Remark: `lean.cpp` is a total mess at this point. It contains
leftovers from the Lean2 and Lean3 designs, and hacks for supporting
the Lean4 transition.
Remark: this commit introduce memory leaks, but this is just an
intermediate step to get modification objects in Lean.
Recall that, we will eventually remove modification objects from Lean.
The result type of a join point is always equal to the function return
type. Moreover, the extra bookkeeping introduces extra work, and doesn't
really help.
@kha I keep finding problems with the float `let` inwards
transformation. It is always a nasty interaction between this
transformation and the `reset/reuse` insertion procedure.
The example I used in the new comment can be modified to a
`casesOn` with more than one branch (e.g., `Option.casesOn`).
Suppose we wrote
```
let o : Option Nat := Array.index a i in
let a := Array.update a i none in
Option.casesOn o
none
(fun n, some (Array.update a i (some (n + 1))))
```
In the example above, the compiler will float
`a := Array.update a i none` inwards.
```
let o : Option Nat := Array.index a i in
Option.casesOn o
none
(fun n,
let a := Array.update a i none in
some (Array.update a i (some (n + 1))))
```
Then, adding reset/reuse:
```
let o : Option Nat := Array.index a i in
Option.casesOn o
none
(fun n,
let o := reset o in
let a := Array.update a i none in
let n := n + 1 in
let o := reuse o (some n)
some (Array.update a i o))
```
Similarly to the example in the new comment, the `reset o` will fail since
the array `a` would still have a reference to `o`.
Remarks:
- Haskell also implements float `let` inwards.
- I am not sure how important the float `let` inward transformation is.
- I can see other nasty interactions after we implement user-defined
simplification rules. For example, I guess many users would find the
following lemma to be a good rewriting rule:
```
(Array.update (Array.update a i v) i w) = (Array.update a i w)
```
However, if we use this lemma in the example above, then `Array.update a i none` will be eliminated,
and `reset o` will fail.
@kha The move `x := val` to `casesOn` branch was producing nasty
problems. I documented the issue, and implemented a simple
and sufficient condition for preventing the problem. The approach is very
similar to the one used at `push_proj_fn` at `llnf.cpp`.
I hope this change will not impact existing benchmarks :)
