ScopedTypeVariables下の1ランク多相で型が合わないとき
まとめ
forallを指定してあげると、一連のa, bが固定されてくれます。
f :: forall a b. Eq b => (a -> b) -> (a -> b) -> a -> Bool解説
以下のようなコードを書くと
{-# LANGUAGE ScopedTypeVariables #-}
f :: Eq b => (a -> b) -> (a -> b) -> a -> Bool
f g h x =
let y = g x :: b
z = h x :: b
in y == z
main :: IO ()
main = pure ()以下のようなエラーがでます。
「g xによって推論されたその型b'と、指定されたその型bが同一ではない」の意です。
意図として
「g x :: bのbは、fの型で指定されたbそのものである」
ということを指定したつもりなのだけど…。
/tmp/nvimNLJJ6T/148.hs:5:11: error:
• Couldn't match expected type ‘b2’ with actual type ‘b’
‘b’ is a rigid type variable bound by
the type signature for:
f :: forall b a. Eq b => (a -> b) -> (a -> b) -> a -> Bool
at /tmp/nvimNLJJ6T/148.hs:3:6
‘b2’ is a rigid type variable bound by
an expression type signature:
forall b2. b2
at /tmp/nvimNLJJ6T/148.hs:5:18
• In the expression: g x :: b
In an equation for ‘y’: y = g x :: b
In the expression:
let
y = g x :: b
z = h x :: b
in y == z
• Relevant bindings include
h :: a -> b (bound at /tmp/nvimNLJJ6T/148.hs:4:5)
g :: a -> b (bound at /tmp/nvimNLJJ6T/148.hs:4:3)
f :: (a -> b) -> (a -> b) -> a -> Bool
(bound at /tmp/nvimNLJJ6T/148.hs:4:1)
/tmp/nvimNLJJ6T/148.hs:6:11: error:
• Couldn't match expected type ‘b2’ with actual type ‘b’
‘b’ is a rigid type variable bound by
the type signature for:
f :: forall b a. Eq b => (a -> b) -> (a -> b) -> a -> Bool
at /tmp/nvimNLJJ6T/148.hs:3:6
‘b2’ is a rigid type variable bound by
an expression type signature:
forall b2. b2
at /tmp/nvimNLJJ6T/148.hs:6:18
• In the expression: h x :: b
In an equation for ‘z’: z = h x :: b
In the expression:
let
y = g x :: b
z = h x :: b
in y == z
• Relevant bindings include
h :: a -> b (bound at /tmp/nvimNLJJ6T/148.hs:4:5)
g :: a -> b (bound at /tmp/nvimNLJJ6T/148.hs:4:3)
f :: (a -> b) -> (a -> b) -> a -> Bool
(bound at /tmp/nvimNLJJ6T/148.hs:4:1)
/tmp/nvimNLJJ6T/148.hs:7:6: error:
• Could not deduce (Eq a0) arising from a use of ‘==’
from the context: Eq b
bound by the type signature for:
f :: Eq b => (a -> b) -> (a -> b) -> a -> Bool
at /tmp/nvimNLJJ6T/148.hs:3:1-46
The type variable ‘a0’ is ambiguous
These potential instances exist:
instance Eq Ordering -- Defined in ‘GHC.Classes’
instance Eq Integer
-- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’
instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
...plus 22 others
...plus 7 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: y == z
In the expression:
let
y = g x :: b
z = h x :: b
in y == z
In an equation for ‘f’:
f g h x
= let
y = ...
z = ...
in y == zということで、確かに
「g x :: bのbは、fの型で指定されたbそのものである」
ということを型検査に伝えてあげます。
{-# LANGUAGE ScopedTypeVariables #-}
f :: forall a b. Eq b => (a -> b) -> (a -> b) -> a -> Bool
f g h x =
let y = g x :: b
z = h x :: b
in y == z
main :: IO ()
main = pure ()OK 🐕
そりゃそうなんですけど、いつも忘れて「！？」ってなりますよね。
