The 6 parameters of (’a, ’b, ’c, ’d, ’e, ’f) format6

The infamous format6 type is the basis of the hackish but damn convenient, type-safe way in which OCaml handles format strings:

Printf.printf "%d) %s -> %f\n"
  3 "foo" 5.12

The first argument of printf in this example is not a string, but a format string (they share the same literal syntax, but have different type, and there is a small hack in the type-inference engine to make this possible). It's type, which you can get by asking ("%d) %s -> %f\n" : (_,_,_,_,_,_) format6) in a toplevel, is a mouthful (and I renamed the variables for better readability):

(int -> string -> float -> 'f, 'b, 'c, 'd, 'd, 'f) format6

What do those six arcane parameters mean? In the process of reviewing Benoît Vaugon's work on using GADTs to re-implement format functions, I've finally felt the need (after years of delighted oblivion) to understand each of those parameters. And that came after several days of hacking on easier-to-understand ten-parameters GADT functions; hopefully most of our readers will never need this information, but the probability that I need to refresh my memory in the future is strong enough for me to write this blog post.

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