141592653's questions

在 GHCi 中运行以下命令会在 上抛出错误，stuff' 4但在 上不会抛出错误stuff 4。为什么？

有办法修复吗？我想使用MkStuff'而不是MkStuff。

ghci> 
:{
class MkStuff s m | m -> s where
  stuff:: s -> m ()

instance MkStuff Int IO where
  stuff = print

class MkStuff' s m | m -> s where
  stuff' :: s -> m

instance MkStuff' Int (IO ()) where
  stuff' = print
:}

ghci> stuff 4
4
ghci> stuff' 4

<interactive>:16:1: error: [GHC-39999]
    • No instance for ‘MkStuff' Integer ()’ arising from a use of ‘it’
    • In the first argument of ‘print’, namely ‘it’   
      In a stmt of an interactive GHCi command: print it