Newbie question on syntax and type signature in Haskell/HappStack
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 instan开发者_如何转开发ce for (FilterMonad Response m0)
arising from a use of `x'
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
.
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