x): return x
> >      def f(x): return 2
> >      f(3) == 2
> >
> > In fact, in pretty every programming language that I know of with a
> > "define" assignment verb, this is so. For example, in Scheme, x is 2
> > at the end::
> >
> >      (define x 1)
> >      (define x 2)
> >      x
> 
> When you write
>    (define x 1)
>    (define x 2)
>    x
> or, in F# and OCaml,
>    let x = 1
>    let x = 2
>    x
> you're saying
>    x = 1
>    {
>       x = 2
>       x
>    }
> You don't modify 'x': you hide it by defining another "value" (not
> variable) with the same name.
> Indeed,
>    let x = 1
>    let x = 2
>    x
> is shorthand for
>    let x = 1 in
>    let x = 2 in
>    x

No, Devin is right about Scheme. On "top level" re-definition is
interpreted as assignment. The following mean the same:

(define x 1) (define x 2) x
(define x 1) (set! x 2) x
       
Local definitions in the beginning of a "body" do not allow duplicate
names at all. The following mean the same:

(let () (define x 1) (define y 2) x)
(letrec* ((x 1) (y 2)) x) ;letrec in older versions (not sure of R6RS)

But (let () (define x 1) (define x 2) x) is still an error. Some
implementations may give it a meaning. Not sure.



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