48 lines
1.6 KiB
Text
48 lines
1.6 KiB
Text
/-
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Copyright (c) 2016 Gabriel Ebner. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Gabriel Ebner
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-/
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prelude
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import init.category.state init.function init.coe
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namespace monad
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class monad_transformer (transformer : ∀m [monad m], Type → Type) :=
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(is_monad : ∀m [monad m], monad (transformer m))
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(monad_lift : ∀m [monad m] α, m α → transformer m α)
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instance transformed_monad (m t) [monad_transformer t] [monad m] : monad (t m) :=
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monad_transformer.is_monad t m
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class has_monad_lift (m n : Type → Type) :=
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(monad_lift : ∀α, m α → n α)
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instance monad_transformer_lift (t m) [monad_transformer t] [monad m] : has_monad_lift m (t m) :=
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⟨monad_transformer.monad_lift t m⟩
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class has_monad_lift_t (m n : Type → Type) :=
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(monad_lift : ∀α, m α → n α)
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def monad_lift {m n} [has_monad_lift_t m n] {α} : m α → n α :=
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has_monad_lift_t.monad_lift n α
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@[reducible] def has_monad_lift_to_has_coe {m n} [has_monad_lift_t m n] {α} : has_coe (m α) (n α) :=
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⟨monad_lift⟩
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instance has_monad_lift_t_trans (m n o) [has_monad_lift n o] [has_monad_lift_t m n] : has_monad_lift_t m o :=
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⟨ λα (ma : m α), has_monad_lift.monad_lift o α $ has_monad_lift_t.monad_lift n α ma ⟩
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instance has_monad_lift_t_refl (m) [monad m] : has_monad_lift_t m m :=
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⟨ λα, id ⟩
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end monad
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namespace state_t
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def state_t_monad_lift (S) (m) [monad m] (α) (f : m α) : state_t S m α :=
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assume state, do res ← f, return (res, state)
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instance (S) : monad.monad_transformer (state_t S) :=
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⟨ state_t.monad S, state_t_monad_lift S ⟩
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end state_t
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