37 lines
1.1 KiB
Text
37 lines
1.1 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.logic init.category.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 : Π {a : Type u}, f a)
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(orelse : Π {a : Type u}, f a → f a → f a)
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section
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variables {f : Type u → Type v} [alternative f] {a : Type u}
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@[inline] def failure : f a :=
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alternative.failure f
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@[inline] def orelse : f a → f a → f a :=
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alternative.orelse
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infixr ` <|> `:2 := orelse
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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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/- 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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| tt := pure ()
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| ff := failure
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@[inline] def optional (x : f a) : f (option a) :=
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some <$> x <|> pure none
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end
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