Less CSS - access part of class name and use in mixin - css

Question

In Less, is it possible to access part of a class name and use within a mixin?
This will be best explained with an example:
I have a grid which i have declared as follows:
.columns (#columns) {
//code here to generate column widths
}
//This is where my problem is:
.column-1 {
.col (1)
}
.column-2 {
.col (2)
}
.column-3 {
.col (3)
}
// etc etc
Obviously there is a lot of repetitive code going on here. Ideally i would like to be able to not have to declare column-1 column-2 etc and have some way, regex perhaps, of parsing the class name, and using the value after the dash to automatically calculate the column width. I am almost certain twitter bootstrap is doing something similar but i cant understand it:
.spanX (#index) when (#index > 0) {
(~".span#{index}") { .span(#index); }
.spanX(#index - 1);
}
.spanX (0) {}

Answer 1

I think you'll understand that :
.columnX (#index) when (#index > 0) { // Guarded mixin: use this mixin when the condition is true (like an if statement)
(~".column-#{index}") { // outputs .column-0 as a class name
.col(#index); // for the contents, use the col mixin
} // which class you are doing
.columnX(#index - 1); // Recursive call to the same mixin, going down by 1
}
.columnX (0) {} // Default implementation so that when .columnX(0) is called, a matching mixin is found.
.col (#index) {
// actual css that will be applied to column-1 if #index is 1
width: #index * 10px; // for example
}
.columnX(3); // number of columns you want
Edit (missed the ; of .columnX(3); )
Edit Added more comments
All this should give the result :
.column-3 {
width: 30px;
}
.column-2 {
width: 20px;
}
.column-1 {
width: 10px;
}

Answer 2

This is essentially the same as #sherbrow's answer, but a little more concise and doesn't error. Consider this a long explanatory comment to support his correct answer - it's just a lot more than fits in a comment!
You'll use a LESS loop mixin like this as an intermediate helper, and then call it specifying the number of classes you want to generate. Here's how to do .column-1, .column-2, and .column-3. If say you wanted to go up to four columns: just do .columns(4) instead of .columns(3) [line 9]. To go up to five columns, just do .columns(5).
1 // we'll call this if we want to build columns
2 .columns(#i) when (#i > 0) {
3 .column-#{i} {
4 .col(#i)
5 }
6 .columns(#i - 1)
7 }
8 // for example, here we build columns 1-3
9 .columns(3);
which will compile to the result of
.column-1 {.col(1)}
.column-2 {.col(2)}
.column-3 {.col(3)}
(Your question assumes there's already a mixin .col(#x) so I'm assuming that too. See 4 for how to skip that extra step.)
Here's what's happening:
The whole first chunk [lines 1-7] just sits there until called.
.columns(3) [line 9] sends us to the .columns(#i) mixin [line 2], assigning the variable #i the value 3.
Because #i (3) is greater than 0 [line 2], we satisfy the guard and are allowed past the { [line 2].
.column-#{i} {...} [lines 3-5] is what this mixin will output.
#i is 3, so the output will be .column-3 {.col(3)}
the syntax #{i} is used to insert the variable's value as a string
If you don't need to use .col(#x) anywhere else, you could also just drop styles in here directly, e.g. (a là #sherbrow) .column-#{i} {width: #i * 10px}
And then the loop: after compiling lines 3-5, call this mixin again but with a different value [line 6]: .columns(#i - 1) ==> .columns(3 - 1) ==> .columns(2).
Back to the top [line 2]: #i, now 2, is greater than zero, so we're allowed in. output .column-2 {.col(2)} (where .col(2) is immediately compiled, so your compiled CSS actually reads .column-2 { the.col(2)styles })
And keep outputting and looping until #i isn't greater than 0 (i.e. stop after .columns(1)).