eec12232e3
We would need to add the following instance to fix this example
```lean
instance ExceptT.monadIO {σ} (m : Type → Type) [Monad m] [MonadIO m] : MonadIO (ExceptT σ m) := {}
```
However, this instance may create problems for polymorphic functions on
`MonadIO m`. If `m` is of the form `ExcepT Except1 ... EIO Exception2`,
then `finally` and `catch` is catching `Except1`. This is particularly
bad for code that uses `finally`.
Right now, we are not using monad stacks with multiple `ExceptT`s for
performance reasons, but we should have a solution for users.
54 lines
1.2 KiB
Text
54 lines
1.2 KiB
Text
def f (v : Nat) : StateRefT Nat IO Nat := do
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IO.println "hello";
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modify fun s => s - v;
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get
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def g : IO Nat :=
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(f 5).run' 20
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#eval (f 5).run' 20
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#eval (do set 100; f 5).run' 0
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def f2 : ReaderT Nat (StateRefT Nat IO) Nat := do
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v ← read;
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IO.println $ "context " ++ toString v;
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modify fun s => s + v;
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get
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#eval (f2.run 10).run' 20
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def f3 : StateT String (StateRefT Nat IO) Nat := do
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s ← get;
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n ← getThe Nat;
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set (s ++ ", " ++ toString n);
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s ← get;
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IO.println s;
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set (n+1);
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getThe Nat
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#eval (f3.run' "test").run' 10
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structure Label {β : Type} (v : β) (α : Type) :=
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(val : α)
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class HasGetAt {β : Type} (v : β) (α : outParam Type) (m : Type → Type) :=
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(getAt : m α)
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instance monadState.hasGetAt (β : Type) (v : β) (α : Type) (m : Type → Type) [Monad m] [MonadStateOf (Label v α) m] : HasGetAt v α m :=
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{ getAt := do a ← getThe (Label v α); pure a.val }
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export HasGetAt (getAt)
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abbrev M := StateRefT (Label 0 Nat) $ StateRefT (Label 1 Nat) $ StateRefT (Label 2 Nat) IO
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def f4 : M Nat := do
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a0 : Nat ← getAt 0;
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a1 ← getAt 1;
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a2 ← getAt 2;
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IO.println $ "state0 " ++ toString a0;
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IO.println $ "state1 " ++ toString a1;
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IO.println $ "state1 " ++ toString a2;
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pure (a0 + a1 + a2)
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#eval ((f4.run' ⟨10⟩).run' ⟨20⟩).run' ⟨30⟩
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