我有一个可扩展AST的代码。我的目标是允许稍后使用Expr
类添加更多表达式类型,而不是在单个数据块中包含所有类型。
data Constant o = Constant o deriving (Show)
data Add l r o = Add (l o) (r o) deriving (Show)
class Expr e where
simplify :: (Expr a, Num o) => (e o) -> (a o)
instance Expr Constant where
simplify (Constant a) = Constant a
instance (Expr l, Expr r) => Expr (Add l r) where
simplify (Add l r) = case (simplify l, simplify r) of
(Conatant a, Constant b) -> Constant $ a + b
(sl@_, sr@_) -> Add sl sr
我遇到了这些错误:
math.hs:10:27: error:
• Couldn't match type ‘a’ with ‘Constant’
‘a’ is a rigid type variable bound by
the type signature for:
simplify :: forall (a :: * -> *) o.
(Expr a, Num o) =>
Constant o -> a o
at math.hs:10:3-10
Expected type: a o
Actual type: Constant o
• In the expression: Constant a
In an equation for ‘simplify’: simplify (Constant a) = Constant a
In the instance declaration for ‘Expr Constant’
• Relevant bindings include
simplify :: Constant o -> a o (bound at math.hs:10:3)
math.hs:15:21: error:
• Couldn't match type ‘a’ with ‘Add l0 r0’
‘a’ is a rigid type variable bound by
the type signature for:
simplify :: forall (a :: * -> *) o.
(Expr a, Num o) =>
Add l r o -> a o
at math.hs:13:3-10
Expected type: a o
Actual type: Add l0 r0 o
• In the expression: Add sl sr
In a case alternative: (sl@_, sr@_) -> Add sl sr
In the expression:
case (simplify l, simplify r) of { (sl@_, sr@_) -> Add sl sr }
• Relevant bindings include
sr :: r0 o (bound at math.hs:15:12)
sl :: l0 o (bound at math.hs:15:6)
simplify :: Add l r o -> a o (bound at math.hs:13:3)
我无法理解为什么这不起作用。再说一次,我对Haskell相当新,在OOP土地上花了将近十年。我希望有人可以告诉我这方面的见解。
我试图在没有任何GHC扩展的情况下这样做,因为我想在开始添加更多内容之前了解vanilla语言。
答案 0 :(得分:2)
class ... where
simplify :: (Expr a, ...) => ... -> (a o)
此声明意味着您需要能够为使用a o
选择的Expr类中的任何类型构造simplify
。您不能在实例中指定具体类型。
编辑:您可以在Expr中引入一个构造:
class Expr e where
...
constant :: a -> e a
add :: e a -> e a -> e a
...
并使用那些。虽然像(add _ _ :: Constant Int)这样的表达式没有意义。
你能做什么是指定具有依赖性的类:
class Simplify e1 e2 | e1 -> e2 where ...
或与类型系列:
class Simplify e where
data Simplified e :: *
....
他们需要相应的扩展,但你不应该害怕它。那就是现代的Haskell。