lean4-htt/tests/elab/splitIssue3.lean
Garmelon 08eb78a5b2
chore: switch to new test/bench suite (#12590)
This PR sets up the new integrated test/bench suite. It then migrates
all benchmarks and some related tests to the new suite. There's also
some documentation and some linting.

For now, a lot of the old tests are left alone so this PR doesn't become
even larger than it already is. Eventually, all tests should be migrated
to the new suite though so there isn't a confusing mix of two systems.
2026-02-25 13:51:53 +00:00

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inductive ListSplit {α : Type u} : List α → Type u
| split l₁ l₂ : ListSplit (l₁ ++ l₂)
def splitList {α : Type _} : (l : List α) → ListSplit l
| [] => ListSplit.split [] []
| h :: t => ListSplit.split [h] t
def len : List α → Nat
| [] => 0
| a :: [] => 1
| l =>
match splitList l with
| ListSplit.split fst snd => len fst + len snd
termination_by l => l.length
decreasing_by
all_goals sorry
-- The equational theorems are
#check @len.eq_1
#check @len.eq_2
#check @len.eq_3 -- It is conditional, and may be tricky to use.
#check @len.eq_def
theorem len_nil : len ([] : List α) = 0 := by
simp [len]
theorem len_1 (a : α) : len [a] = 1 := by
simp [len]
theorem len_2 (a b : α) (bs : List α) : len (a::b::bs) = 1 + len (b::bs) := by
conv => lhs; unfold len
cases bs <;> simp [splitList, len_1]
theorem len_cons (a : α) (as : List α) : len (a::as) = 1 + len as := by
cases as with
| nil => simp [len_1, len_nil]
| cons b bs => simp [len_2]
theorem listlen : ∀ l : List α, l.length = len l := by
intro l
induction l with
| nil => simp [len]
| cons h t ih =>
simp [List.length, len_cons, ih]
rw [Nat.add_comm]