72 lines
2.2 KiB
Text
72 lines
2.2 KiB
Text
/-
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Copyright (c) 2017 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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Authors: Leonardo de Moura
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-/
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prelude
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import init.meta.interactive init.category.transformers
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universes u v
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def option_t (m : Type u → Type v) [monad m] (α : Type u) : Type v :=
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m (option α)
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@[inline] def option_t_bind {m : Type u → Type v} [monad m] {α β : Type u} (a : option_t m α) (b : α → option_t m β)
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: option_t m β :=
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show m (option β), from
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do o ← a,
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match o with
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| none := return none
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| (some a) := b a
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end
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@[inline] def option_t_return {m : Type u → Type v} [monad m] {α : Type u} (a : α) : option_t m α :=
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show m (option α), from
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return (some a)
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instance {m : Type u → Type v} [monad m] : monad (option_t m) :=
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{pure := @option_t_return m _, bind := @option_t_bind m _,
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id_map := begin
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intros,
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simp [option_t_bind, function.comp],
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assert h : option_t_bind._match_1 option_t_return = @pure m _ (option α),
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{ apply funext, intro s, cases s, refl, refl },
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{ simp [h], apply @monad.bind_pure _ (option α) m },
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end,
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pure_bind := begin
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intros,
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simp [option_t_bind, option_t_return, monad.pure_bind]
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end,
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bind_assoc :=
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begin
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intros,
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simp [option_t_bind, option_t_return, monad.bind_assoc],
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apply congr_arg, apply funext, intro x,
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cases x,
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{ simp [option_t_bind, monad.pure_bind] },
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{ refl }
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end}
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def option_t_orelse {m : Type u → Type v} [monad m] {α : Type u} (a : option_t m α) (b : option_t m α) : option_t m α :=
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show m (option α), from
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do o ← a,
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match o with
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| none := b
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| (some v) := return (some v)
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end
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def option_t_fail {m : Type u → Type v} [monad m] {α : Type u} : option_t m α :=
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show m (option α), from
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return none
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instance {m : Type u → Type v} [monad m] : alternative (option_t m) :=
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{ @option_t.monad m _ with
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failure := @option_t_fail m _,
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orelse := @option_t_orelse m _ }
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def option_t.lift {m : Type u → Type v} [monad m] {α : Type u} (a : m α) : option_t m α :=
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(some <$> a : m (option α))
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instance option_t.monad_transformer : monad.monad_transformer option_t :=
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{ is_monad := @option_t.monad,
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monad_lift := @option_t.lift }
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