From 1180def181d7c74dc8229e4d2534054ed2d41d9b Mon Sep 17 00:00:00 2001
From: piux2 <90544084+piux2@users.noreply.github.com>
Date: Fri, 19 Jul 2024 10:54:59 -0700
Subject: [PATCH] feat: named and unnamed type assignment 3 of 3 (#2367)
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This is the last part of the solution for issue #1141.
The 1 of 3 of the solution can be found in PR #1143.
The 2 of 3 of the solution can be found in PR #1246
It decomposes function calls that return multiple values in the
preprocess.
### Here is the problem to solve:
` u1, n2 = x() `
How do we ensure that the returned multiple values from a function call
adhere to named and unnamed type assignment specifications?
Additionally, we want to solve this problem during preprocessing instead
of at runtime to minimize the impact on runtime performance.
### The main ideas:
u1, n2 = x() << decompose the statement to the following two lines
// .tmp_1, .tmp_2 := x()
// u1, n2 = .tmp_1, .tmp_2
then we can apply name and unname type conversion specs to the second
line.
u1, n2 = _tmp_1, _tmp_2
### Here are the example code and the explanation
```
// decompose_filetest.gno
package main
type nat []int
func x() (nat, []int) {
a := nat{1}
b := []int{2}
return a, b
}
func main() {
var u1 []int
var n2 nat
u1, n2 = x()
// .tmp_1, .tmp_2 := x()
// u1, n2 = .tmp_1, .tmp_2
println(u1)
println(n2)
}
// Output:
// slice[(1 int)]
// (slice[(2 int)] main.nat)
```
### Here is the simplified recursive tree of the transformation in the
preprocess
### Here are the major steps involved in this decomposition during
preprocessing:
- Create hidden temporary name expressions .tmp1, .tmp2. In Go, a
leading dot is not valid in variable names, ensuring that users cannot
create names that clash with these hidden variables.
- Create two statements in the block: one for defining and one for
assigning.
```
.tmp1, .tmp2 := x()
u1, n2 = .tmp_1, .tmp_2
```
- Preprocess each newly created statements
- Replace the original statement with the two newly created statements.
### Here are two additional changes to facilitate above.
- Update the FuncValue's body in `updates := pn.PrepareNewValues(pv)
`since its source Body has been changed during preprocessing.
- Replace all ` for index := range Body` with `for i:=0; i < len(Body);
i++` in transcribe.go since the body length might change due to the
decomposition.
Contributors' checklist...
- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [x] No breaking changes were made
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).