How can we generate simple "for loop"s when iterating through lists?

[ceremcem]

I need to generate the following output, wher i is from [0 to 5]:

demo = async function() {
  for (let i = 0; i < 5; i++) {
    console.log(`Waited ${i} seconds...`);
    await sleep(1000);
  }
  console.log('Done');
}

What is the Livescript equivalent of above Js code?

[ceremcem]

The equivalent code in Coffeescript is:

demo = ->
  for i in [5..1]
    console.log "Waited #{i} seconds..."
    await sleep 1000 # wait one second
  console.log "Done"

which produces the following output:

var demo;

demo = async function() {
  var i, j;
  for (i = j = 5; j >= 1; i = --j) {
    console.log(`Waited ${i} seconds...`);
    await sleep(1000); // wait one second
  }
  return console.log("Done");
};

[ceremcem]

Solution:

sleep = (ms) ->
  new Promise (resolve) ->
    setTimeout resolve, ms

demo = (seconds) !->>
  for i from seconds to 1 by -1
    console.log "hey #i"
    await sleep 1000 # wait one second
  console.log "Blastoff!"

demo 3

Correctly (expectedly) compiles to:

var sleep, demo;
sleep = function(ms){
  return new Promise(function(resolve){
    return setTimeout(resolve, ms);
  });
};
demo = async function(seconds){
  var i$, i;
  for (i$ = seconds; i$ >= 1; --i$) {
    i = i$;
    console.log("hey " + i);
    (await sleep(1000));
  }
  console.log("Blastoff!");
};
demo(3);

Problem

If we use for i in [seconds to 1] instead of for i from seconds to 1 by -1, generated output changes dramatically and is far from desired output:

demo = (seconds) !->>
  for i in [seconds to 1]
    console.log "hey #i"
    await sleep 1000 # wait one second
  console.log "Blastoff!"

compiles to:

var demo;
demo = async function(seconds){
  var i$, ref$, len$, i, fn$ = async function(){
    var i$, results$ = [];
    for (i$ = seconds; i$ <= 1; ++i$) {
      results$.push(i$);
    }
    return results$;
  };
  for (i$ = 0, len$ = (ref$ = (await (fn$()))).length; i$ < len$; ++i$) {
    i = ref$[i$];
    console.log("hey " + i);
    (await sleep(1000));
  }
  console.log("Blastoff!");
};

@rhendric Is this change also expected or this is some sort of bug?

[rhendric]

First, you almost certainly don't want [seconds to 1]; you probably meant [seconds to 1 by -1] instead. LiveScript loops and ranges alike always default to counting up, even if you give a number that is greater than the upper limit.

But allowing for that mistake, this result looks right to me. That's what a for loop that iterates over an unknown array looks like when compiled. Arguably, LiveScript could special-case this when the array is a range expression, but... why? The for-loop syntax already gives you a way to express this.

[ceremcem]

Thanks for the answer.

Is there any use case where for i in [x to y] form is preferable over for i from x to y form? If there isn't any, can I explicitly forbid the i in [x to y] form in my applications?

[rhendric]

I can't think of any case where the former is better, no.

[ceremcem]

FYI:

For those who wants to refactor the entire project to get rid of for i in [x to y] like usages:

find . -name "*.ls" -and -not -path "*/node_modules/*" | xargs grep -PHn 'for\b.*\s\[(?!(\d+\s+(to|til)\s+\d+)).*\]'

The regex and its test cases are here: https://regex101.com/r/Bj9pZE/3

[sourcevault]

FYI:

For those who wants to refactor the entire project to get rid of for i in [x to y] like usages:

find . -name "*.ls" -and -not -path "*/node_modules/*" | xargs grep -PHn 'for\b.*\s\[(?!(\d+\s+(to|til)\s+\d+)).*\]'

The regex and its test cases are here: https://regex101.com/r/Bj9pZE/3

better to do it manually, no ?

[ceremcem]

better to do it manually, no ?

Yes. The Regex pattern is quite basic, but it works for me. I decided to integrate it into my build tool, so a warning message can be displayed via a popup as the project is saved.