On 2012-07-24 10:10, Christian Sternagel wrote:
Dear all,
with respect to formal verification of Haskell code I was wondering whether (==) of the Eq class is intended to be commutative (for many classes such requirements are informally stated in their description, since Eq does not have such a statement, I'm asking here). Or are there any known cases where commutativity of (==) is violated (due to strictness issues)?
Strictness plays no role for Eq, since to test for equality both sides will have to be fully evaluated. I think (==) is supposed to be equivalence relation, which is symmetric (i.e. commutative); as well as reflexive and transitive. There are some cases of (==) not being reflexive, Floats being the most notable one, but many people consider that to be a bug. I can't think of any instances that violate symmetry. Twan