9ba41a692d
This PR implements better support for unfolding class fields marked as `reducible`. For example, we want to mark fields that are types such as ```lean MonadControlT.stM : Type u -> Type u ``` The motivation is similar to our heuristic that type definitions should be abbreviations. Now, suppose we want to unfold `stM m (ExceptT ε m) α` using the `.reducible` transparency setting, we want the result to be `stM m m (MonadControl.stM m (ExceptT ε m) α)` instead of `(instMonadControlTOfMonadControl m m (ExceptT ε m)).1 α`. The latter would defeat the intent of marking the field as reducible, since the instance `instMonadControlTOfMonadControl` is `[instance_reducible]` and the resulting term would be stuck when using `.reducible` transparency mode. **Remark**: This feature introduces a few breakages in core and Mathlib. So, it is disabled for now in this PR. To enable, we must use `set_option backward.whnf.reducibleClassField true`
48 lines
1.5 KiB
Text
48 lines
1.5 KiB
Text
notation "#(" a ")" => ⟨a, by decreasing_tactic⟩
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def List.unfoldr {α β : Type u} [sz : SizeOf β] (f : (b : β) → Option (α × { b' : β // sizeOf b' < sizeOf b})) (b : β) : List α :=
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match f b with
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| none => []
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| some (a, ⟨b', h⟩) => a :: unfoldr f b'
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def tst1 (n : Nat) : List Nat :=
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List.unfoldr (b := n) fun
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| 0 => none
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| b+1 => some (3*n - b*2, #(b))
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#guard tst1 10 == [12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
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def tst2 (n : Nat) : List Nat :=
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-- Similar example where we provide our custom `SizeOf` instance
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List.unfoldr (sz := ⟨fun b => n - b⟩) (b := 0) fun b =>
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if h : b < n then
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some (b*2, ⟨b+1, by simp [sizeOf]; omega⟩)
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else
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none
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#guard tst2 10 == [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
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-- More general (and less convenient to use) version that can take an arbitrary well-founded relation.
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def List.unfoldr' {α β : Type u} [w : WellFoundedRelation β] (f : (b : β) → Option (α × { b' : β // w.rel b' b})) (b : β) : List α :=
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match f b with
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| none => []
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| some (a, ⟨b', h⟩) => a :: unfoldr' f b'
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termination_by b
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-- We need the `master` branch to test the following example
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def tst3 (n : Nat) : List Nat :=
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List.unfoldr' (b := n) fun
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| 0 => none
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| b+1 => some (3*n - b*2, #(b))
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#guard tst3 10 == [12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
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def tst4 (n : Nat) : List Nat :=
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List.unfoldr' (w := invImage (fun b => n - b) inferInstance) (b := 0) fun b =>
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if h : b < n then
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some (2*b, #(b+1))
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else
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none
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#guard tst4 10 == [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
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