In reading over the Debugging draft (i.e., the future S20), I ran across the concept of the infectious trait - that is, a trait that doesn't just get applied to the thing to which it is explicitly applied; rather, it tends to spread to whatever else that thing comes in contact with. "Taint" is the primary example of this, although not the only one.
In reading about functional programming, I ran across the concept of the "pure function" - i.e., a function that doesn't produce side effects. The article went on to say that "While most compilers for imperative programming languages detect pure functions, and perform common-subexpression elimination for pure function calls, they cannot always do this for pre-compiled libraries, which generally do not expose this information, thus preventing optimisations that involve those external functions. Some compilers, such as gcc, add extra keywords for a programmer to explicitly mark external functions as pure, to enable such optimisations." It occurred to me that this business of marking functions as pure could be done in perl by means of traits - that is, any function might be defined as "is pure", promising the compiler that said function is subject to pure function optimization. It further occurred to me that a variation of the contagious trait concept (operating on code blocks and their component statements instead of objects; operating at compile time rather than run time; and spreading via "all participants" rather than "any participant") could be used to auto-tag new pure functions, provided that all so-called "primitive pure functions" are properly tagged to begin with. The compiler could then rely on this tagging to perform appropriate optimization. Such auto-tagging strikes me as being in keeping with Perl's virtue of laziness: writers of new code don't have to care about tagging pure functions (or even know that they exist) in order for the tagging to take place, leading to a potentially more robust library of functions overall (in the sense of "everything in the library is properly annotated"). As well, I could see it having some additional benefits, such as in concurrent programming: "is critical" could be similarly infectious, in that any function that might call a critical code block should itself be tagged as critical. Indeed, "is critical" and "is pure" seem to be mutually exclusive concepts: a given code block might be critical, pure, or neither; but it should never be both. Am I onto something, or is this meaningless drivel? -- Jonathan "Dataweaver" Lang
