[Haskell-cafe] list comprehension with multiplegenerator|targets

[Donn Cave]

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