28cf146d00
This PR fixes a bug where the monad lift coercion elaborator would partially unify expressions even if they were not monads. This could be taken advantage of to propagate information that could help elaboration make progress, for example the first `change` worked because the monad lift coercion elaborator was unifying `@Eq _ _` with `@Eq (Nat × Nat) p`: ```lean example (p : Nat × Nat) : p = p := by change _ = ⟨_, _⟩ -- used to work (yielding `p = (p.fst, p.snd)`), now it doesn't change ⟨_, _⟩ = _ -- never worked ``` As such, this is a breaking change; you may need to adjust expressions to include additional implicit arguments.
55 lines
1.4 KiB
Text
55 lines
1.4 KiB
Text
namespace Stream
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variable [Stream ρ τ] (s : ρ)
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def take (s : ρ) : Nat → List τ × ρ
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| 0 => ([], s)
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| n+1 =>
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match next? s with
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| none => ([], s)
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| some (x,rest) =>
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let (L,rest) := take rest n
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(x::L, rest)
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def isEmpty : Bool :=
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Option.isNone (next? s)
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def lengthBoundedBy (n : Nat) : Prop :=
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isEmpty (take s n).2
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def hasNext : ρ → ρ → Prop
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:= λ s1 s2 => ∃ x, next? s1 = some ⟨x,s2⟩
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def isFinite : Prop :=
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∃ n, lengthBoundedBy s n
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instance hasNextWF : WellFoundedRelation {s : ρ // isFinite s} where
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rel := λ s1 s2 => hasNext s2.val s1.val
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wf := ⟨λ ⟨s,h⟩ => ⟨Subtype.mk s h, by
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simp only [Subtype.forall]
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cases h; case intro w h =>
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induction w generalizing s
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case zero =>
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intro s' h' h_next
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simp [hasNext] at h_next
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cases h_next; case intro x h_next =>
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simp [lengthBoundedBy, isEmpty, Option.isNone, take, h_next] at h
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case succ n ih =>
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intro s' h' h_next
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simp [hasNext] at h_next
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cases h_next; case intro x h_next =>
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simp [lengthBoundedBy, take, h_next] at h
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have := ih s' h
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exact Acc.intro (⟨s',h'⟩ : {s : ρ // isFinite s}) (by simpa only [Subtype.forall])
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⟩⟩
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def mwe [Stream ρ τ] (acc : α) : {l : ρ // isFinite l} → α
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| ⟨l,h⟩ =>
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match h:next? l with
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| none => acc
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| some (x,xs) =>
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have h_next : hasNext l xs := by exists x
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mwe acc ⟨xs, by sorry⟩
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termination_by l => l
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end Stream
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