On Sun, Sep 13, 2009 at 3:40 PM, Brent Yorgey
On Sat, Sep 12, 2009 at 09:13:58PM +0300, Michael Snoyman wrote:
I often times have to write a lookup function that returns its value into any monad instead of just Maybe. For example:
mLookup :: (Eq k, Monad m) => k -> [(k, v)] -> m v mLookup k pairs = case lookup k pairs of Nothing -> fail "mLookup: nothing found" Just v -> return v
This is actually the type that the lookup function USED to have, but it was changed since monads actually have nothing to do with failing (the fail method is just a hack used to handle pattern-match failures in do-notation). Probably a better implementation of this would be
mLookup :: (Eq k, MonadPlus m) => k -> [(k,v)] -> m v mLookup k pairs = maybe mzero return (lookup k pairs)
I understand that fail being in Monad is controversial, but my version of the function works in *all* monads. This is very useful for: 1) Quickly writing up code in the IO monad (ie, for a shell script) 2) Check out the data-objects library; having an mLookup function makes dealing with mappings very convenient. Michael