Erlang article #1 on programming.reddit.com
Richard A. O'Keefe
Fri Aug 18 04:16:20 CEST 2006
I mentioned the
(let N = 0 then N+1, S = 0 then S+X
for X <- List, X > 0
construct that had been designed recently in this mailing list.
"Ryan Rawson" <ryanobjc@REDACTED> wrote:
That's more of a list comprehension than a loop now is it?
It's as loopy a for loop as ever there was.
List comprehensions make lists.
This kind of loop *may* make a list, but needn't.
[ E(X) || X <- L, G(X) ]
can be expressed in this form as
(let R =  then [E(X)|R]
for X <- L, G(X)
You can think of this as a multi-result fold + map + filter.
In other words, you can think of it as a for loop.
If you think a 'for' loop must have something to do with counting,
reflect that a compiler might recognise X <- lists:seq(1, N) and
do something special with it (if you promise faithfully not to change
the semantics of seq/2 next time you reload the lists module, of course...).
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