56 lines
2.1 KiB
Text
56 lines
2.1 KiB
Text
/-
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Copyright (c) 2018 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Sebastian Ullrich
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Recursion monad transformer
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-/
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prelude
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import init.lean.parser.parsec init.fix
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namespace Lean.Parser
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/-- A small wrapper of `ReaderT` that simplifies introducing and invoking
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recursion points in a computation. -/
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def RecT (α δ : Type) (m : Type → Type) (β : Type) :=
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ReaderT (α → m δ) m β
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namespace RecT
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variables {m : Type → Type} {α δ β : Type} [Monad m]
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local attribute [reducible] RecT
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/-- Continue at the recursion point stored at `run`. -/
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@[inline] def recurse (a : α) : RecT α δ m δ :=
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λ f, f a
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/-- Execute `x`, executing `rec a` whenever `recurse a` is called.
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After `maxRec` recursion steps, `base` is executed instead. -/
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@[inline] protected def run (x : RecT α δ m β) (base : α → m δ) (rec : α → RecT α δ m δ) : m β :=
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x (fixCore base (λ a f, rec f a))
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@[inline] protected def runParsec {γ : Type} [MonadParsec γ m] (x : RecT α δ m β) (rec : α → RecT α δ m δ) : m β :=
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RecT.run x (λ _, MonadParsec.error "RecT.runParsec: no progress") rec
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-- not clear how to auto-derive these given the additional constraints
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instance : Monad (RecT α δ m) := inferInstance
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instance [Alternative m] : Alternative (RecT α δ m) := inferInstance
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instance : HasMonadLift m (RecT α δ m) := inferInstance
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instance (ε) [MonadExcept ε m] : MonadExcept ε (RecT α δ m) := inferInstance
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instance (μ) [MonadParsec μ m] : MonadParsec μ (RecT α δ m) :=
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inferInstance
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-- NOTE: does not allow to vary `m` because of its occurrence in the Reader State
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instance [Monad m] : MonadFunctor m m (RecT α δ m) (RecT α δ m) :=
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inferInstance
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end RecT
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class MonadRec (α δ : outParam Type) (m : Type → Type) :=
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(recurse {} : α → m δ)
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export MonadRec (recurse)
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instance MonadRec.trans (α δ m m') [HasMonadLift m m'] [MonadRec α δ m] [Monad m] : MonadRec α δ m' :=
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{ recurse := λ a, monadLift (recurse a : m δ) }
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instance MonadRec.base (α δ m) [Monad m] : MonadRec α δ (RecT α δ m) :=
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{ recurse := RecT.recurse }
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end Lean.Parser
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