38 lines
1.3 KiB
Text
38 lines
1.3 KiB
Text
/-
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Copyright (c) 2016 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: Leonardo de Moura
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-/
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prelude
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import init.core init.control.applicative
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universes u v
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class Alternative (f : Type u → Type v) extends Applicative f : Type (max (u+1) v) :=
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(failure : ∀ {α : Type u}, f α)
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(orelse : ∀ {α : Type u}, f α → f α → f α)
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instance alternativeHasOrelse (f : Type u → Type v) (α : Type u) [Alternative f] : HasOrelse (f α) :=
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⟨Alternative.orelse⟩
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section
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variables {f : Type u → Type v} [Alternative f] {α : Type u}
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@[inline] def failure : f α :=
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Alternative.failure f
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@[inline] def guard {f : Type → Type v} [Alternative f] (p : Prop) [Decidable p] : f Unit :=
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if p then pure () else failure
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@[inline] def assert {f : Type → Type v} [Alternative f] (p : Prop) [Decidable p] : f (Inhabited p) :=
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if h : p then pure ⟨h⟩ else failure
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/- Later we define a coercion from Bool to Prop, but this version will still be useful.
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Given (t : tactic Bool), we can write t >>= guardb -/
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@[inline] def guardb {f : Type → Type v} [Alternative f] : Bool → f Unit
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| true := pure ()
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| false := failure
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@[inline] def optional (x : f α) : f (Option α) :=
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some <$> x <|> pure none
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end
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