新手問題/ HappStack

Question

爲什麼我不能做

z = x? 

，但我能做到這一點？

y s = x s 

我是一個新手哈斯克爾這 是我一直在GHCI一直在努力：

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

貌似monomorphism restriction的另一種情況。

您可以明確包含參數，即y s = x s包含明確的類型簽名，或者運行GHCi與-XNoMonomorphismRestriction。