Consider
let f1 x = x + 1 in f1 1 -- ==> 2 ok let f2 x y = x + y in (f2 2) 3 -- ==> 5 ok let (f2 x) y = x + y in f2 3 4 -- ==> 7 ok let (f1 x) = x + 1 in f1 4
The last gives a syntax error `Parse error in pattern: f1`. This is in line with the Language Report, but why? Something to do with patterns?
let ( x ) = 1 in x -- ==> 1 ok let (Just x) = Just 6 in x -- ==> 6, pattern binding for x ok let Just x = Just 7 in x -- ==> 7 ok let (CP x y) = (CP 1) 2 in x -- ==> 1.0 ok let (CP x) y = (CP 3) 4 in x -- `Parse error in pattern: (CP x)`
So the parens are optional for a pattern binding. But if you use them they must enclose the whole pattern -- unlike partially applying a data constr on rhs. OTOH for a function binding, on lhs there must be at least one var (or pattern) not appearing inside outermost parens. Why not optional parens around a whole function binding? It might be your lhs is declaring an operator. Then that can't be inside parens, fair enough:
let (Just x) +? (Just y) = (x +) y in ... -- ok let ((Just x) +?) (Just y) = x + y in ... -- rejected
Lexically we can tell a function (starts lower case) from a data constr. Why the extra restriction with the parens? AntC