Details
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Improvement
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Status: Closed
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Major
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Resolution: Fixed
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None
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None
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None
Description
Overview
As compilation proceeds, the compiler's data structures typically gain additional information as the various parts of the compiler enrich that data. Ideally, AST transforms should be invoked once all of the compiler mdata they need is ready. However, the order of processing becomes a secondary factor in determining what phase a transform will run.
AST Transforms target a particular phase with transforms in earlier phases running first. For transforms in the same phase, the order they appear in the source code determines which ones run first. If framework writers rely on certain transforms to run first, they might delay some transforms to later stages. This places additional constraints on framework writers which are further compounded when using annotation collectors (meta-annotations) since they list component transforms in a particular order.
This issue is to allow framework developers some ability to order transforms in the same phase using a priority. Transforms with a higher priority (determined by an integer value) run first.
If no priority is given, a priority of 0 is assumed. Transforms with the same priority run in the order in which they appear. To make a transform run before others, give it a higher positive value. A negative priority will send the transform to the end of the list. AST transforms need to implement the TransformWithPriority interface if they want to alter their priority. Positive values will run ahead of those with priority 0 which run ahead of those with negative priorities. Transforms typically run after all of the inbuilt compiler steps have been undertaken. This behavior hasn't changed, just the order within the normal AST transformation processing steps is potentially changed.
Out of Scope
- Widescale changing many of the internal Groovy transforms away from the default priority of 0 (TupleConstructor has a slightly raised priority so that constructors it creates will be available to other transforms also at the same phase)
- Any mechanism for negotiating priority between frameworks (frameworks can choose how they assign priorities)
- Any mechanism for end users to alter the priority (it is baked into the AST transform definition not usage of the transform annotation)
