On 2009 Jan 5, at 21:40, dcmorse+haskell@gmail.com wrote:
Learning from the example of "read" and also Real World Haskell, I come across the idea to overload my function's return types. Trying to think of an application for this, I've always wanted to write == applications like in Icon, that is
a === b === c means a == b && b == c.
This requires === to sense what context it is called in. If it's being called for a Boolean value, it needs to return a Boolean value. If it's being called as a parameter to another === application, then it needs to somehow remember both it's truthiness and if true what value its already seen.
My thought is that Icon's notion of failure as an out-of-band result is best captured by the Monad instance for Maybe (or, perhaps more generally, MonadZero or whatever we're going to call it this time around; at the moment that means Monad). Unfortunately, this can't be made especially clean: given
(<==) :: Eq a => a -> a -> m Bool a <== b = if a <= b then b else fail ">"
which causes -1 <== x <== 1 (say) to do the right thing, you have to either escape the monad to use it as a comparison or create a lifted if-then-else. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH