100 lines
3.2 KiB
Idris
100 lines
3.2 KiB
Idris
data ExprType
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= IntType
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| BoolType
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| StringType
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repr : ExprType -> Type
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repr IntType = Int
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repr BoolType = Bool
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repr StringType = String
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intBoolImpossible : IntType = BoolType -> Void
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intBoolImpossible Refl impossible
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intStringImpossible : IntType = StringType -> Void
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intStringImpossible Refl impossible
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boolStringImpossible : BoolType = StringType -> Void
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boolStringImpossible Refl impossible
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decEq : (a : ExprType) -> (b : ExprType) -> Dec (a = b)
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decEq IntType IntType = Yes Refl
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decEq BoolType BoolType = Yes Refl
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decEq StringType StringType = Yes Refl
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decEq IntType BoolType = No intBoolImpossible
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decEq BoolType IntType = No $ intBoolImpossible . sym
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decEq IntType StringType = No intStringImpossible
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decEq StringType IntType = No $ intStringImpossible . sym
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decEq BoolType StringType = No boolStringImpossible
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decEq StringType BoolType = No $ boolStringImpossible . sym
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data Op
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= Add
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| Subtract
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| Multiply
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| Divide
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data Expr
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= IntLit Int
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| BoolLit Bool
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| StringLit String
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| BinOp Op Expr Expr
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| IfElse Expr Expr Expr
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data SafeExpr : ExprType -> Type where
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IntLiteral : Int -> SafeExpr IntType
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BoolLiteral : Bool -> SafeExpr BoolType
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StringLiteral : String -> SafeExpr StringType
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BinOperation : (repr a -> repr b -> repr c) -> SafeExpr a -> SafeExpr b -> SafeExpr c
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IfThenElse : SafeExpr BoolType -> SafeExpr t -> SafeExpr t -> SafeExpr t
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typecheckOp : Op -> (a : ExprType) -> (b : ExprType) -> Either String (c : ExprType ** repr a -> repr b -> repr c)
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typecheckOp Add IntType IntType = Right (IntType ** (+))
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typecheckOp Subtract IntType IntType = Right (IntType ** (-))
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typecheckOp Multiply IntType IntType = Right (IntType ** (*))
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typecheckOp Divide IntType IntType = Right (IntType ** div)
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typecheckOp _ _ _ = Left "Invalid binary operator application"
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requireBool : (n : ExprType ** SafeExpr n) -> Either String (SafeExpr BoolType)
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requireBool (BoolType ** e) = Right e
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requireBool _ = Left "Not a boolean."
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typecheck : Expr -> Either String (n : ExprType ** SafeExpr n)
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typecheck (IntLit i) = Right (_ ** IntLiteral i)
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typecheck (BoolLit b) = Right (_ ** BoolLiteral b)
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typecheck (StringLit s) = Right (_ ** StringLiteral s)
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typecheck (BinOp o l r) = do
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(lt ** le) <- typecheck l
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(rt ** re) <- typecheck r
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(ot ** f) <- typecheckOp o lt rt
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pure (_ ** BinOperation f le re)
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typecheck (IfElse c t e) =
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do
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ce <- typecheck c >>= requireBool
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(tt ** te) <- typecheck t
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(et ** ee) <- typecheck e
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case decEq tt et of
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Yes p => pure (_ ** IfThenElse ce (replace p te) ee)
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No _ => Left "Incompatible branch types."
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eval : SafeExpr t -> repr t
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eval (IntLiteral i) = i
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eval (BoolLiteral b) = b
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eval (StringLiteral s) = s
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eval (BinOperation f l r) = f (eval l) (eval r)
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eval (IfThenElse c t e) = if (eval c) then (eval t) else (eval e)
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resultStr : {t : ExprType} -> repr t -> String
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resultStr {t=IntType} i = show i
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resultStr {t=BoolType} b = show b
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resultStr {t=StringType} s = show s
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tryEval : Expr -> String
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tryEval ex =
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case typecheck ex of
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Left err => "Type error: " ++ err
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Right (t ** e) => resultStr $ eval {t} e
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main : IO ()
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main = putStrLn $ tryEval $ BinOp Add (IfElse (BoolLit True) (IntLit 6) (IntLit 7)) (BinOp Multiply (IntLit 160) (IntLit 2))
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