e9d85f49e6
Based on the discussion at https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/for.2C.20unexpected.20need.20for.20type.20ascription/near/269083574 The consensus seemed to be that "auto pure" is more confusing than its worth.
35 lines
905 B
Text
35 lines
905 B
Text
namespace TryCatchWithAutolift
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structure M (α : Type) where
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σ : ExceptT String IO α
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instance : Monad M where
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pure x := { σ := pure x }
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bind x f := { σ := do (f (← x.σ)).σ }
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map f x := { σ := return f (← x.σ) }
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instance : MonadLiftT IO M where
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monadLift {α : Type} (act : IO α) : M α :=
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{ σ := monadLift act }
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instance : MonadFinally M where
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tryFinally' := fun x h => M.mk do
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tryFinally' x.σ fun e => (h e).σ
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def t : M Unit := do
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liftM $ IO.println "bad"
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try do pure () -- Error
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catch ex : IO.Error => do pure ()
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finally do pure ()
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instance : MonadExceptOf IO.Error M where
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throw e := { σ := throwThe IO.Error e }
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tryCatch x handle := { σ := tryCatchThe IO.Error x.σ fun e => handle e |>.σ }
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def t2 : M Unit := do
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liftM $ IO.println "bad"
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try do pure ()
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catch ex : IO.Error => do pure ()
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finally do pure ()
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end TryCatchWithAutolift
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