Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!
If you're targeting JavaScript, the API docs for BuckleScript includes all of below, plus JS-specific APIs.
sig val is_in_char_set : CamlinternalFormatBasics.char_set -> char -> bool val rev_char_set : CamlinternalFormatBasics.char_set -> CamlinternalFormatBasics.char_set type mutable_char_set = bytes val create_char_set : unit -> CamlinternalFormat.mutable_char_set val add_in_char_set : CamlinternalFormat.mutable_char_set -> char -> unit val freeze_char_set : CamlinternalFormat.mutable_char_set -> CamlinternalFormatBasics.char_set type ('a, 'b, 'c, 'd, 'e, 'f) param_format_ebb = Param_format_EBB : ('x -> 'a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmt -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormat.param_format_ebb val param_format_of_ignored_format : ('a, 'b, 'c, 'd, 'y, 'x) CamlinternalFormatBasics.ignored -> ('x, 'b, 'c, 'y, 'e, 'f) CamlinternalFormatBasics.fmt -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormat.param_format_ebb type ('b, 'c) acc_formatting_gen = Acc_open_tag of ('b, 'c) CamlinternalFormat.acc | Acc_open_box of ('b, 'c) CamlinternalFormat.acc and ('b, 'c) acc = Acc_formatting_lit of ('b, 'c) CamlinternalFormat.acc * CamlinternalFormatBasics.formatting_lit | Acc_formatting_gen of ('b, 'c) CamlinternalFormat.acc * ('b, 'c) CamlinternalFormat.acc_formatting_gen | Acc_string_literal of ('b, 'c) CamlinternalFormat.acc * string | Acc_char_literal of ('b, 'c) CamlinternalFormat.acc * char | Acc_data_string of ('b, 'c) CamlinternalFormat.acc * string | Acc_data_char of ('b, 'c) CamlinternalFormat.acc * char | Acc_delay of ('b, 'c) CamlinternalFormat.acc * ('b -> 'c) | Acc_flush of ('b, 'c) CamlinternalFormat.acc | Acc_invalid_arg of ('b, 'c) CamlinternalFormat.acc * string | End_of_acc type ('a, 'b) heter_list = Cons : 'c * ('a, 'b) CamlinternalFormat.heter_list -> ('c -> 'a, 'b) CamlinternalFormat.heter_list | Nil : ('b, 'b) CamlinternalFormat.heter_list type ('b, 'c, 'e, 'f) fmt_ebb = Fmt_EBB : ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmt -> ('b, 'c, 'e, 'f) CamlinternalFormat.fmt_ebb val make_printf : ('b -> ('b, 'c) CamlinternalFormat.acc -> 'd) -> 'b -> ('b, 'c) CamlinternalFormat.acc -> ('a, 'b, 'c, 'c, 'c, 'd) CamlinternalFormatBasics.fmt -> 'a val output_acc : Pervasives.out_channel -> (Pervasives.out_channel, unit) CamlinternalFormat.acc -> unit val bufput_acc : Buffer.t -> (Buffer.t, unit) CamlinternalFormat.acc -> unit val strput_acc : Buffer.t -> (unit, string) CamlinternalFormat.acc -> unit val type_format : ('x, 'b, 'c, 't, 'u, 'v) CamlinternalFormatBasics.fmt -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmtty -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmt val fmt_ebb_of_string : ?legacy_behavior:bool -> string -> ('b, 'c, 'e, 'f) CamlinternalFormat.fmt_ebb val format_of_string_fmtty : string -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmtty -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.format6 val format_of_string_format : string -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.format6 -> ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.format6 val char_of_iconv : CamlinternalFormatBasics.int_conv -> char val string_of_formatting_lit : CamlinternalFormatBasics.formatting_lit -> string val string_of_formatting_gen : ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.formatting_gen -> string val string_of_fmtty : ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmtty -> string val string_of_fmt : ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.fmt -> string val open_box_of_string : string -> int * CamlinternalFormatBasics.block_type val symm : ('a1, 'b1, 'c1, 'd1, 'e1, 'f1, 'a2, 'b2, 'c2, 'd2, 'e2, 'f2) CamlinternalFormatBasics.fmtty_rel -> ('a2, 'b2, 'c2, 'd2, 'e2, 'f2, 'a1, 'b1, 'c1, 'd1, 'e1, 'f1) CamlinternalFormatBasics.fmtty_rel val trans : ('a1, 'b1, 'c1, 'd1, 'e1, 'f1, 'a2, 'b2, 'c2, 'd2, 'e2, 'f2) CamlinternalFormatBasics.fmtty_rel -> ('a2, 'b2, 'c2, 'd2, 'e2, 'f2, 'a3, 'b3, 'c3, 'd3, 'e3, 'f3) CamlinternalFormatBasics.fmtty_rel -> ('a1, 'b1, 'c1, 'd1, 'e1, 'f1, 'a3, 'b3, 'c3, 'd3, 'e3, 'f3) CamlinternalFormatBasics.fmtty_rel val recast : ('a1, 'b1, 'c1, 'd1, 'e1, 'f1) CamlinternalFormatBasics.fmt -> ('a1, 'b1, 'c1, 'd1, 'e1, 'f1, 'a2, 'b2, 'c2, 'd2, 'e2, 'f2) CamlinternalFormatBasics.fmtty_rel -> ('a2, 'b2, 'c2, 'd2, 'e2, 'f2) CamlinternalFormatBasics.fmt end