相关疑难解决方法(0)

考虑这个例子:

applyKTimes :: Integral i => i -> (a -> a) -> a -> a
applyKTimes 0 _ x = x
applyKTimes k f x = applyKTimes (k-1) f (f x)

applyThrice :: (a -> a) -> a -> a
applyThrice = applyKTimes 3

该3在applyThrice通过GHC默认为一个Integer与编译时如图所示-Wall:

Warning: Defaulting the following constraint(s) to type 'Integer'
         'Integral t'
           arising from a use of 'applyKTimes'

所以我猜这Integer是默认的Integral a => a.

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