[Haskell-cafe] Re: Practise fingerspelling with Haskell! (Code cleanup request)

[apfelmus]

Dougal Stanton wrote:
> I worked out that [ (a,b) | a <- as, b <- bs ] must be equivalent to
> 
>> comp = concatMap (\x -> map ((,) x) ys) xs
> 
> but I can't really say how conditions like "a /= b" get slotted in to
> that style. Is there a reference for that?

Here's an example translation

   [ (a,b) | a <- as, b <- bs, a /= b ]
 = concatMap (\a -> [ (a,b) | b <- bs, a /= b ]) as
 = concatMap (\a -> concatMap (\b -> [ (a,b) | a /= b ]) bs) as
 = concatMap (\a ->
     concatMap (\b -> if a /= b then [(a,b)] else []) bs) as

The exact specification can be found at

  http://www.haskell.org/onlinereport/exps.html#list-comprehensions

Of course, this is not very different from monadic expressions in the
[]-monad.

Regards,
apfelmus