I don't like notElem either
* quite useful for cleaning up guards:
f x | not (x `Set.member` map) && foo = ... is hard to read.
You may swap quards (or the "then" and "else" branches of "if"), or avoid the infix notation (which is horrible in this case anyway).
Although I'd normally agree with negated versions of things being somewhat redundant, the infix version here is rather important, since one often wants the code to look the same way as the mathematics. The symbols \in and \not\in are both usually written infix, and that's not going to change soon. Putting the membership operation in between is nice, because we read the guard here as "If x is a member of xs then..." rather than "If member x xs then..." anyway. Perhaps if the proposal to allow somewhat more general expressions as infix operators goes through (which I'm actually ambivalent about), I'd feel different about this case. If you want to reduce redundancy, let's unify the 3 separate ways to say 'map'. :) - Cale