refactor: add mli to our fmt module

lib/core.ml

@@ -484,6 +484,7 @@ module View = struct
 end
 
 let rec pp fmt t =
+  let open Format in
   let open Fmt in
   let var s re = sexp fmt s pp re in
   let seq s rel = sexp fmt s (list pp) rel in

lib/fmt.mli

@@ -0,0 +1,12 @@
+type formatter := Format.formatter
+type 'a t = formatter -> 'a -> unit
+
+val pp_str_list : string list t
+val sexp : formatter -> string -> 'a t -> 'a -> unit
+val str : string t
+val optint : int option t
+val int : int t
+val pair : 'a t -> 'b t -> ('a * 'b) t
+val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t
+val list : ?pp_sep:unit t -> 'a t -> 'a list t
+val to_to_string : 'a t -> 'a -> string

lib/group.ml

@@ -50,6 +50,7 @@ let pp fmt t =
     let strs = all t in
     Array.to_list (Array.init (Array.length strs) (fun i -> strs.(i), offsets.(i)))
   in
+  let open Format in
   let open Fmt in
   let pp_match fmt (str, (start, stop)) = fprintf fmt "@[(%s (%d %d))@]" str start stop in
   sexp fmt "Group" (list pp_match) matches