问题
All of the following expressions get evaluated without mishap:
(+2) 1 -- 3
(*2) 1 -- 2
((-)2) 1 -- 1
(2-) 1 -- 1
(/2) 1 -- 0.5
(2/) 1 -- 2.0
but not this one:
(-2) 1 -- the inferred type is ambiguous
GHC throws some error about the inferred type being ambiguous. Why?
回答1:
Each of these parenthesized expressions but (-2) (edit: and ((-) 2)) are sections, i.e. functions that take one argument and "put it on the missing side of the infix operator" (see this haskell.org wiki).
(-2) is not a function, but a number (negative 2):
λ> :t (-2)
(-2) :: Num a => a
If you write
λ> (-2) 1
it looks like you're trying to apply (-2) (a number) to 1 (which is not possible), and GHCi rightfully complains:
Could not deduce (Num (a0 -> t))
arising from the ambiguity check for ‘it’
from the context (Num (a -> t), Num a)
bound by the inferred type for ‘it’: (Num (a -> t), Num a) => t
at <interactive>:3:1-6
The type variable ‘a0’ is ambiguous
When checking that ‘it’
has the inferred type ‘forall a t. (Num (a -> t), Num a) => t’
Probable cause: the inferred type is ambiguous
If you want a function that subtracts 2 from another number, you can use
(subtract 2)
Compare its type,
λ> :t (subtract 2)
(subtract 2) :: Num a => a -> a
to that of (-2) (see above).
Terminology addendum (after OP's edit)
Parenthesizing the minus operator turns it into a normal (prefix) function that takes two arguments; therefore ((-) 2) is not a section, but a partially applied function.
来源：https://stackoverflow.com/questions/28890335/why-is-the-unary-minus-operator-problematic-in-this-expression-2-1