Simon Peyton-Jones
| The problem I see is that head/fromJust errors are usually |caused by *beginner* Haskellers, who don't know the |techniques for statically avoiding them.
I don't agree. My programs have invariants that I can't always express in a way that the type system can understand. E.g. I know that a variable is in scope, so searching for it in an environment can't fail: head [ v | (n,v) <- env, n==target ] (Maybe if I had an Oleg implant I could express all this in the type system -- but I don't.)
But instead of “blah (head [ v | (n,v) <- env, n==target ]) blah”, you could write blah the_v_in_scope blah where (the_v_in_scope:_) = [ v | (n,v) <- env, n==target ] and get a source-code located error message, couldn't you? It's not very high tech, but it's what you would write if head didn't exist, and it doesn't seem /that/ great an imposition. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk