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Tuesday, June 12, 2012

CSS and Rails - Migrating from Less to Sass

In my Rails projects over the past couple of years I have been using Less to make my CSS files easier to work with. Less is a CSS pre-processor that lets you use variables for color, etc.

Less is pretty similar to Sass and I didn't see any real reason to move away from it. But then Rails 3.1 made the decision to uses Sass and CoffeeScript as CSS and JavaScript preprocessors. So now I want to migrate my Less files over to Scss (the v2 format of Sass) so I can go with the flow in current Rails versions.

Sass comes with a conversion tool called sass-convert that you can find in the bin directory of the gem.
You can find where that is located with 'gem which sass'. But this gave me this error:
NameError: undefined method `build' for module `Less::StyleSheet::Mixin4'
From the sass github site it seems like sass -convert has not kept up with changes in Less and there seems little motivation to fix the problem.

Never mind - the Less and Scss syntaxes are pretty similar and my files do not include anything too complex, so manually converting them in an editor is quite feasible. In fact the main difference is that Less prefixes variables with @ and Sass uses $. The important exception here is that @import should not be changed as it is exactly the same in Sass.

Less has an equivalent to Sass Mixins. You create a custom class and then insert that into other CSS blocks. Here is an example followed by the Sass equivalent.
.my-mixin {
  color: red;
.my-class {
  width: 200px;

In Sass you would write:
@mixin my-mixin {
  color: red;
.my-class {
  width: 200px;
  @include my-mixin;
Making this conversion is easy enough to do with find and replace in your editor.

The easiest way to validate the modified files is to run sass on them and check the css.
$ sass my.scss my.css

You can try validating the CSS using a service like http://jigsaw.w3.org/css-validator/validator

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