[Haskell-cafe] list comprehension with multiple generator|targets

[Mateusz Kowalczyk]

On 11/10/2014 02:58 AM, Donn Cave wrote:
> I'm guessing this isn't supported, but might be worth asking -
> can I extend a list comprehension like  ['A' | A <- s] to multiple values?
> Like,
> 
> data V = A | B | C
> 
> pv :: [V] -> [Char]
> pv [] = []
> pv (A:x) = 'A':(pv x)
> pv (B:x) = 'B':(pv x)
> pv (_:x) = pv x
> 
> -- can that be a list comprehension, like
> 
> pv s = [
>           'A' | A <- s
>           -- ??
>           ]
> 
> thanks,
> 	Donn

You basically want map and filter. Moreover, you are also inlining a
toChar function which complicates matters.

If you have ‘Eq V’ instance and ‘toChar’ function then you could write it as

[ toChar y | y <- [ x | x <- s, x /= C ] ]

Where inner comprehension is just filter and outer is just map. It
doesn't make much sense to do it this way and it imposes an extra
constraint, Eq. Alternative (with LambdaCase):

map toChar $ filter (\case { C -> False; _ -> True }) s

But that's ugly and we still need toChar. Further, although not really
applicable here, there might not be a reasonable toChar :: V -> Char for
every constructor of V.

So in conclusion, the way you have now is pretty good: it avoids Eq
constraint and it doesn't force us to write (possibly partial) toChar.

So to answer your question, no, you can't extend this very easily to
multiple without effectively inlining your existing ‘pv’ function into
the comprehension.

-- 
Mateusz K.