Optimize literal checks in untagged variants

[zth]

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zth]

This is still a thing I'd like us to have a look at eventually. It's about untagged variants sometimes producing excessive code. Example:

let v = JSON.Array([String("hello")])

let x = x =>
  switch x {
  | JSON.Array(arr) =>
    arr
    ->Array.filterMap(a =>
      switch a {
      | String(s) => Some(s)
      | _ => None
      }
    )
    ->Array.join(",")
  | _ => "-"
  }

Produces this JS:

import * as Core__Array from "./stdlib/core__Array.js";

var v = ["hello"];

function x(x$1) {
  if (!Array.isArray(x$1) && (x$1 === null || typeof x$1 !== "object") && typeof x$1 !== "number" && typeof x$1 !== "string" && typeof x$1 !== "boolean" || !Array.isArray(x$1)) {
    return "-";
  } else {
    return Core__Array.filterMap(x$1, (function (a) {
                    if (!Array.isArray(a) && (a === null || typeof a !== "object") && typeof a !== "number" && typeof a !== "string" && typeof a !== "boolean" || typeof a !== "string") {
                      return ;
                    } else {
                      return a;
                    }
                  })).join(",");
  }
}

2 main things:

1. The if that checks the array: if (!Array.isArray(x$1) && (x$1 === null || typeof x$1 !== "object") && typeof x$1 !== "number" && typeof x$1 !== "string" && typeof x$1 !== "boolean" || !Array.isArray(x$1)). This checks all the ways it can't be an array, but it could just do Array.isArray(x$1).
2. Checking each element f (!Array.isArray(a) && (a === null || typeof a !== "object") && typeof a !== "number" && typeof a !== "string" && typeof a !== "boolean" || typeof a !== "string") it's looking for a string, so it could just check that it's a string via typeof a === "string". But instead it checks all of the negative cases.

This might not be trivial to fix and is probably a property of how the pattern matching engine is written, but I figured I'd log it anyway so we have it in the tracker.

Playground link: https://rescript-lang.org/try?version=v11.1.3&code=DYUwLgBAbhC8ECkDKB5AcgOgIICccEMBPACgG0kwcBLAOwHNiAiACxGGAHtGBKAXW4BQA0JAAecCONgA+ARAgBnAO5UwAY2aSIAbzkQAPolSZcBEvjzc4s+fIs498gLTTTRDADMqwMCBwBZfAAHYnxrR1tlVQ0IMN1bW0MKanpiBSsZCCQOAFsQNMEE+UMAfWsINA4aEAj5AF8IwtsXN0IMACsOWiYAGh49UvLGJ0Y9BqA

[zth]

This has been solved. Latest v12 beta now gives this nice output instead:

import * as Stdlib_Array from "./stdlib/Stdlib_Array.js";

let v = ["hello"];

function x(x$1) {
  if (Array.isArray(x$1)) {
    return Stdlib_Array.filterMap(x$1, a => {
      if (typeof a === "string") {
        return a;
      }
      
    }).join(",");
  } else {
    return "-";
  }
}