http://www.haskell.org/~simonmar/haskell-2010-draft-report-2/haskellch3.html infixexp → lexp qop infixexp (infix operator application) | - infixexp (prefix negation) | lexp This grammar rule describes a right associative nesting of (any) infix operators "qop" and prefix negation as binding weaker than any infix. Thus a parser would create from "- 1 /= 1 && a" the tree "- (1 /= (1 && a))". Would it not be better to give an ambiguous grammar and leave it to the infix resolution algorithm to allow only the intended trees, rather than letting the infix resolution algorithm correct a wrong tree? My suggestion would be to change the rule to: infixexp → infixexp qop infixexp (infix operator application) | - infixexp (prefix negation) | lexp thus only replacing the first lexp by infixexp. Cheers Christian