Conditional catch clause

Brendan Eich brendan at
Wed Dec 19 11:14:54 PST 2012

David Bruant wrote:
> Le 19/12/2012 16:38, Claude Pache a écrit :
>> Le 19 déc. 2012 à 15:32, David Bruant <bruant.d at> a écrit :
>>> Le 19/12/2012 14:13, Claude Pache a écrit :
>>>> Hello,
>>>> In SpiderMonkey (and perhaps other JS engines?), there are 
>>>> conditional catch clauses:
>>>>     catch (exception if condition)
>>>> Could such a feature added to ECMAScript?
>>>> Rationale: Although try/catch mechanism was initially intended for 
>>>> treating errors, it could be used in normal control flow with 
>>>> non-error condition.
>>> Why not just use normal control flow when you want to express 
>>> non-error conditions?
>>> throw/try/catch was indeed intented for treating errors and when I 
>>> read code, I expect an error path when encountering a try/catch.
>> As I've said, iterators throw StopIteration, which may be considered 
>> as a non-error.
> I completely agree. I have said elsewhere [1] that StopIteration could 
> be its own statement. What we want is a way to stop the current call 
> frame. The language provides 2 such ways, return and throw. Return 
> can't be used because it may result in ambiguities. It could be the 
> same thing for throw, but normal use of throw make that throwing a 
> specific object is an acceptable hack on StopIteration.

This is a red herring. It's very rare to have to catch StopIteration.

The catch (e if cond) syntax is in SpiderMonkey and Rhino, dates from 
ES3 where it was proposed, prototyped, and shot down in Ecma TC39 TG1, 
but kept in the implementations. Again, this syntax helps discriminate 
on e's duck type or instanceof without requiring a hand-coded if-else 
chain or switch, with mandatory re-throw in final-else or default.

In TC39 we had a discussion, I think in March 2011, about generalized 
refutable match as proposed by dherman:

This was considered not baked enough to put in ES6. The tension with 
destructuring is an ongoing issue in my view. Also, Dave's strawman 
changes catch from how it works in SpiderMonkey and Rhino, so a pattern 
implies an 'if'.

I think we need to talk about refutable patterns and matching again, 
before we beat the catch-if dead horse once again. Suggest new spin-off 
thread (different subject).


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