Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
71e09ca883
This PR improves `mvcgen invariants?` to suggest concrete invariants
based on how invariants are used in VCs.
These suggestions are intentionally simplistic and boil down to "this
holds at the start of the loop and this must hold at the end of the
loop":
```lean
def mySum (l : List Nat) : Nat := Id.run do
let mut acc := 0
for x in l do
acc := acc + x
return acc
/--
info: Try this:
invariants
· ⇓⟨xs, letMuts⟩ => ⌜xs.prefix = [] ∧ letMuts = 0 ∨ xs.suffix = [] ∧ letMuts = l.sum⌝
-/
#guard_msgs (info) in
theorem mySum_suggest_invariant (l : List Nat) : mySum l = l.sum := by
generalize h : mySum l = r
apply Id.of_wp_run_eq h
mvcgen invariants?
all_goals admit
```
It still is the user's job to weaken this invariant such that it
interpolates over all loop iterations, but it *is* a good starting point
for iterating. It is also useful because the user does not need to
remember the exact syntax.
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