62. Learning a compiler’s rejection rule

The preceding selector chooses among three supplied compiler strategies. It changes which algorithm will be used, but does not compose a new decision rule. We now move one step inside a compiler. A small learner constructs a predicate that tells the compiler when it can stop inspecting an unpromising query. The learned object is neither a hidden-cell answer nor a completed diagnostic tree. It is an operation on the compiler’s own partial information, reused while constructing trees for further problems.