The following passages differ on the status of (a+b+): 3 Expressions aexp -> ... | ( expi+1 qop(a,i) ) (left section) | ( qop(a,i) expi+1 ) (right section) 3.5 Sections Syntactic precedence rules apply to sections as follows. (op e) is legal if and only if (x op e) parses in the same way as (x op (e)); and similarly for (e op). For example, (*a+b) is syntactically invalid, but (+a*b) and (*(a+b)) are valid. Because (+) is left associative, (a+b+) is syntactically correct, but (+a+b) is not; the latter may legally be written as (+(a+b)). ghc follows the grammar, rejecting (a+b+) Hugs accepts (op a op' b) and rejects (a op b op') regardless of associativity and precedence, while nhc accepts them both. The grammar could be made to match the text by adding two alternatives: | ( lexpi qop(l,i) ) (left section) | ( qop(r,i) rexpi ) (right section) Changing the text to match the grammar seems less attractive. But the text says more than the revised grammar. For example, it deals with the expression glitch I mentioned the other week: together with the disambiguation meta-rule, it implies that the expression (let x = 10 in x `div`) is invalid, because let x = 10 in x `div` z doesn't parse as (let x = 10 in x) `div` z (might be worth adding this as an example) but it's no help with SimonM's example let x = 10 in x == x == True