[Haskell-cafe] Simple "Maybe" question

Matthew Walton matthew at alledora.co.uk
Tue Jan 27 13:04:01 EST 2004

Jim Lewis wrote:

> I'm new to Haskell and can't find an example to solve a trivial problem.
> I have code like this:
> findBlank :: [] -> Int
> findBlank str = findIndex (==' ') str
> But interpreter complains elsewhere of mismatch of Int with Maybe Int. I want to handle the maybe only here and nowhere else.
> This just makes matters worse:
> findBlank str = Just findIndex (==' ') str
> Thanks.

The result type of findIndex (==' ') str is Maybe Int, therefore you 
need to be prepared to handle both sides of a Maybe result, which could 
be Just n (where n is some Int), or Nothing. The simplest solution is to 
use 'case' to handle when findIndex returns Just n, and when findIndex 
returns Nothing, although you'll need to find a suitable Int for 
findBlank to return when findIndex returns Nothing.

Although were I writing the code, I'd be tempted to make findBlank 
return a Maybe Int as well, and let the code which calls that deal with 
Nothing as an error condition.

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