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haskelloperator-precedence

Weird behavior for infix functions regarding precedence

I am using the bool :: a -> a -> Bool -> a function. I wanted to use the infix version, because I though it more readable, but I noticed that:

(-1) `bool` 1 True

is an error

(-1) `bool` 1 $ True

works. Even

(-1) `bool` 1 (True)

doesn't work, which I thought was an equal alternative until now (i.e. using $ versus wrapping in parentheses from this location till the end)

How can this even make a difference? In the first version there is only one single operation.

Solution

  • Infix operators bind loosely, applications bind tightly.

    (-1) `bool` 1 True
-- means
(-1) `bool` (1 True)


(-1) `bool` 1 $ True
-- means
((-1) `bool` 1) $ True


(-1) `bool` 1 (True)
-- means
(-1) `bool` (1 (True))

    You might want:

    ((-1) `bool` 1) True