[Swift-devel] Problem with iterate

[Mihael Hategan]

On Sun, 2010-02-21 at 08:37 +0000, Ben Clifford wrote:
> > And then ML would just have full static scoping, so whatever is visible
> > in the scope of foo's definition is visible in its invocation.
> 
> But that only works if your variables are visible. For static parameters, 
> sure. But:
> 
> foreach p1 in [1,2,3] { foreach p2 in [10,20,30] { o[p1][p2] = p1 + p2 }}
> 
> o = map outerbody [1,2,3]
> 
> outerbody p = map innerbody [10,20,30]

You're not using p there

> 
> innerbody q = q + ?

outerbody p = map innerbody zip [10, 20, 30] replicate 3 p
innerbody p q = p + q

Or you could deal with nested foreaches by merging the values into
tuples:

body p q = p + q
p = map body cross [1, 2, 3] [10, 20, 30]

where we'd define:
cross a b = map (\c -> zip b replicate length b c) a

Either way it kinda looks like gibberish if you're not used to it.

> 
> So you end up wanting to define innerbody inside the scope of outerbody,

Right, so we'd need proper closures.

> 
> outputbody p = let
>   innerbody q = q + p
>  in map innerbody [10,20,30]
> 
> which looks pretty un-c-like indeed.
>  But I guess something like that is 
> necessary if you want to access loop variables from nested loops (which I 
> think is desirable).

There's also the list comprehension way with C-like syntax, which
involves no array indices:

o = list(
  foreach p in [1, 2, 3] {
    list(
      foreach q in [10, 20, 30] {
        p + q;
      }
    )
  }
)