To my

Anyway, if a compiler `knows' the word CommutativeRing, [..] expr = (2*a + b) - (a+a) :: T

and sees the language version (compiler) flag -fignore-bottom, (something of this sort) then, has it right to convert

expr --> b :: T ? [..]

D. Tweed <tweed@compsci.bristol.ac.uk> writes

On a purely pragmatic note ( :-) ) these optimisations can't necessarily be safely used in three of the four most common cases, namely Int, Float and Double because of the restricted range of intermediates. [..]

Int, Float, Double are not supposed to be provided with the CommutativeRing instance. And most common cases are, to my mind, different: Integer, Rational, (Integer, Rational), Polynomial Integer ... Regards, ----------------- Serge Mechveliani mechvel@botik.ru