5 ms·

What is misguided - rtti or type-based dispatch - and why?

• colanderman 12y ago
  RTTI. Specifically the "runtime" part is misguided. Languages which permit runtime type inspection are much more difficult to compile efficiently, since type information must be kept at runtime. (There is no method to identify all cases when such information may be elided, and in any such language which permits "eval", there is no such way to elide RTTI.) Without loss of generality, any program using RTTI can be trivially transformed into one which doesn't use RTTI by tagging types, or by performing type-level computations at compile time. By performing one of these transformations, the compiler is free to generate much more efficient code. (Technically, RTTI includes runtime type-based dispatch; e.g. object-oriented method calls. I similarly avoid such code, preferring instead less-general mechanisms such as OCaml's functors or Haskell's type classes; both of which eliminate, or, in concert with existential types or first-class modules, make the overhead of RTTI explicit. Fortunately in languages such as Java where not everything is an object, the damage of RTTI is somewhat limited.)