haskell - 在负输入 Haskell 上崩溃

Question

谁能解释为什么这会在负输入时崩溃？

add :: Integral a => (a -> a) -> a -> a
add f n | n<0  = error "only non-negative integers allowed as input"
    | otherwise     = sum[ f x |x<-[1..n] ]


foo:: Int -> Int
foo x = x

---

*Main> add foo -5

<interactive>:82:9:
No instance for (Num (Int -> Int))
arising from a use of `-'
Possible fix: add an instance declaration for (Num (Int -> Int))
In the expression: add foo - 5
In an equation for `it': it = add foo - 5

Answer 1

你必须写 (-5) 来表明你使用的是一元 -，否则 haskell 会读取 -作为二元运算符:

add foo (-5)