Added ability to use inline

[taras]

• Added implementation
• Added test
• Updated README
• Added test for respecting scoping rules
• Added support for multiple declarations
• Added error reporting for incorrect use

Note: this feature requires Ember 2.0+

Fixes #7

[rwjblue]

This is pretty unlike the language concept of let. Since this is setting a bound property on the template context, some really weird/unexpected things happen.

{{let name post.author.name}}
{{name}}
{{#each post.comments as |comment|}}
  {{let name comment.author.name}}
  {{name}}
{{/each}}
{{name}}

All of these {{name}} usages will be the value of the last commenter. This is pretty WAT.

[rwjblue]

I also see no mention in the README that using {{let foo would clobber foo on whatever the template context is...

[cowboyd]

All of these {{name}} usages will be the value of the last commenter. This is pretty WAT.

@rwjblue Agree, so how to fix?

As it stands, it's basically straight up variable assignment which is sub-optimal, but the question is, how to make it mask (and then restore) the previous declaration in the underlying runtime.

Even if the semantics are not just-so, I'm still very curious about what the style entails. (I am not currently using the inline form)

[rwjblue]

@cowboyd - @mmun and I have been chatting about the best fix. I believe it is to use an AST transform to change:

{{name}} <- should be whatever {{name}} is on the template context
<div>
  {{name}} <- should be whatever {{name}} is on the template context
  {{let name (some-sexpr)}}
  {{#foo-bar}}
    {{name}} <- should be some-sexpr's value
  {{/foo-bar}}

  {{#foo-bar as |name|}}
    {{name}} <- should be the yielded value from foo-bar value
  {{/foo-bar}}

  {{name}} <- should be some-sexpr's value
</div>
{{name}} <- should be whatever {{name}} is on the template context

Into:

{{name}} <- should be whatever {{name}} is on the template context
<div>
  {{name}} <- should be whatever {{name}} is on the template context
  {{#let (some-sexpr) as |name|}}
    {{#foo-bar}}
      {{name}} <- should be some-sexpr's value
    {{/foo-bar}}

    {{#foo-bar as |name|}}
      {{name}} <- should be the yielded value from foo-bar value
    {{/foo-bar}}

    {{name}} <- should be some-sexpr's value
  {{/let}}
</div>
{{name}} <- should be whatever {{name}} is on the template context

With this solution, nothing is set on the template context. Follows much closer to JS's version of let (available for the duration of the block, but not before, etc).

[cowboyd]

Ok, so TIL AST transforms are a thing.

Thank you for that. This sounds like exactly what it should be: sugar for the block form. Which would also give us a clean path for implementing let*

[mmun]

Yes, I've already implemented the transform. Just cooking up a PR now...

https://jsbin.com/yebitem/edit?html,js,output

[taras]

Awesome. Thank you @mmun!

On Sun, Aug 28, 2016 at 8:41 AM, Martin Muñoz notifications@github.com wrote:

Yes, I've already implemented the transform. Just cooking up a PR now...

https://jsbin.com/nuwebe/edit?html,js,output

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[mmun]

Also I'd suggest making the inline form be {{let foo=bar}} so that the parser can ensure that the name "foo" is a valid identifier more easily (and the user is more likely to make the connection themselves). But I'll keep that in a separate PR.

[cowboyd]

Also I'd suggest making the inline form be {{let foo=bar}}

Yeah, I like that. The "positionality" of binding expressions was one of the really frustrating things to me about learning to work with let in lisp. It always (and sometimes still does) make my eyes go screwy, so if we can avoid it, I'm all for it.

[taras]

Yeah, this is more obvious.

On Sun, Aug 28, 2016 at 9:05 AM, Charles Lowell notifications@github.com wrote:

Also I'd suggest making the inline form be {{let foo=bar}}

Yeah, I like that. The "positionality" of binding expressions was one of the really frustrating things to me about learning to work with let in lisp. It always (and sometimes still does) make my eyes go screwy, so if we can avoid it, I'm all for it.

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