A compile-time generated named function that lives in the ruleset module.

Expressions are the only executable part of the IR. The compiler emits them as public, side-effect-free functions, in the module that used Rete.Ruleset. This lets them be captured, compared, and shared between rules.

:code is a stable, readable unique id. Two structurally identical conditions in one module context produce the same code. This is what lets the network share nodes. :kind fixes the calling convention: :alpha is (fact) -> bindings_map | nil, :test is (bindings_map) -> boolean, and :join_filter is (token_bindings, fact_bindings) -> boolean. :fun is nil until the expression is escaped. :__ast__ is compile-time only.

:share says whether two modules that produce this code may be put on one node. It states the conclusion, not the reason. Today one reason sets it: an unqualified call hashes as its bare name, so ok?(amt) under two different imports gives one code for two functions. A later reason to refuse a share sets the same field.

Rete.DSL.Codegen decides it while :__ast__ is still in hand, because escape/1 drops the AST and Rete.Compiler.disambiguate_codes/1 runs after that. It defaults to false, so an expression built somewhere this is not set is kept apart rather than merged on an assumption nobody checked.