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--- language:langs:refine [2024/08/11 10:21] – rajit
+++ language:langs:refine [2025/04/21 19:58] (current) – [Refinement overrides] rajit
@@ Line 73: / Line 73: @@
 ===== Nested refinements =====
+Since ''refine { ... }'' blocks contain normal ACT statements, they can also include refine blocks within them.
+Hence, you can say:
+<code act>
+refine {
+    inst i1;
+    chp { ... }
+    refine { inst i2; }
+}
+</code>
+When ACT selects the outer refinement block for the process, it decrements the refinement level before proceeding; the level is restored at the end of the refinement block. Hence, if  ''-ref=1'' were specified, then
+the block:
+<code act>
+  inst i1;
+  chp { ... }
+  refine { inst i2; }
+</code>
+would be processed with the refinement level set to zero; hence, the ''chp { ... }'' body would be selected, and
+not the nested refinement. To replace the CHP with the nested refinement block, use ''-ref=2''. Note that in both cases the instance ''i1'' would be processed.
 ===== Multiple refinement options =====
+While the nested refinement approach works nicely when designs are recursively refined, it may not
+be suitable for transformations where an existing refinement block needs to be completely replaced.
+ACT has a mechanism to support this in the following manner:
+<code act>
+refine {
+    inst i1;
+    chp { ... }
+}
+refine<2> {
+    inst j1;
+    prs { ... }
+}
+</code>
+In this case, "-ref=2" will //replace// the first refine block with the second. In other words, instance ''i1'' will not
+be created, and ''-ref=2'' will select instance ''j2'' and the ''prs { ... }'' body.
+In general, a refine block has an associated integer refinement level (the default is one). When ACT processes a block with refinement level //r//, it searches for the largest refinement block with level that is at most //r//. (Note: this may be no block, in which case the block is processed as normal.) If a refine block with level //l// is found, then it processes all non-language constructs within the block and then expands the refinement block with level //l// by decrementing the current refinement level by //l//.
+===== Refinement overrides =====
+Sometimes a refinement body needs to [[language:impl#overrides|override]] types. To support this, refinement bodies can be accompanied by refinement overrides. These are overrides that are only applied when the refinement is used.
+The syntax for providing refinement overrides is below:
+<code act>
+refine<2>  +{ e1of2 l; } {
+       inst1 i1;
+       ...
+ }
+</code>
+This will apply the refinement override for ''l'' when this particular refinement body is selected.
+</code>