1
几周前,我读了Writing an interpreter using fold。我尝试将这种方法应用到我正在处理的项目上,但由于GADT存在错误。这是产生相同问题的玩具代码。在GADT上使用翻译解释器
{-# 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)
这是错误消息。
/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)
我觉得编译器不能推断出n
和n1
是相同类型的,所以答案可能会提升内部变量数据类型的签名。但是,不像在这个例子中,它不能用在原始代码上。原始代码在Expr
中具有全量化类型变量,并且类型签名必须处理特定信息。
+这里是原代码
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)
正确的定义是'Alg ::(n - > a) - > ... - > Alg n a' - 类型'存在n。 n - > a'与'a'是同构的,因为你可以用函数做的唯一事情就是应用它,但你对'n'类型一无所知,所以你只能将这个函数应用到'undefined'。但似乎你知道这一点 - “所以答案可能是提升内部变量的数据类型签名”。 “但是,不像在这个例子中,它不能用在原始代码上” - 那么它就是你应该发布的原始代码。 – user2407038