@avigad, @fpvandoorn, @rlewis1988, @dselsam
This commit modifies how have-expressions are elaborated.
Now, to process
have H : <type>, from <proof>,
<rest>
we first process the constraints in <type> and <proof> simultaneously.
After all these constraints are solved, the elaborator performs
a Prolog-like cut, and process the constraints in <rest>.
So, all overloads, type classes and coercions in <type> and <proof> are solved
before we start processing <rest>. Moreover, while processing <rest>, we
cannot backtrack to <type> and <proof> anymore.
I fixed all affected proofs in the standard and HoTT libraries in
previous commits pushed today and yesterday. I think most affected proofs were not using a good
style and/or were easy to fix. Here is a common pattern that does not
work anymore.
structure has_scalar [class] (F V : Type) :=
(smul : F → V → V)
infixl ` • `:73 := has_scalar.smul
proposition smul_zero (a : R) : a • (0 : M) = 0 :=
have a • 0 + a • 0 = a • 0 + 0, by rewrite [-smul_left_distrib, *add_zero],
!add.left_cancel this
The `have` doesn't work because Lean can't figure out the type of 0 before
it starts processing `!add.left_cancel this`. This is easy to fix, we just have to
annotate one of the `0`s in the `have`:
proposition smul_zero (a : R) : a • (0 : M) = 0 :=
have a • (0:M) + a • 0 = a • 0 + 0, by rewrite [-smul_left_distrib, *add_zero],
!add.left_cancel this
BTW, all tactics are still being executed after all constraints are solved.
We may change that in the future. I didn't want to execute
the tactics at <proof> before <rest> because of universe
meta-variables. In Lean, unassigned universe meta-variables become
parameters. Moreover, we perform this conversion *before*
we start processing tactics. Reason: universe meta-variables
create many problems for tactics such as `rewrite`, `blast` and `simp`.
Finally, we can recover the previous behavior using the option
set_option parser.checkpoint_have false
04eaf184a9