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

[Raphaël Mongeau]

Wow, didn't know about the LambdaCase.

Here is the code with LambdaCase, filter and Eq

{-# LANGUAGE LambdaCase #-}

data V = A | B | C deriving (Eq)

f :: [V] -> String
f l = flip map (filter (/= C) l) $ \case
    A -> 'A'
    B -> 'B'

main = print $ f [A,B,C,C,A]


2014-11-09 22:28 GMT-05:00 Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk>:

> 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.
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>



-- 
Viva Cila
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