关于 Haskell/HappStack 中语法和类型签名的新手问题
为什么我做不到，
z = x?
但我却能做到这一点？
y s = x s
我是哈斯克尔新手 这是我在 GHCi 中一直在尝试的：
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'
Why is it that I can't do
z = x?
but I can do this?
y s = x s
I'm a Haskell newbie
This is what I've been trying in GHCi:
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'
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看起来像是单态限制的另一种情况。
您可以显式包含参数，即
ys = x s，包含显式类型签名，或者使用-XNoMonomorphismRestriction运行 GHCi。Looks like another case of the monomorphism restriction.
You can either include the argument explicitly, i.e.
y s = x s, include an explicit type signature, or run GHCi with-XNoMonomorphismRestriction.