Support ad-hoc filtering of directives #258
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isolates other directives from the no-op filter strangeness, and opens up some possibilities of useful filtering.
When a directive is constructed via `when ... orElse` or `Directive#orElse`, we can filter a successful result and fall back to the orElse directive. This allows for ad-hoc extention of directive requirements in the `if` clause of a for expression. Added a demonstration to DirectiveSpec.
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Seems to fit the typical 'withfilter' bill. Most impls of this pattern define and inner class of the type being filtered called 'WhithFilter'. Any thoughts on making 'FilterDirective' and inner class of 'Directive' named 'Filtered' or something? |
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If you link to an example we can confirm, but I think the difference is that this FilterDirective class provides a withFilter method, while WithFilter are typically classes of objects returned by such a method. |
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Ah, I think I missed that. Here's an example in travsersablelike https://github.com/scala/scala/blob/2.11.x/src/library/scala/collection/TraversableLike.scala. What I like about inner classes is that it reduces the number of names brought into scope at the package level which is good if they aren't intended to be directly instantiated in user code. Do you think users will ever be instantiating instances of FilterDirectives outside the context of an instance of a Directive? |
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Yes, I've done that in this pull request. |
Support ad-hoc filtering of directives
Directives currently have no-op
filtermethod that was added to support extraction of parameter directive tuples. This pull request moves filter functionality to aFilterDirectivesubtype so that the potentially surprising behavior of a no-op filter is now limited to and-ed parameter directives.Further, these changes allow us to support proper filtering in cases where
orElseis used. We can now construct a directive that knows how to fail for some conditions, and add more failure conditions via theifclause of a for expression. This behavior is demonstrated in an example added toDirectiveSpec.