[Haskell-cafe] nested maybes

[Mattias Bengtsson]

On Sun, 2007-02-04 at 19:54 +0530, Martin DeMello wrote:
> I have a Data.Map.Map String -> (Layout, [String]) as follows:
> 
> type Anagrams = [String]
> type Cell = (Layout, Anagrams)
> type WordMap = Map.Map String Cell
> 
> exists str wmap =
>   let a = Map.lookup (sort str) wmap in
>       case a of
>            Nothing -> False
>            Just x -> case (find (== str) (snd x)) of
>                           Nothing -> False
>                           _       -> True
> 
> the existence test looks ugly - any more compact way to write it?
> 

Using the Maybe Monad is one solution i think (as in: i _think_ this
should work):

findIt str wmap = do 
  a <- Map.lookup (sort str) wmap
  return $ find (== str) (snd a)

exists str wmap = 
  case findIt str wmap of
    Nothing -> False
    Just _ -> True

The Maybe monad is very nice for abstracting away all those
case-expressions.
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