Implicit parameter and function

问题

I have a problem considering implicit parameters in Haskell (GHC). I have a function f, that assumes the implicit parameter x, and would like to encapsulate it in a context by applying f to g

f :: (?x :: Int) => Int -> Int
f n = n + ?x

g :: (Int -> Int) -> (Int -> Int)
g t = let ?x = 5 in t

But when i try to evaluate

g f 10

I get an error that x is not bound, e.g.:

Unbound implicit parameter (?x::Int)
  arising from a use of `f'
In the first argument of `g', namely `f'
In the second argument of `($)', namely `g f 10'

Can anybody tell me, what I am doing wrong?

(I am trying to get the WordNet Interface for Haskell to work - http://www.umiacs.umd.edu/~hal/HWordNet/ - and it uses on implicit parameters in the above manner, and I keep getting errors as the one above when I try to compile it)

回答1:

The first parameter of g must be of type ((?x::Int) => Int -> Int) to clarify that ?x should be passed to f. This can be dony be enabling Rank2Types (or RankNTypes). Unfortunately, GHC cannot infer this type.

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE Rank2Types #-}

f :: (?x::Int) => Int -> Int
f n = n + ?x

g :: ((?x::Int) => Int -> Int) -> (Int -> Int)
g f = let ?x = 5 in f`

Now g f 10 works.

回答2:

The problem here is that ?x isn't bound at the point it's referenced. You and I can see that ?x will be bound within g, but the compiler can't. One (confusing) solution is to change

g f 10

to

g (let ?x = 5 in f) 10

来源：https://stackoverflow.com/questions/11141600/implicit-parameter-and-function