The difference between +1 and -1

Question

> :t (+1)
(+1) :: Num a => a -> a

> :t (-1)
(-1) :: Num a => a

How come the second one is not a function? Do I have to write (+(-1)) or is there a better way?

Selfmade

Possible duplicate: https://mcmap.net/q/534498/-currying-subtraction – Enrol

Answer 1

This is because (-1) is interpreted as negative one, however (+1) is interpreted as the curried function (\x->1+x).

In haskell, (a **) is syntactic sugar for (**) a, and (** a) is (\x -> x ** a). However (-) is a special case since it is both a unary operator (negate) and a binary operator (minus). Therefore this syntactic sugar cannot be applied unambiguously here. When you want (\x -> a - x) you can write (-) a, and, as already answered in Currying subtraction, you can use the functions negate and subtract to disambiguate between the unary and binary - functions.

Enrol

"Negation is the only prefix operator in Haskell" haskell.org/onlinereport/exps.html – Perquisite

@mjg123, I think writing "the unary operator" rather then "a unary operator" here only makes it harder to get the message which is "its both unary and binary." Therefor I think I will leave it as it is. =D – Enrol

Answer 2

Do I have to write (+(-1)) or is there a better way?

I just found a function called subtract, so I can also say subtract 1. I find that quite readable :-)

Selfmade

+1 subtract was made specifically for use in sections. Haskell 2010 / Expressions # Sections "Because - is treated specially in the grammar, (- exp) is not a section, but an application of prefix negation, as described in the preceding section. However, there is a subtract function defined in the Prelude such that (subtract exp) is equivalent to the disallowed section. The expression (+ (- exp)) can serve the same purpose." – Tannie

Answer 3

(-1) is negative one, as others have noted. The subtract one function is \x -> x-1, flip (-) 1 or indeed (+ (-1)).

- is treated as a special case in the expression grammar. + is not, presumably because positive literals don't need the leading plus and allowing it would lead to even more confusion.

Edit: I got it wrong the first time. ((-) 1) is the function "subtract from one", or (\x -> 1-x).

Speiss

the usual "subtract one" function is just subtract 1 – Pasha