有谁知道为什么这段代码会失败?
{-# LANGUAGE NoMonomorphismRestriction,
TypeFamilies #-}
module Test where
asExprTyp :: Expr γ =>
γ α
-> α
-> γ α
asExprTyp x _ = x
int = undefined :: Integer
class Expr γ where
a :: γ α
-- this works fine
b = a `asExprTyp` int
-- this fails
mcode = do
return ()
where b = a `asExprTyp` int
错误如下,
Test.hs:23:15:
Ambiguous type variable `γ0' in the constraint:
(Expr γ0) arising from a use of `a'
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `asExprTyp', namely `a'
In the expression: a `asExprTyp` int
In an equation for `b': b = a `asExprTyp` int
Failed, modules loaded: none.