关于Haskell/HappStack中语法和类型签名的新手问题

Question

为什么我不能

z = x?

但我能做到吗？

y s = x s

我是Haskell的新手，这是我在GHCi上尝试过的：

Prelude> import Happstack.Server
Prelude Happstack.Server> let x s = ok $ toResponse $ "Some string"
Prelude Happstack.Server> :t x
x :: FilterMonad Response m => t -> m Response

Prelude Happstack.Server> let y s = x s
Prelude Happstack.Server> :t y
y :: FilterMonad Response m => t -> m Response

Prelude Happstack.Server> let z = x
<interactive>:1:9:
    No instance for (FilterMonad Response m0)
      arising from a use of `x'

Answer 1

看起来像是monomorphism restriction的另一个病例。

您可以显式包含参数，即y s = x s，包含显式类型签名，或者使用-XNoMonomorphismRestriction运行GHCi。