问题
I was hoping someone could explain the following behavior in GHCi, when using the function fromIntegral:
Prelude> let x = 1 :: Integer
Prelude> :t x
x :: Integer
Prelude> sqrt $ fromIntegral x
1.0
Prelude> let y = fromIntegral x
Prelude> sqrt y
<interactive>:181:1:
No instance for (Floating Integer)
arising from a use of `sqrt'
Possible fix: add an instance declaration for (Floating Integer)
In the expression: sqrt y
In an equation for `it': it = sqrt y
Why does it matter whether I set y and then take its sqrt or just directly take the sqrt?
回答1:
fromIntegral is polymorphic in its return type. So the type of y in your code could be expected to be Num a => a. This type would allow you to use y as the argument to sqrt without problem.
However due to the monomorphism restriction, the type of y is not allowed to be polymorphic. Therefore it is defaulted to the default Num type, which is Integer.
When you do sqrt $ fromIntegral x the monomorphism restriction does not apply because it only applies to global variables and you don't store the result of fromIntegral in a variable this time.
You can fix this issue by either adding a type signature to y (let y :: Num a => a; y = fromIntegal x) or by disabling the monomorphism restriction.
来源：https://stackoverflow.com/questions/11439163/inconsistent-behavior-with-fromintegral-in-ghci