[英]Newbie question on syntax and type signature in Haskell/HappStack
Why is it that I can't do 为什么我不能这样做
z = x?
but I can do this? 但我可以这样做吗?
y s = x s
I'm a Haskell newbie This is what I've been trying in GHCi: 我是Haskell新手这是我在GHCi中尝试过的:
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'
Looks like another case of the monomorphism restriction . 看起来像单态限制的另一种情况。
You can either include the argument explicitly, ie ys = xs
, include an explicit type signature, or run GHCi with -XNoMonomorphismRestriction
. 您可以显式包含参数,即ys = xs
,包含显式类型签名,或使用-XNoMonomorphismRestriction
运行GHCi。
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