我想将我输入的 GADT (GExpr) 放入一个 hashmap,所以我首先将其转换为相应的单态 ADT (Expr)。当我从哈希映射中查找时,我无法将单态 ADT 转换回 GADT。
以下是简化版。基本上有两个函数,“dim”和“gexprOfExpr”,我只能让其中一个同时工作。我想做的事是不可能的吗?
{-# OPTIONS_GHC -Wall #-}
{-# Language GADTs #-}
type ListDim = [Int]
data DIM0 = DIM0
data DIM1 = DIM1
class Shape sh where
shapeOfList :: ListDim -> sh
instance Shape DIM0 where
shapeOfList _ = DIM0
instance Shape DIM1 where
shapeOfList _ = DIM1
data Expr = EConst ListDim Double
| ESum ListDim Int
data GExpr sh where
GRef :: sh -> Int -> GExpr sh
GConst :: sh -> Double -> GExpr sh
GSum :: GExpr DIM1 -> GExpr DIM0 -- GADT, this works for "dim"
-- GSum :: GExpr DIM1 -> GExpr sh -- phantom type, this works for "gexprOfExpr"
dim :: GExpr sh -> sh
dim (GRef sh _) = sh
dim (GConst sh _) = sh
dim (GSum _) = DIM0
gexprOfExpr :: Shape sh => Expr -> GExpr sh
gexprOfExpr (EConst lsh x) = GConst (shapeOfList lsh) x
gexprOfExpr (ESum lsh k) = GSum $ GRef (shapeOfList lsh) k
注意:我确实知道我要恢复的类型。如果它有帮助,这会很好:
gexprOfExpr :: Shape sh => sh -> Expr -> GExpr sh