54 lines
1.5 KiB
Text
54 lines
1.5 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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Authors: Leonardo de Moura
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-/
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prelude
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import init.monad init.meta.format
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/-
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Remark: we use a function that produces a format object as the exception information.
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Motivation: the formatting object may be big, and we may create it on demand.
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-/
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inductive exceptional (A : Type)
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| success : A → exceptional
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| exception : (options → format) → exceptional
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section
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open exceptional
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variables {A : Type}
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variables [has_to_string A]
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protected meta definition exceptional.to_string : exceptional A → string
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| (success a) := to_string a
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| (exception .A e) := "Exception: " ++ to_string (e options.mk)
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meta instance : has_to_string (exceptional A) :=
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has_to_string.mk exceptional.to_string
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end
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namespace exceptional
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variables {A B : Type}
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attribute [inline]
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protected meta definition fmap (f : A → B) (e : exceptional A) : exceptional B :=
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exceptional.cases_on e
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(λ a, success (f a))
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(λ f, exception B f)
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attribute [inline]
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protected meta definition bind (e₁ : exceptional A) (e₂ : A → exceptional B) : exceptional B :=
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exceptional.cases_on e₁
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(λ a, e₂ a)
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(λ f, exception B f)
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attribute [inline]
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protected meta definition return (a : A) : exceptional A :=
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success a
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attribute [inline]
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meta definition fail (f : format) : exceptional A :=
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exception A (λ u, f)
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end exceptional
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meta instance : monad exceptional :=
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⟨@exceptional.fmap, @exceptional.return, @exceptional.bind⟩
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