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There is this index function in “Erlang Programming”:
index(0, [X|_]) -> X;
index(N, [_|Xs]) when N>0 -> index(N-1, Xs)
Isn’t the guard “when N>0” superfluous because of the pattern matching? Calling index(0, List) will never end up in the second clause so N will always be > 0. Or am I totally wrong here?
The function works correctly for N>=0. Without a guard, for N<0 it would traverse the whole list:
index(-2,[1,2,3]) -> index(-3,[2,3]) -> … -> index(-5,[]) -> error.
That isn’t a large problem, only you might get a confusing exception. In languages with infinite lists (Haskell, Ocaml), forgetting about that guard might lead to infinite loop: index(-1, [0,0,0..]).