On 3/26/2012 13:13, Jussi Piitulainen wrote:
Kiuhnm writes:
On 3/26/2012 10:52, Devin Jeanpierre wrote:
On Sun, Mar 25, 2012 at 11:16 AM, Kiuhnm
<kiuhnm03.4t.yahoo...@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.
Thanks for the correction. I haven't written a line of code in Scheme
for 15 years and it shows :(
Kiuhnm
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