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46 lines
950 B
Haskell
46 lines
950 B
Haskell
{-# LANGUAGE RankNTypes #-}
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data ExprP a
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= VarP a
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| AppP (ExprP a) (ExprP a)
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| LamP (a -> ExprP a)
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| LitP Integer
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data Value
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= VLit Integer
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| VFun (Value -> Value)
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fromVFun :: Value -> (Value -> Value)
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fromVFun val = case val of
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VFun f -> f
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_ -> error "not a function"
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fromVLit :: Value -> Integer
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fromVLit val = case val of
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VLit n -> n
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_ -> error "not an integer"
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eval :: Expr -> Value
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eval e = ev (unExpr e) where
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ev (LamP f) = VFun(ev . f)
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ev (VarP v) = v
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ev (AppP e1 e2) = fromVFun (ev e1) (ev e2)
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ev (LitP n) = VLit n
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newtype Expr = Expr { unExpr :: forall a . ExprP a }
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i :: ExprP a
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i = LamP (\a -> VarP a)
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k :: ExprP a
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k = LamP (\x -> LamP (\y -> VarP x))
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s :: ExprP a
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s = LamP (\x -> LamP (\y -> LamP (\z -> AppP (AppP (VarP x) (VarP z)) (AppP (VarP y) (VarP z)))))
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skk :: ExprP a
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skk = AppP (AppP s k) k
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example :: Integer
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example = fromVLit $ eval $ Expr (AppP skk (LitP 3))
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