Jan Stolarek
Hi all,
consider this simple reimplementation of 'elem' function:
member :: Eq a => a -> [a] -> Bool member _ [] = False member y (x:xs) | x == y = True | otherwise = member y xs
If Haskell allowed to write pattern matching similar to Prolog then we could write this function like this:
member :: Eq a => a -> [a] -> Bool member _ [] = False member x (x:_) = True member x (_:xs) = member x xs
This kind of pattern matching was considered and rejected by the very first Haskell committee. Others have given some of the reasoning, and I don’t recall the rest so won’t attempt to rehearse them here. What I would like to say (without meaning to attack you personally, Jan) is that we really need to make a rule that this sort of small convenience should never be considered. Every now and a language feature will be invented that really makes a large difference to a large number of programmes (do notation was a prime example), so the language should evolve. But against that, there is considerable value in having the basic syntax remain unchanged for as long as possible. I don’t know how to formulate it formally, but the rule should be something like, unless a new feature shortens programmes that can use it by a significant factor, it shouldn’t be included. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk