make 'throw foo' an expression

[DartBot]

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

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If throwStatement was an expression instead, this would allow:

() => throw new Exception("foo")
z = foo ? bar : throw new BazException()

If it was of static type ⊥ (like null) then these should be warning-free, too.

[DartBot]

This comment was originally written by drfibonacci@google.com

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

[gbracha]

Generally, expressions compose better than statements and should be preferred. One reason why this might be tricky in Dart is because we need to compile to Javascript.

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Set owner to @gbracha.
Added Accepted label.

[rakudrama]

JavaScript is not a problem - other issues already cause the compiler to generate JavaScript that calls a helper function to do the actual JavaScript 'throw', so the compiler is already generating code suitable for an expression context. Even if the other issues went away, we could use a helper expressly for converting from statement to expression context when needed.

I would welcome the combination of => and throw. It would clean up the appearance of code that is incomplete:

   String get prettyVersion() => throw const NotImplementedException();

or throws on mutators:

   add(E elem) => throw new UnsupportedOperationException('Cannot add to immutable list');

I don't see it as a high priority as I believe the statement could be promoted to an expression after v1 without breaking anything.

[gbracha]

That's all good news. I'll see if/when we can act on this.

[anders-sandholm]

Added this to the Later milestone.

[anders-sandholm]

Removed this from the Later milestone.
Added this to the M1 milestone.

[rakudrama]

Issue #3370 has been merged into this issue.

[lexprfuncall]

I tripped over this today when trying to validate constructor arguments during the assignment to a final field within an initializer list.

class _Float32ArrayView extends _ByteArrayViewBase implements Float32List {
  _Float32ArrayView(ByteArray array, [int offsetInBytes = 0, int length])
    : _array = array,
      _offset = (offsetInBytes is int) ? offsetInBytes : (throw new IllegalArgumentException()),
      _length = ((length === null) ?
                 ((array.lengthInBytes() - offsetInBytes) % _BYTES_PER_ELEMENT) : length) { ... }

cc file. out/DebugIA32/obj/gen/snapshot_gen.cc
 'bootstrap': Error: line 2953 pos 54: unexpected token 'throw'
    _offset = offsetInBytes is int ? offsetInBytes : throw IllegalArgumentException(),

[gbracha]

Fixed in 0.11 draft.

Corresponding bugs:

Analyzer issue #4872
VM issue #4873
dart2js issue #4874

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Added Done label.