44 lines
1.5 KiB
Haskell
44 lines
1.5 KiB
Haskell
module LambdaCalculus.Expression
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( Expr (..), ExprF (..), DefF (..), VoidF, Text
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, Eval, EvalExpr, EvalX, EvalXF (..), Identity (..)
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, pattern AppFE, pattern Cont, pattern ContF, pattern CallCC, pattern CallCCF
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, Parse, AST, ASTF, NonEmptyDefFs (..), NonEmpty (..), simplify
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, pattern LetFP
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, ast2eval, eval2ast
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) where
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import LambdaCalculus.Evaluator.Base
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import LambdaCalculus.Evaluator
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import LambdaCalculus.Syntax.Base
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import Data.Functor.Foldable (cata, hoist)
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import Data.HashSet qualified as HS
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import Data.List (foldl')
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import Data.List.NonEmpty (toList)
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-- | Convert from an abstract syntax tree to an evaluator expression.
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ast2eval :: AST -> EvalExpr
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ast2eval = substitute "callcc" CallCC . cata \case
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VarF name -> Var name
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AppF ef exs -> foldl' App ef $ toList exs
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AbsF ns e -> foldr Abs e $ toList ns
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LetF ds e ->
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let letExpr name val body' = App (Abs name body') val
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in foldr (uncurry letExpr) e $ getNonEmptyDefFs ds
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-- | Convert from an evaluator expression to an abstract syntax tree.
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eval2ast :: EvalExpr -> AST
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-- Because all `ast2eval` replaces all free instances of `callcc`,
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-- all instances of `callcc` must be bound;
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-- therefore, we are free to alpha convert them,
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-- freeing the name `callcc` for us to use for the built-in again.
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eval2ast = hoist go . alphaConvert (HS.singleton "callcc")
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where
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go :: EvalExprF r -> ASTF r
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go = \case
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VarF name -> VarF name
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CallCCF -> VarF "callcc"
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AppFE ef ex -> AppF ef (ex :| [])
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AbsF n e -> AbsF (n :| []) e
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ContF e -> AbsF ("!" :| []) e
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