Add support for nesting stache helpers

[dylanrtt]

Handlebars supports putting the result of one stache helper in another. They call it Subexpressions.

{{outer-helper (inner-helper 'abc') 'def'}}

Angular also allows chaining filters but there is currently no way to do this type of thing in the CanJS implementation of mustache/stache.

[asavoy]

+1 I'd like this.

[justinbmeyer]

@dylanrtt @asavoy any interest in helping implement it? I can walk through what's needed.

Anyone think that:

{{outer-helper inner-helper('abc') 'def'}}

Would be more natural?

[justinbmeyer]

The place to start implementing this is to first get the parsing of sub-expressions working. Currently, the parsing happens here:

https://github.com/bitovi/canjs/blob/master/view/stache/mustache_core.js#L133

For something like {{outer-helper 1 name zed='ted' foo=bar}} this returns a data structure that looks like:

{
  name: "outer-helper",
  args: [1, {get: 'name'}],
  hashes: {zed: 'ted', foo: {get: 'bar'}}
}

To make sub-expressions work, we'd want to make this recursive. Something like {{outer-helper inner-helper('abc') 'def'}} might return:

{
  name: "outer-helper",
  args: [{name: 'inner-helper', args: ['def']}],
  hashes: {}
}

With that in place, the next work would need to be done in makeEvaluator: https://github.com/bitovi/canjs/blob/master/view/stache/mustache_core.js#L182

There is looks in args and in hashes for objects "getter" objects:

https://github.com/bitovi/canjs/blob/master/view/stache/mustache_core.js#L208

If it found an object like {name, args, hashes}, it could actually call itself (makeEvaluator) with that object as exprData. makeEvaluator returns a function that will need to be converted into a compute.

All in all .. pretty simple actually.

[justinbmeyer]

Related to #1888, I'm thinking that function calls are more natural when you might have multiple of them. Compare:

{{#each ( (game.statsForPlayerId player.id).filter type) }}

vs

{{#each gameStatsForPlayerId(player.id).filter(type) }}

[dylanrtt]

I like the function call method filter(type) better than the way handlebars does it.

[dylanrtt]

This works now, with helpers and functions. I think it was added via #1832.

Working example with 2.3.1:
http://jsbin.com/dakesokuva/edit?html,js,output