17:23 <Serpent7776> why is format_of_string's signature format6 -> format6?

17:28 <def> Serpent7776: format_of_string is just a convenience to make sure a string literal is typed as a format

17:28 <def> if you do let x = "%d" in ..., x defaults to type string

17:29 <def> rather you should do let x = format_of_string "%d" in ...

17:29 <def> as far as I known, there is no way to dynamically check a format type.

17:31 <Serpent7776> so where does the actual conversion take place? before format_of_string? is it some compiler magic?

17:31 <def> it is compiler magic.

17:32 <Serpent7776> ah

17:32 <def> it is done by the typechecker because it needs to check that the format string conforms to the arguments

17:33 <Serpent7776> so format_of_string just invokes that compiler magic?

17:33 <def> [format_of_string e] does nothing more than (e : _ format6)

17:34 <def> (the format string is actually a value of an internal gadt and you can build values of that type so in theory it is possible to implement a dynamic format checker at runtime)

17:34 <Serpent7776> so this function does nothing at runtime? wholo conversion takes place at compilation time?

17:36 <def> yes

17:36 <Serpent7776> ok, thanks for clarification, it's kinda surprising at first

17:39 <def> ("%s" : _ format) is a convenient syntax for something that looks like CamlinternalFormatBasics.(Int (Int_d, No_padding, No_precision, End_of_format))

17:39 <def> it looks like a string in the source but beside that, it doesn't have much to do with a string.

17:40 <def> (Older versions of OCaml would parse the format twice: first at compile time to check types, then at runtime to interpret the format, this was buggy)

17:45 <Serpent7776> oh, I didn't know _ can be used as type parameter placeholder

20:33 <octachron> Serpent7776, note that there is a limited form of dynamical checking of format string in https://ocaml.org/releases/4.10/htmlman/libref/Scanf.html#VALformat_from_string

20:52 <Serpent7776> does that function check that given string yields exactly given format type? Can't think of use case for that.