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37 lines
1.1 KiB
Idris
37 lines
1.1 KiB
Idris
module InterpE
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data Expr = Var String -- variables
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| Val Int -- values
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| Add Expr Expr -- addition
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data Eval : Type -> Type where
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MkEval : (List (String, Int) -> Maybe a) -> Eval a
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fetch : String -> Eval Int
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fetch x = MkEval (\e => fetchVal e) where
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fetchVal : List (String, Int) -> Maybe Int
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fetchVal [] = Nothing
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fetchVal ((v, val) :: xs) = if (x == v)
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then (Just val)
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else (fetchVal xs)
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Functor Eval where
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map f (MkEval g) = MkEval (\e => map f (g e))
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Applicative Eval where
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pure x = MkEval (\e => Just x)
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(<*>) (MkEval f) (MkEval g) = MkEval (\x => app (f x) (g x)) where
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app : Maybe (a -> b) -> Maybe a -> Maybe b
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app (Just fx) (Just gx) = Just (fx gx)
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app _ _ = Nothing
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eval : Expr -> Eval Int
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eval (Var x) = fetch x
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eval (Val x) = [| x |]
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eval (Add x y) = [| eval x + eval y |]
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runEval : List (String, Int) -> Expr -> Maybe Int
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runEval env e = case eval e of
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MkEval envFn => envFn env
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