Question

Is there any way to pass one mixin or style’s declaration to another mixin as an input parameter?

Let’s take a look at an example with animation keyframes. Following is how we define keyframes in pure CSS:

@-moz-keyframes some-name
{
    from { color: red; }
    to { color: blue; }
}

@-webkit-keyframes some-name
{
    from { color: red; }
    to { color: blue; }
}

@keyframes some-name
{
    from { color: red; }
    to { color: blue; }
}

Idea is to simplify these declarations using mixins, so we can have something like following:

.keyframes(name, from, to)
{
    // here we need somehow to reproduce structure
    // that we have in an example above
}

// define one animation
.my-from() { color: red; }
.my-to() { color: blue; }
// the following won't work because you cannot pass mixin as a parameter
// in way I have here, so I am looking for a way to solve this problem
.keyframes('some-name', .my-from, .my-to);

// define another animation
.another-from() { font-size: 1em; }
.another-to() { font-size: 2em; }
.keyframes('another-name', .another-from, .another-to);

The system will have different modules that could be dynamically attached to application as well as removed. So, don’t suggest me to use @import because it’s not the case. Output CSS is dynamically compiled on-fly using information about modules and their own LESS styles as well as base LESS dependencies like mixins library and etc.

Note: it will work for me if you know a way to pass class definition instead of mixin. In an example above it would be .my-from instead of .my-from() and etc.

Answer 1

UPDATED for LESS 1.7.0+ (WAY Simpler)

We can do this far more directly now with the 1.7.0 update and the ability to create rulesets, and to use variables in setting @keyframes.

Now we really can pass a mixin through a parameter by a ruleset, or we can pass in the property stings themselves. So consider this:

LESS (using 1.7)

.keyframes(@name, @from, @to) {
    @frames: {
        from { @from(); }
        to { @to(); }
    };
    @pre: -moz-keyframes;
    @-moz-keyframes @name
    {
       @frames();
    }

    @-webkit-keyframes @name
    {
       @frames();
    }

    @keyframes @name
    {
       @frames();
    }
}

.keyframes(testName, {color: red; .myMix(0);}, {color: blue; .myMix(1);});

.myMix(@value) {opacity: @value;}

Note that I am passing both a property setting and a mixin call, and my output is:

CSS Output

@-moz-keyframes testName {
  from {
    color: red;
    opacity: 0;
  }
  to {
    color: blue;
    opacity: 1;
  }
}
@-webkit-keyframes testName {
  from {
    color: red;
    opacity: 0;
  }
  to {
    color: blue;
    opacity: 1;
  }
}
@keyframes testName {
  from {
    color: red;
    opacity: 0;
  }
  to {
    color: blue;
    opacity: 1;
  }
}

Note how the rulesets are passed, in brackets {...}, and then called, via @from() and @to() (looking a lot like a mixin call). I’m using these passed rule sets to set another ruleset of @frames which is then itself called to fill the keyframes definitions.

More Generically

Here I pass a private mixin to another mixin and then call it from that other mixin:

LESS

.someMixin(@class; @expectedMixin) {
    .@{class} {
      @expectedMixin();
      .myPrivateMix(0.6);
      test: 1;
    }
}

.someMixin(newClass; {.myClass;});

.myClass {
  .myPrivateMix(@value) {opacity: @value;}
}

CSS Output

.newClass {
  opacity: 0.6;
  test: 1;
}

---

Kept the below for legacy info.

Updated (added LESS 1.4.0+ support)

Wow, this took some doing, but I think I have something you can work with. However, it does take some special defining of your mixins in your modules, specifically, using pattern matching. So…

First, Define Your Module Mixins

Note how the module mixins intended to be used in a specific future mixin are defined with the same mixin name, but with a different pattern name. This was key to making this work.

// define one animation in a module
.from(my-from){ color: red; }
.to(my-to) { color: blue; }

// define one animation in another module
.from(another-from){ font-size: 1em; }
.to(another-to) { font-size: 2em; }

If you also want individual mixin names in the modules, you should be able to do this:

// define one animation in a module
.my-from(){ color: red; }
.my-to() { color: blue; }

.from(my-from){ .my-from() }
.to(my-to) { .my-to() }   

// define one animation in another module
.another-from(){ font-size: 1em; }
.another-to() { font-size: 2em; }

.from(another-from){ .another-from() }
.to(another-to) { .another-to() }

This should allow one to call either the straight mixin .my-from() or, to make it variably accessible within later mixins that access the singular .from() mixin group through the pattern matching.

