如何在Haskell中组合多态函数？

我有以下文件：

module SimpleComposition where

class Domain a where
    f :: a -> a
    g :: a -> a

h = f . g

尝试在 ghci 中加载它时，出现错误

srcplay.hs:7:5: error:
    * No instance for (Domain c0) arising from a use of `f'
    * In the first argument of `(.)', namely `f'
      In the expression: f . g
      In an equation for `h': h = f . g

我认为问题在于fand的类型gisforall a. Domain a => a ->a而不仅仅是a -> a，所以这些 foralls 是碍事的。但我还是想创作它们。如何？

回答

{-# LANGUAGE NoMonomorphismRestriction #-}

h :: Domain a => a -> a
h = f . g