Proposal: Symbol.templateTag

Alexander Jones alex at weej.com
Sun Nov 20 15:38:56 UTC 2016


As promised (to myself!) I've written a stage-0.

https://github.com/alex-weej/es-symbol-template-tag

Your feedback is greatly appreciated.

Thanks

On 18 November 2016 at 00:30, Alexander Jones <alex at weej.com> wrote:

> OK, I was thinking about this incorrectly and I think you're right - it
> doesn't Break The Web. It's exactly the same as when @@isConcatSpreadable
> and @@toStringTag were introduced, as far as I can tell.
>
> If no-one has any obvious objections to save me the effort, I'll try to
> write up a stage 0 this weekend.
>
> Thanks
>
>
>
> On 17 November 2016 at 04:58, Mark S. Miller <erights at google.com> wrote:
>
>> Hi Alexander, I have run into this myself. It is a real problem, and I
>> like the nature of your proposed solution. But I don't understand how it
>> might break the web. If Symbol.templateTag is a new symbol, guaranteed
>> unequal to any prior value, how can introducing it, and new behavior
>> conditional on it, change any existing behavior?
>>
>>
>>
>>
>> On Wed, Nov 16, 2016 at 5:02 PM, Alexander Jones <alex at weej.com> wrote:
>>
>>> Hi es-discuss!
>>>
>>> Template tags are a great feature with many novel applications, IMO
>>> approaching macro-level expressiveness. But due to their nature in this
>>> context, it's helpful if tags are named quite tersely. Unfortunately, if
>>> you have existing API, this can be challenging.
>>>
>>> Let's say I have a DSL (domain-specific language), and I *may*,
>>> unsafely, generate typed snippets of code containing that DSL like so:
>>>
>>> ```js
>>> function dsl(code: string) {
>>>     // `code` MUST be well-formed
>>> }
>>>
>>> const someStatement = dsl("val foo = " + dslEncode(foo));
>>> ```
>>>
>>> Let's say I'd like to add support in my library for ES6 clients such
>>> that they can do:
>>>
>>> ```js
>>> const someStatement = dsl`val foo = ${foo}`;
>>> ```
>>>
>>> where the `dsl` template tag would be implemented such that `dslEncode`
>>> is used on each interpolated expression.
>>>
>>> Unfortunately, the `dsl` function's arguments are already defined and
>>> can't be changed. I'd need a new name, like `dsl.tag`, which would end up
>>> being a fair bit more verbose.
>>>
>>> Would it be possible, at this point, to introduce a new behaviour into
>>> ES such that instead of merely calling the tag object, the implementation
>>> first checks for a `Symbol.templateTag` property on the tag object, and if
>>> it exists, it is invoked? I'd guess this can't Break The Web because no-one
>>> can realistically be using an existing function as a template tag if it was
>>> not already designed for it.
>>>
>>> ```js
>>> const someStatement = dsl`val foo = ${foo}`;
>>> // desugars to, approximately
>>> const someStatement = (dsl[Symbol.templateTag] || dsl)(["val foo =",
>>> ""], foo);
>>> ```
>>>
>>> To be honest, I am kind of surprised it wasn't already implemented like
>>> this, but maybe there were performance concerns with the branching.
>>> Interested in your thoughts.
>>>
>>> Thanks
>>>
>>> Alex
>>>
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>>
>>
>>
>> --
>>     Cheers,
>>     --MarkM
>>
>
>
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