Hello all, I'm a newbie at Haskell and I was not aware of this problem. So, equality comparison can run into an infinite-loop? My current knowledge of the language tells me that everything is Haskell is a thunk until it's value is really needed. Is it possible to implement (==) that first check these thunks before evaluating it? (Considering both arguments has pure types). E.g., Equivalent thunks, evaluates to True, does not need to evaluate its arguments: [1..] == [1..] Another case: fib = 1:1:zipWith (+) fib (tail fib) fibA = 1:tail fib fib == fibA -- True Evaluating: 1:1:zipWith (+) fib (tail fib) == 1:tail fib -- first item match, check further 1:zipWith (+) fib (tail fib) == tail fib -- thunks do not match, evaluate arguments 1:zipWith (+) fib (tail fib) == 1:zipWith (+) fib (tail fib) -- thunks matches, comparison stops and the value is True As I said before, I'm a newbie at Haskell. Sorry if my question or examples makes no sense. Thanks, Thiago.