@Kha @dselsam:
This hack was preventing us from making `Expr` a "real" Lean type.
This was bad for a few reasons:
- It was hard to extend/modify `Expr` in Lean since we would also have
to modify the C++ code that creates the `Expr` objects with the hidden
fields.
- `Expr.lam` and `Expr.forallE` were not following the Lean layout
standard where we sort fields by size. @Kha: recall we used that to
avoid a UB. The issue with `Expr.lam` and `Expr.forallE` is that they
have a "visible" field (`BinderInfo`), which is smaller than
hidden fields such as hash code.
- `Expr.fvar` had only one field at `Expr.lean,` but four behind the
scenes.
I added a new constructor `Local` that is only accessible from C++.
It is only used in legacy code we inherited from Lean2.
We will eventually delete it.
This refactoring was quite painful since many parts of the codebase
were mixing the new `Expr.fvar` with the old `Expr.local`.
I doubt I would be able to do it without the new staging framework
@Kha built.
BTW, some of the patches are horrible. I didn't care much since we
are going to deleted the super ugly files. That being said,
you should expect new weird bevaior due to `Expr.fvar` vs `Expr.local`.
Next step: use the new `ExprCachedData` to make all `Expr` hidden visibles
accessible from Lean.
checkpoint
We need this feature for:
1) Defining nonlinear search patterns. Example: (?m <= ?m + 1)
2) Preprocessing recursive equations and support the pattern
refinement approach used in Agda. Example: in Agda, they accept
```
def append {A : Type} : Π (m n : nat), Vec A m -> Vec A n -> Vec A (m + n)
| m n nil ys := ys
| m n (cons m' x xs) ys := cons x (append m' n xs ys)
```
These equations have to be refined. For example, `m` has to be
replaced with `0` (in the first equation), and `succ m'` in the
second. To implement this kind of refinement, we need to convert
the pattern variables (local constants) into metavariables during
elaboration. Then, the unassigned metavariables become local constants
again. This preprocessing step will fix some of the issues on #1594.
To completely fix#1594, we will need yet another preprocessing step
which will implement "complete transition" used in the equation
compiler before we start elim_match.cpp
