1
幾周前,我讀了Writing an interpreter using fold。我嘗試將這種方法應用到我正在處理的項目上,但由於GADT存在錯誤。這是產生相同問題的玩具代碼。在GADT上使用翻譯解釋器
{-# LANGUAGE GADTs, KindSignatures #-}
data Expr :: * -> * where
Val :: n -> Expr n
Plus :: Expr n -> Expr n -> Expr n
data Alg :: * -> * where
Alg :: (n -> a)
-> (a -> a -> a)
-> Alg a
fold :: Alg a -> Expr n -> a
fold [email protected](Alg val _) (Val n) = val n
fold [email protected](Alg _ plus) (Plus n1 n2) = plus (fold alg n1) (fold alg n2)
這是錯誤消息。
/home/mossid/Code/Temforai/src/Temforai/Example.hs:16:36: error:
• Couldn't match expected type ‘n1’ with actual type ‘n’
‘n’ is a rigid type variable bound by
the type signature for:
fold :: forall a n. Alg a -> Expr n -> a
at /home/mossid/Code/Temforai/src/Temforai/Example.hs:15:9
‘n1’ is a rigid type variable bound by
a pattern with constructor:
Alg :: forall a n. (n -> a) -> (a -> a -> a) -> Alg a,
in an equation for ‘fold’
at /home/mossid/Code/Temforai/src/Temforai/Example.hs:16:11
• In the first argument of ‘val’, namely ‘n’
In the expression: val n
In an equation for ‘fold’: fold [email protected](Alg val _) (Val n) = val n
• Relevant bindings include
n :: n
(bound at /home/mossid/Code/Temforai/src/Temforai/Example.hs:16:27)
val :: n1 -> a
(bound at /home/mossid/Code/Temforai/src/Temforai/Example.hs:16:15)
fold :: Alg a -> Expr n -> a
(bound at /home/mossid/Code/Temforai/src/Temforai/Example.hs:16:1)
我覺得編譯器不能推斷出n
和n1
是相同類型的,所以答案可能會提升內部變量數據類型的簽名。但是,不像在這個例子中,它不能用在原始代碼上。原始代碼在Expr
中具有全量化類型變量,並且類型簽名必須處理特定信息。
+這裏是原代碼
data Form :: * -> * where
Var :: Form s
Prim :: (Sat s r) => Pred s -> Marker r -> Form s
Simple :: (Sat s r) => Pred s -> Marker r -> Form s
Derived :: Form r -> Suffix r s -> Form s
Complex :: (Sat s r, Sat t P) =>
Form s -> Infix r -> Form t -> Form s
data FormA a where
FormA :: (Pred s -> Marker t -> a)
-> (Pred u -> Marker v -> a)
-> (a -> Suffix w x -> a)
-> (a -> y -> a -> a)
-> FormA a
foldForm :: FormA a -> Form s -> a
foldForm [email protected](FormA prim _ _ _) (Prim p m) = prim p m
foldForm [email protected](FormA _ simple _ _) (Simple p m) = simple p m
foldForm [email protected](FormA _ _ derived _) (Derived f s) =
derived (foldForm alg f) s
foldForm [email protected](FormA _ _ _ complex) (Complex f1 i f2) =
complex (foldForm alg f1) i (foldForm alg f2)
正確的定義是'Alg ::(n - > a) - > ... - > Alg n a' - 類型'存在n。 n - > a'與'a'是同構的,因爲你可以用函數做的唯一事情就是應用它,但你對'n'類型一無所知,所以你只能將這個函數應用到'undefined'。但似乎你知道這一點 - 「所以答案可能是提升內部變量的數據類型簽名」。 「但是,不像在這個例子中,它不能用在原始代碼上」 - 那麼它就是你應該發佈的原始代碼。 – user2407038