Early error vs. error on first call to function vs. runtime error

[Domenic Denicola]

As a user, not implementer, I really want early errors. Perf costs of startup are negligible especially long-term. By the time ES6 is in browsers computers and phones should be faster by enough of a factor to mitigate any costs, whereas omitting early errors hurts developers indefinitely into the future.

On Sep 28, 2012, at 4:02, "Brendan Eich" <brendan at mozilla.org> wrote:

> Brendan Eich wrote:
>> We have not discussed error-on-first-call in this thread at all! 
> 
> This needs a separate thread. The idea from last week's TC39 meeting was to have not only
> 
> * Early error, thrown before any code in the Program (grammar goal symbol) containing the error, required by specific language in Clause 16.
> 
> * Runtime error, all the other kinds.
> 
> and now
> 
> * Error on first call to a function, where the function contains what would be an early error but for the supposed cost of early error analysis.
> 
> The last case is really just a runtime error: a function with what should be a static error becomes a booby trap: if your tests happen to miss calling it, you'll feel ok, but a user who tickles the uncovered path will get a runtime error.
> 
> TC39 heard from some implementors who wanted to avoid more early error requirements in ES6, or at least any that require analysis, e.g. reaching definitions.
> 
> That's fair as input to the committee, but implementation concerns are not the only ones we weigh. And we were far from agreed on adding the "Error on first call" category.
> 
> The example you imply here would be
> 
>  function f(a, b = c, a = d) {
>  }
> 
> and the duplicate formal a would be illegal because of the novel default-parameter syntax.
> 
> Making f into a proximity-fused bomb does not see either good or necessary. The analysis requires to notice duplicate formals is trivial, and as I keep pointing out, ES5 already requires it:
> 
>  function g(a, a) {
>    "use strict";
>  }
> 
> This must be an early error per ES5 clause 16.
> 
> Given the ES5-strict sunk cost, there's no added implementation tax beyond the logic conjoining duplicate detection with novel-syntax detection, which is trivial.
> 
> It'd be good to hear from Luke on this.
> 
> /be
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