Next, Define Your Mixin

For your @keyframes example, that was extremely difficult. In fact, a stack overflow answer was vital to helping me solve an issue with applying the @name, which was not applying under normal LESS rules because of it following the @keyframes definition. The solution to apply the @name looks nasty, but it works. It does have the, perhaps, unfortunate necessity of also defining a selector string to play the animation by (because it uses that string to help build the last } of the keyframes). This naming limitation would only be true of css strings that begin with @ like @keyframes and probably @media.

Further, because we have a standard mixin name used in our module files, we can access that consistently within our new mixin, while at the same time passing a variable in to select the proper variation of that mixin through a pattern match. So we get:

LESS 1.3.3 or under

// define mixin in mixin file

.keyframes(@selector, @name, @from, @to) {
    @newline: `"\n"`; // Newline
    .setVendor(@pre, @post, @vendor) {
        (~"@{pre}@@{vendor}keyframes @{name} {@{newline}from") {
            .from(@from); 
        }    
        to  { 
            .to(@to);
        }
       .Local(){}
       .Local() when (@post=1) {
           (~"}@{newline}@{selector}") {
              -moz-animation: @name;
              -webkit-animation: @name;
              -o-animation: @name;
              -ms-animation: @name;
              animation: @name;
           } 
       }    
       .Local;
    } 
    .setVendor(""            , 0,    "-moz-");
    .setVendor(~"}@{newline}", 0, "-webkit-");
    .setVendor(~"}@{newline}", 0,      "-o-");
    .setVendor(~"}@{newline}", 0,     "-ms-");
    .setVendor(~"}@{newline}", 1,         "");
}

LESS 1.4.0+

.keyframes(@selector, @name, @from, @to) {
    @newline: `"\n"`; // Newline
    .setVendor(@pre, @post, @vendor) {
        @frames: ~"@{pre}@@{vendor}keyframes @{name} {@{newline}from";
        @{frames} {
            .from(@from); 
        }    
        to  { 
            .to(@to);
        }
       .Local(){}
       .Local() when (@post=1) {
           @animationSector: ~"}@{newline}@{selector}";
           @{animationSector} {
              -moz-animation: @name;
              -webkit-animation: @name;
              -o-animation: @name;
              -ms-animation: @name;
              animation: @name;
           } 
       }    
       .Local;
    } 
    .setVendor(""            , 0,    "-moz-");
    .setVendor(~"}@{newline}", 0, "-webkit-");
    .setVendor(~"}@{newline}", 0,      "-o-");
    .setVendor(~"}@{newline}", 0,     "-ms-");
    .setVendor(~"}@{newline}", 1,         "");
}

Now Call Your Mixin

You can give it your own name, and just pass the straight pattern (all are no dot [.] and no quotes) for the pattern matches on the module mixins, but don’t forget that you also need a selector string (which is quoted) to get the mixin to work right:

.keyframes('.changeColor', some-name, my-from, my-to);
.keyframes('.changeFontSize', another-name, another-from, another-to);

Which Gives You the Desired Output

@-moz-keyframes some-name {
from {
  color: red;
}
to {
  color: blue;
}
}
@-webkit-keyframes some-name {
from {
  color: red;
}
to {
  color: blue;
}
}
@-o-keyframes some-name {
from {
  color: red;
}
to {
  color: blue;
}
}
@-ms-keyframes some-name {
from {
  color: red;
}
to {
  color: blue;
}
}
@keyframes some-name {
from {
  color: red;
}
to {
  color: blue;
}
}
.changeColor {
  -moz-animation: some-name;
  -webkit-animation: some-name;
  -o-animation: some-name;
  -ms-animation: some-name;
  animation: some-name;
}
@-moz-keyframes another-name {
from {
  font-size: 1em;
}
to {
  font-size: 2em;
}
}
@-webkit-keyframes another-name {
from {
  font-size: 1em;
}
to {
  font-size: 2em;
}
}
@-o-keyframes another-name {
from {
  font-size: 1em;
}
to {
  font-size: 2em;
}
}
@-ms-keyframes another-name {
from {
  font-size: 1em;
}
to {
  font-size: 2em;
}
}
@keyframes another-name {
from {
  font-size: 1em;
}
to {
  font-size: 2em;
}
}
.changeFontSize {
  -moz-animation: another-name
  -webkit-animation: another-name;
  -o-animation: another-name;
  -ms-animation: another-name;
  animation: another-name;
}