catch syntax is confusing (looks like an annotation)

[DartBot]

This issue was originally filed by sammcca...@google.com

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Types are both used as annotations (don't affect behaviour) and as references (do affect behaviour), in different contexts:

Annotations:
  T local = 42;
  T method() { }
  method(T param);

References:
  new T();
  x is T
  List<T>
  extends T
  catch (T foo)

Note that catch syntax fits the pattern of an annotation, but it's actually a behaviour-affecting reference to the type!
This confuses people (myself, and others on the mailing list).

The catch syntax originates with languages that don't have this inconsistency, because types are enforced rather than annotations. In these languages, the most similar syntactic construct (method parameter declarations) is used for a pattern matching construct (overloading) similar to catch. This similarity is not present in Dart, so the similar syntax is misleading.

I think the following syntax is an improvement:

  } catch (x is T) {

Pros:
  * syntax is already reserved
  * type appears at the end of the expression, like other type references
  * meaning is obvious
Cons:
  * looks like a boolean expression
  * less familiar than current syntax to Java/C++/C# developers

There's probably even better syntax to be found, though.

[anders-sandholm]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

I agree the use of types in the catch syntax is at odds with the rest of the language.

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Set owner to @gbracha.
Added Accepted label.

[gbracha]

Added this to the M1 milestone.

[gbracha]

The next rev of the spec (0.11) updates the catch syntax to resolve this issue and make it cleaner.

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Added Done label.

[gbracha]

Analyzer issue #3576
VM issue #3577
dart2js issue #3578

[sethladd]

News post for this update: http://news.dartlang.org/2012/06/darts-catch-syntax-revisited.html

[DartBot]

This comment was originally written by @mhausner

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Gilad, the spec does not say whether "on" is a reserved word or a built-in identifier. Please update the spec accordingly.

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Added NeedsInfo label.

[gbracha]

That is because it is neither. The word "on" was deemed too important in web APIs etc.

[DartBot]

This comment was originally written by @aam

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Correcting links to relevant issues:
  Analyzer http://code.google.com/p/dart/issues/detail?id=3756
  VM http://code.google.com/p/dart/issues/detail?id=3757
  dart2js http://code.google.com/p/dart/issues/detail?id=3758

[gbracha]

Added Done label.