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--- language:langs:refine [2022/09/16 12:52] – rajit
+++ language:langs:refine [2024/08/11 10:31] (current) – [Multiple refinement options] rajit
@@ Line 35: / Line 35: @@
  }
 </code>
+At this point, a tool can select the level of abstraction/detail for the process by picking one of the two sub-languages provided.
-Now imagine that instead of writing the production rules directly, you'd like to instantiate a set of gates to implement the circuit. If you wrote:
+Now imagine that instead of writing the production rules directly, you'd like to instantiate a set of gates to implement the circuit. Suppose we write the following:
 <code act>
 defproc example(...)
@@ Line 49: / Line 50: @@
 }
 </code>
-then this means that the process has a CHP definition, and //in addition// it has the specified instances! This is not what was intended; we'd like to use the CHP definition //or// the instances. To provide support for this, the ''refine'' body is used. The example would be written:
+As written, this means that the process has a CHP definition, and //in addition// it has the specified instances! This is not what we intended to specify; we'd like to use the CHP definition //or// the instances.
+To provide support for this, the ''refine'' body is used. The example would be written:
 <code act>
 defproc example(...)
@@ Line 64: / Line 67: @@
 }
 </code>
-Now you can //pick// the refined version of the process by using the ACT command-line option ''-ref=1''.
+Now you can //pick// the refined version of the process by using the ACT command-line option ''-ref=1'', and so a tool can pick either the ''chp'' or the ''refine'' sub-language, giving us the desired effect.
+This approach is used in the ''chp2prs'' tool to provide an implementation of the CHP. This is the standard convention used in any tool that takes CHP and generates a circuit-level description. By augmenting the definition of the process in this manner, you can model the process at both the CHP level of abstraction as well as a more detailed representation containing a collection of instances that implement the CHP.
+===== 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//.