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

On 11/10/2014 03:32 AM, Raphaël Mongeau wrote:

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]

The problem with this solution is that your pattern match is partial. Add a D constructor and you get a pattern match failure. You could extend to ‘\case { A -> Just 'A'; …; _ -> Nothing }’ and use mapMaybe instead of map but it doesn't answer the question of using list comprehensions.

2014-11-09 22:28 GMT-05:00 Mateusz Kowalczyk :

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On 11/10/2014 02:58 AM, Donn Cave wrote:

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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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Mateusz K.