RE: [Caml-list] type-safe printf

[David Allsopp <dra-news@metastack.com>]

Андрей Бергман wrote: 
> is there a good article or just short explanation how compile-type
> checking of Printf arguments works?
>
>
> Does it require special compiler support like in C++ compilers/analyzers,
> or one can somehow do the trick of parsing string literal using Ocaml
> language itself? (actually latter does not look possible to me)

It does require a certain amount of special support, yes - see the Pexp_constant(Pconst_string _) case in typing/typecore.ml, in particular the comment "Terrible hack for format strings"! In essence, a string is "magically" interpreted as a format string if it is used in a context where one would be expected (e.g. the usual Printf.printf). It's one of the reasons Pervasives.format_of_string exists (because they're type- rather than syntax-driven).

From that point onwards, it's a lot of GADTs, but doesn't require any special language support!

If you were adding Printf to OCaml for the first time now, you'd probably use quoted string literals instead (see http://caml.inria.fr/pub/docs/manual-ocaml/extn.html#sec249) - and the entire thing would then be in OCaml with no special case hacks.


David