Proposal: Symbol.templateTag

Andrea Giammarchi andrea.giammarchi at gmail.com
Fri Nov 18 01:03:19 UTC 2016


fix: forgot of course i would never be 0

`const dsl = (chunks, ...rest) => chunks.reduce((s, c, i) => s + rest[i-1]
+ c);`

VS

`const dsl = (chunks, ...rest) => chunks.reduce((s, c, i) => s +
(dsl[Symbol.templateTag]
|| String)(rest[i-1]) + c);`



On Fri, Nov 18, 2016 at 12:56 AM, Andrea Giammarchi <
andrea.giammarchi at gmail.com> wrote:

> I'm not fully sure I understand ... instead of:
>
> ```js
> const dsl = (chunks, ...rest) => chunks.reduce((s, c, i) => s + (i ?
> rest[i-1] : '') + c);
> ```
>
> you are looking for something similar ?
>
> ```js
> const dsl = (chunks, ...rest) => chunks.reduce((s, c, i) => s + (
>   i ? (dsl[Symbol.templateTag] || String)(rest[i-1]) : ''
> ) + c);
> ```
>
> if that's the case, is `Symbol.templateTag` really needed ?
>
> It looks to me it's possible to define your own behavior through your own
> transformer already and this proposal easily brings parsing-method clashing
> in the plate.
>
> Or maybe not ... like I've said, I haven't fully understood the issue.
>
> Thanks for any sort of extra clarification.
>
> Regards
>
>
> On Fri, Nov 18, 2016 at 12:30 AM, 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
>>>
>>
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
>
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