Concatenate String in LESS in loop

I'm working on converting Unsemantic from SASS to LESS and while I'm building the loop to create my classes:

    .populateGridClasses (@index, @interval) when (@index > 0) {
    @num: @index * @interval;
    (~".eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}") {
        .grid();
    }
   .populateGridClasses(@index - 1, @interval);
}
.populateGridClasses (0, @interval) {}

// Create the grids in an inverval of 5
.populateGridClasses(20, 5);

// Create the grids in thirds .populateGridClasses(3, 33);

It creates the classes as so:

.eh-grid-100, .eh-mobile-grid-100, .eh-tablet-grid-100 {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 10px;
}
.eh-grid-95, .eh-mobile-grid-95, .eh-tablet-grid-95 {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 10px;
}
...

Obviously, that could be condesnsed so that all 6 of those classes are defined at once. So my idea is to use the loop to create a giant string that I'll then add the .grid() mixin to:

@test: "";
.populateGridClasses4 (@index, @interval) when (@index > 0) {
    @num: @index * @interval;
    @ntest: ".eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}";
    @test: "@{test}@{ntest}";
.populateGridClasses4(@index - 1, @interval);
}
.populateGridClasses4 (0, @interval) {}
.populateGridClasses4(20, 5);

("@{test}"){
    padding-left: 1px;
}

But that gives me a LESS error LESS: Out of stack space. Any idea on how I can create this massive string so I can create 69 classes and only define them once? Programmatically, of course.

You could try passing another attribute to the mixin ... like this, where I added to your code the @t1 to the arguments and define the @t2 in the loop, and pass it on. Now you'll be writing to a variable only in the scope of one loop step, and not trying to overwrite the same variable over again in the recursion (does not agree with less). So this is your code, that should not get the error you mention anymore:

    @test: "";

    .populateGridClasses4 (@index, @interval, @t1) when (@index > 0) {
        @num: @index * @interval;
        @ntest: ".eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}";
        @t2: ~"@{t1}@{ntest}";
    .populateGridClasses4(@index - 1, @interval,@t2);
    }

    .populateGridClasses4 (0, @interval,@t1) {}

    .populateGridClasses4(20, 5, @test);

    @{t2} {
        padding-left: 1px;
    }

Also you need use ~ for class interpolation, not to return the class names between quotation marks.

Edit: The above will only work in 1.3.3, but for your approach to work in 1.4 you need to tweak it a little. Also I noticed that the way you were joining the strings did not add commas between class names of each loop, so I added another step here, this should now do the right thing in1.4 and previous versions of LESS.

    .populateGridClasses4(1,@num,@t1) {
        @test: ~"@{t1}, .eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}";
    }

    .populateGridClasses4(@index, @interval, @t1) when (@index > 1) {
        @num: (@index * @interval);
        @t2: "@{t1}, .eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}";
        .populateGridClasses4((@index - 1),@interval,@t2);
    }

    .populateGridClasses4(@index,@interval) {
        @num: (@index * @interval);
        @t2: ".eh-grid-@{num}, .eh-mobile-grid-@{num}, .eh-tablet-grid-@{num}";
       .populateGridClasses4((@index - 1), @interval, @t2);
    }

    .populateGridClasses4(20, 5);
    @{test} { padding-left: 1px; }

the output CSS is:

  .eh-grid-100, .eh-mobile-grid-100, .eh-tablet-grid-100, .eh-grid-95, .eh-mobile-grid-95, .eh-tablet-grid-95, .eh-grid-90, .eh-mobile-grid-90, .eh-tablet-grid-90, .eh-grid-85, .eh-mobile-grid-85, .eh-tablet-grid-85, .eh-grid-80, .eh-mobile-grid-80, .eh-tablet-grid-80, .eh-grid-75, .eh-mobile-grid-75, .eh-tablet-grid-75, .eh-grid-70, .eh-mobile-grid-70, .eh-tablet-grid-70, .eh-grid-65, .eh-mobile-grid-65, .eh-tablet-grid-65, .eh-grid-60, .eh-mobile-grid-60, .eh-tablet-grid-60, .eh-grid-55, .eh-mobile-grid-55, .eh-tablet-grid-55, .eh-grid-50, .eh-mobile-grid-50, .eh-tablet-grid-50, .eh-grid-45, .eh-mobile-grid-45, .eh-tablet-grid-45, .eh-grid-40, .eh-mobile-grid-40, .eh-tablet-grid-40, .eh-grid-35, .eh-mobile-grid-35, .eh-tablet-grid-35, .eh-grid-30, .eh-mobile-grid-30, .eh-tablet-grid-30, .eh-grid-25, .eh-mobile-grid-25, .eh-tablet-grid-25, .eh-grid-20, .eh-mobile-grid-20, .eh-tablet-grid-20, .eh-grid-15, .eh-mobile-grid-15, .eh-tablet-grid-15, .eh-grid-10, .eh-mobile-grid-10, .eh-tablet-grid-10, .eh-grid-5, .eh-mobile-grid-5, .eh-tablet-grid-5 {
    padding-left: 1px;
  }