JavaScript Evaluation

[SomMeri]

Known syntaxes:

  color: `javascript`;

[SomMeri]

Postponed since may deprecate it in the future. This will be implemented only if they will decide that they do want it.

[SomMeri]

Removed javascript tests from less.js 1.3.3 test suit. It needs to be added back before closing this issue.

[SomMeri]

Escaping in JavaScript is also supported:

~`"@{arguments}".replace(/[\[\]]|\,\sX/g, '')`;

Found in twitter bootstrap v2.1.1 #127

[karlvr]

We have a need for javascript evaluation in backticks. I could look into implementing this. If you have a moment, would you be able to do a brain dump of thoughts about implementing it? Where things will need to be added etc. Does ` need to be added to the grammar?

[SomMeri]

Hi, that would be really awesome.

Braindump has two parts, one is about API and another explains less4j code. This comment has only API part in it, less4j architecture will go into second comment.

API:
There has been some discussion on less.js side about whether keep javascript in less, making it possible to turn it off or at least advise against it.

So, maybe we could do it plugginable? The new Configuration object could expose some setExpressionCompiler(OtherCompiler compiler) method. JavaScript interpreter would implement OtherCompiler interface and compile JavaScript strings into strings.

If the compiler would be unavailable, less4j would issue warning/error and leave expression unchanged.

Advantages:

• Flexibility, anybody could add compiler (python) of his choice.
• Keeping number of dependencies low. JavaScript executor dependency would be optional in this case. Only those that really need it would have to download it.

I do not mind publishing that into maven or maintaining that other repo, if you do not want to bother with bureaucracy.

[SomMeri]

Less4j uses ANTLR. If you never used it, here are tutorials:

• http://meri-stuff.blogspot.sk/2011/08/antlr-tutorial-hello-word.html
• http://meri-stuff.blogspot.sk/2011/09/antlr-tutorial-expression-language.html

The grammar file:

• /less4j/src/main/antlr3/com/github/sommeri/less4j/core/parser/Less.g

The general overwiew of how less4j works is explained here. You do not have to read the whole post, only the chapter I linked.

JavaScript expressions could be parsed and evaluated as other expressions, e.g. almost like functions. Advantage: they would be automatically allowed everywhere where expressions goes (variable values, selectors, functions, properties.) Plus, they are expressions.

You have to:

• Modify grammar in Less.g. New parser rule should probably be referenced as term alternative.
• Create new node in AstCssNode which would correspond to JavaScript expressions. Most likely as subclass of Expression.
• Modify ASTBuilderSwitch to transform new antlr tokens into new AstCssNode nodes and put them into tree.
• Assuming new node is a subclass of Expression: add new public Expression evaluate(NewNode input) method into ExpressionEvaluator node. It could return an instance of CssString.

You can look at how functions are done, it should be almost the same.

Unit tests:

• Add new javascript sub-folder into src/test/resources/compile-basic-features/.
• Add some .less, .css, .err files into it.
• Create test class similar to FunctionsTest. It will compile all .less files, compare results with .css and issued errors with .err.

Good luck :)

[SomMeri]

This is going to be possible using custom functions https://github.com/SomMeri/less4j/wiki/Extending-Less-Language which are already implemented, but do not support ~... and ... syntaxes.

Starting with next release:

• ~... will be treated as function named ~`
• ... will be treated as function named `

The advantage is that anyone will be able to implement them however he likes and use any language he finds useful.

I'm not sure whether or when I will create "official" javascript implementation, but anyone will be able to do so. In any case, it will be stored in separate repository. I do not want less4j to depend on Rhino or other javascript runtime, because only some projects will use it.

[SomMeri]

It might make sense to test lesshat compatibility #192 once custom javaascript function is released.

[SomMeri]

This will have to wait until decisions for less/less.js#1939 , less/less.js#1941 and less/less.js#1943 are taken.

I will do some javascript plugin, but it will be subject to changes later on.

[SomMeri]

JavaScript plugin is here: https://github.com/SomMeri/less4j-javascript