Assign matched expressions to variables in generated JavaScript ⚡

[jmackie]

Updates the JavaScript code generator to assign expressions being matched against. This avoids potentially expensive re-evaluations.

Given the following module:

module Example exports (..);


some = fn (a) -> a;

long = fn (a) -> a;

convoluted = fn (a) -> a;

function = fn (a) -> a;

type ABC =
    | A(Int)
    | B(Int)
    | C(Int);

example = fn (abc) ->
    match abc
    |> some
    |> long
    |> convoluted
    |> function with
    | A(i) -> i
    | B(i) -> i
    | C(i) -> i
    end;

Before this PR would generate:

function example(abc) {
  if ($function(convoluted(long(some(abc))))[0] === "A") {
    const i = $function(convoluted(long(some(abc))))[1];
    return i;
  }
  if ($function(convoluted(long(some(abc))))[0] === "B") {
    const i = $function(convoluted(long(some(abc))))[1];
    return i;
  }
  if ($function(convoluted(long(some(abc))))[0] === "C") {
    const i = $function(convoluted(long(some(abc))))[1];
    return i;
  }
  throw new Error("Pattern match error");
}
export { convoluted, example, $function, long, some, A, B, C };

But now generates:

function example(abc) {
  const $0 = $function(convoluted(long(some(abc))));
  if ($0[0] === "A") {
    const i = $0[1];
    return i;
  }
  if ($0[0] === "B") {
    const i = $0[1];
    return i;
  }
  if ($0[0] === "C") {
    const i = $0[1];
    return i;
  }
  throw new Error("Pattern match error");
}

[codecov]

Codecov Report

Merging #114 (8975da7) into main (8ac4c06) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   84.34%   84.37%   +0.02%     
==========================================
  Files          83       83              
  Lines       15214    15242      +28     
==========================================
+ Hits        12833    12861      +28     
  Misses       2381     2381              

Impacted Files	Coverage Δ
crates/ditto-codegen-js/src/convert.rs	93.94% <100.00%> (+0.24%)	⬆️
crates/ditto-codegen-js/src/optimize/mod.rs	95.47% <100.00%> (ø)

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