Haskell Language Design Questions

Tom Pledger Tom.Pledger@peace.com
Tue, 9 Jan 2001 11:04:38 +1300

Doug Ransom writes:
 > 2.	It seems to me that the Maybe monad is a poor substitute for
 > exception handling because the functions that raise errors may not
 > necessarily support it.

It sometimes helps to write such functions for monads in general,
rather than for Maybe in particular.  Here's an example adapted from
the standard List module:

    findIndex       :: (a -> Bool) -> [a] -> Maybe Int
    findIndex p xs   = case findIndices p xs of
                           (i:_) -> Just i
                           []    -> Nothing

It generalises to this:

    findIndex       :: Monad m => (a -> Bool) -> [a] -> m Int
    findIndex p xs   = case findIndices p xs of
                           (i:_) -> return i
                           []    -> fail "findIndex: no match"

The price of the generalisation is that you may need to add some type
signatures to resolve any new overloading at the top level of the