@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 :)
The current commit only detects lambda expressions that should be
lifted eagerly.
@kha I added a comment describing why this optimization is useful.
Right now, we are not writing code that benefits from this optimization,
but it seems very useful for implementing combinators that return
a tuple containing functions. I think this is a useful idiom, for
example, the combinator may have two parts: one that is the actual
action, and another that collects "static properties".
Last summer, if I remember correctly, you considered building this
kind of combinator for the new Lean parser, but gave up because we
did not have compiler support for it. In your case, the "static
property" would be the set of tokens. It could also contain the left
most token for initializing the Pratt parser table, etc.
This commit is the first step to make this kind of code efficient.
It will also improve the experiment at `tests/playground/parser/`
It was retuning true for declarations such as
```lean
def f (n : Nat) :=
g._rarg n
```
It should only return true if the nested application is of the form
`f._rarg ...`
