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

This : pv a = [t | Just t <- pvc a] is strange, can we really do pattern matching inside list comprehension? If I try to make your code work its lead me to this: import Data.Maybe data V = A | B | C pv l = catMaybes [pvc e | e <- l] where pvc A = Just 'A' pvc B = Just 'B' pvc _ = Nothing main = print $ pv [A,B,C,C,A] As you can see, [pvc e | e <- l] is just "map (plv) l" and I think the where is more clear with a lambdaCase. {-# LANGUAGE LambdaCase #-} import Data.Maybe data V = A | B | C pv l = catMaybes $ flip map l $ \case A -> Just 'A' B -> Just 'B' _ -> Nothing main = print $ pv [A,B,C,C,A] No, this solution does not use list comprehension, but your problem need some form of pattern matching and as Mateusz said, inlining it inside the function would be ugly. Since all the real work of your problem is in the case with the Maybe and the _ I think list comprehension can't offer much. I think its interesting how the case is doing the job of the earlier discussed filter AND the mapping to a char. And as a bonus it support adding D, E, F ... to the V data without much trouble. 2014-11-09 22:47 GMT-05:00 Donn Cave :

quoth Mateusz Kowalczyk [... re someone else's example ]

...

...

{-# 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.

Indeed, I'd rigged up something with Maybe for this, like

pv a = [t | Just t <- pvc a] where pvc A = Just 'A' pvc B = Just 'B' _ = Nothing

... when it occurred to me that I might be wasting the power of the list comprehensions that I so rarely use. Guess not! Thanks,

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