Add "allowedExtensions" to Transformer.

Add "allowedExtensions" to Transformer.

BUG=
R=nweiz@google.com, sigmund@google.com

Committed: https://code.google.com/p/dart/source/detail?r=26385

[Bob Nystrom, 2013-08-20 00:16:03]

Siggy, this lets you replace the verbose:

  Future<bool> isPrimary(Asset asset) =>
      new Future.value(asset.extension == ".html");

with just:

  String get allowedExtensions = "html";

[Siggi Cherem (dart-lang), 2013-08-20 00:19:53]

nice, I like it =)

lgtm

[nweiz, 2013-08-20 00:35:43]

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
File pkg/barback/lib/src/transformer.dart (right):

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
pkg/barback/lib/src/transformer.dart:20: /// (without leading `.`) that are
allowed for the primary inputs to this
It's confusing that this doesn't use the leading ".", but all the AssetId
methods do.

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
pkg/barback/lib/src/transformer.dart:26: String get allowedExtensions => null;
It's weird that this uses a space-separated list when it's just going to be
defined in code. It feels like it should be able to return either a string (for
a single extension) or a list of strings (for multiple).

[Bob Nystrom, 2013-08-20 16:15:37]

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
File pkg/barback/lib/src/transformer.dart (right):

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
pkg/barback/lib/src/transformer.dart:20: /// (without leading `.`) that are
allowed for the primary inputs to this
On 2013/08/20 00:35:43, nweiz wrote:
> It's confusing that this doesn't use the leading ".", but all the AssetId
> methods do.

Changed to require the leading ".".

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
pkg/barback/lib/src/transformer.dart:26: String get allowedExtensions => null;
On 2013/08/20 00:35:43, nweiz wrote:
> It's weird that this uses a space-separated list when it's just going to be
> defined in code. It feels like it should be able to return either a string
(for
> a single extension) or a list of strings (for multiple).

I thought about that, but then I wouldn't be able to type the parameter. I could
always require a list, but that punishes the common case of only allowing a
single extension. This was the best compromise I could think of. Since this is
really just a convenience anyway, I wanted to optimize for brevity.

[nweiz, 2013-08-20 19:20:53]

I still don't like [Transformer.allowedExtension] always being a String, but
otherwise lgtm.

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
File pkg/barback/lib/src/transformer.dart (right):

https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
pkg/barback/lib/src/transformer.dart:26: String get allowedExtensions => null;
On 2013/08/20 16:15:37, Bob Nystrom wrote:
> On 2013/08/20 00:35:43, nweiz wrote:
> > It's weird that this uses a space-separated list when it's just going to be
> > defined in code. It feels like it should be able to return either a string
> (for
> > a single extension) or a list of strings (for multiple).
> 
> I thought about that, but then I wouldn't be able to type the parameter. I
could
> always require a list, but that punishes the common case of only allowing a
> single extension. This was the best compromise I could think of. Since this is
> really just a convenience anyway, I wanted to optimize for brevity.

If the way you specify a list of extensions is as a string, you're not really
typing the parameter now. I'd rather have the signature always explicitly
indicate that this can return multiple types of values.

[Bob Nystrom, 2013-08-20 19:41:53]

On 2013/08/20 19:20:53, nweiz wrote:
> I still don't like [Transformer.allowedExtension] always being a String, but
> otherwise lgtm.
> 
>
https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
> File pkg/barback/lib/src/transformer.dart (right):
> 
>
https://codereview.chromium.org/23050009/diff/1/pkg/barback/lib/src/transform...
> pkg/barback/lib/src/transformer.dart:26: String get allowedExtensions => null;
> On 2013/08/20 16:15:37, Bob Nystrom wrote:
> > On 2013/08/20 00:35:43, nweiz wrote:
> > > It's weird that this uses a space-separated list when it's just going to
be
> > > defined in code. It feels like it should be able to return either a string
> > (for
> > > a single extension) or a list of strings (for multiple).
> > 
> > I thought about that, but then I wouldn't be able to type the parameter. I
> could
> > always require a list, but that punishes the common case of only allowing a
> > single extension. This was the best compromise I could think of. Since this
is
> > really just a convenience anyway, I wanted to optimize for brevity.
> 
> If the way you specify a list of extensions is as a string, you're not really
> typing the parameter now. I'd rather have the signature always explicitly
> indicate that this can return multiple types of values.

I'll check it in like this, and if you don't end up warming up to it, we can
always change it.

- bob

[Bob Nystrom, 2013-08-20 19:47:18]

Committed patchset #2 manually as r26385 (presubmit successful).