[Haskell-cafe] list comprehension with multiplegenerator|targets

[Raphaël Mongeau]

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 <donn at avvanta.com>:

> quoth Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk>
> [... 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,
>
>         Donn
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-- 
Viva Cila
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