an edited version of simple_example.ml which shows the formatter break at width 68

breaking_margin.re

/*
   A fairly complete demonstration of the features provided
   by Easy-format.
 */

open Easy_format;

let list = {
  ...list,
  list_style: Some "list",
  opening_style: Some "op",
  body_style: Some "body",
  separator_style: Some "sep",
  closing_style: Some "cl"
};

let atom = {atom_style: Some "atom"};

let label = {...label, label_style: Some "label"};

let tuple_param = {
  ...list,
  space_after_opening: false,
  space_before_closing: false,
  align_closing: false
};

let operator_param = {
  ...list,
  space_after_opening: false,
  space_before_closing: false,
  separators_stick_left: false,
  space_before_separator: true,
  space_after_separator: true,
  align_closing: true
};

let html_escape_string s => {
  let buf = Buffer.create (2 * String.length s);
  for i in 0 to (String.length s - 1) {
    switch s.[i] {
    | '&' => Buffer.add_string buf "&"
    | '<' => Buffer.add_string buf "&lt;"
    | '>' => Buffer.add_string buf "&gt;"
    | c => Buffer.add_char buf c
    }
  };
  Buffer.contents buf
};

let html_escape = `Escape_string html_escape_string;

let html_style = [
  ("atom", {tag_open: "<a>", tag_close: "</a>"}),
  ("body", {tag_open: "<lb>", tag_close: "</lb>"}),
  ("list", {tag_open: "<l>", tag_close: "</l>"}),
  ("op", {tag_open: "<op>", tag_close: "</op>"}),
  ("cl", {tag_open: "<cl>", tag_close: "</cl>"}),
  ("sep", {tag_open: "<sep>", tag_close: "</sep>"}),
  ("label", {tag_open: "<la>", tag_close: "</la>"})
];

let format_tuple f l => List ("(", ",", ")", tuple_param) (List.map f l) [@implicit_arity];

let format_int x => Atom (string_of_int x) atom [@implicit_arity];

let format_float x => Atom (Printf.sprintf "%.5f" x) atom [@implicit_arity];

let format_sum wrap::wrap=`Wrap_atoms l =>
  List ("(", "+", ")", {...operator_param, wrap_body: wrap}) (List.map format_int l)
  [@implicit_arity];

let format_array align_closing::align_closing wrap::wrap f a => {
  let l = Array.to_list (Array.map f a);
  List ("[|", ";", "|]", {...list, align_closing, wrap_body: wrap}) l [@implicit_arity]
};

let format_matrix
    align_closing1::align_closing1=true
    align_closing2::align_closing2=true
    wrap1::wrap1=`Wrap_atoms
    wrap2::wrap2=`Wrap_atoms
    m =>
  format_array
    align_closing::align_closing1
    wrap::wrap1
    (format_array align_closing::align_closing2 wrap::wrap2 format_float)
    m;

let format_record f l0 => {
  let l =
    List.map
      (fun (s, x) => Label (Atom (s ^ ":") atom [@implicit_arity], label) (f x) [@implicit_arity])
      l0;
  List ("{", ";", "}", list) l [@implicit_arity]
};

let begin_style = (
  {...label, indent_after_label: 0},
  ("begin", ";", "end", {...list, stick_to_label: false})
);

let curly_style = (label, ("{", ";", "}", list));

let format_function_definition (body_label, body_param) name param body =>
  Label
    (
      Label
        (Atom ("function " ^ name) atom [@implicit_arity], label)
        (
          List
            ("(", ",", ")", tuple_param) (List.map (fun s => Atom s atom [@implicit_arity]) param)
          [@implicit_arity]
        )
      [@implicit_arity],
      body_label
    )
    (List body_param (List.map (fun s => Atom s atom [@implicit_arity]) body) [@implicit_arity])
  [@implicit_arity];

/*
    Illustrate the difference between `Force_break and `Force_breaks_rec on
    labels.
 */
let label_one_atom = Atom "reallyLongLabelOne" atom [@implicit_arity];

let label_two_atom = Atom "reallyLongLabelTwo" atom [@implicit_arity];

let label_three_atom = Atom "reallyLongLabelABC" atom [@implicit_arity];

let make_list_in_labels wrap =>
  Label
    (label_one_atom, label)
    (
      Label
        (label_two_atom, label)
        (
          Label
            (label_three_atom, label)
            (
              List
                ("[", ",", "]", {...list, wrap_body: wrap})
                [
                  Atom "1.23456" atom [@implicit_arity],
                  Atom "9.87654" atom [@implicit_arity],
                  Atom "9.87654" atom [@implicit_arity],
                  Atom "9.87654" atom [@implicit_arity]
                ]
              [@implicit_arity]
            )
          [@implicit_arity]
        )
      [@implicit_arity]
    )
  [@implicit_arity];

/*
    Illustrate the difference between `Force_break and `Force_breaks_rec
 */
let make_heterogenous_list (container_wrap, wrap) =>
  List
    ("[", ",", "]", {...list, wrap_body: container_wrap})
    [
      Atom "0" atom [@implicit_arity],
      List
        ("[", ",", "]", {...list, wrap_body: wrap})
        [Atom "1.23456" atom [@implicit_arity], Atom "9.87654" atom [@implicit_arity]]
      [@implicit_arity],
      Atom "1" atom [@implicit_arity],
      Atom "2" atom [@implicit_arity],
      Atom "3" atom [@implicit_arity]
    ]
  [@implicit_arity];

let print_margin fmt () => {
  let margin = Format.pp_get_margin fmt ();
  for i in 1 to margin {
    print_char '+'
  };
  print_newline ()
};

let with_margin html::html=false margin f x => {
  let fmt = Format.formatter_of_out_channel stdout;
  Format.pp_set_margin fmt margin;
  if html {
    Pretty.define_styles fmt html_escape html_style
  };
  print_margin fmt ();
  f fmt x;
  Format.pp_print_flush fmt ();
  print_newline ()
};

let print s => Printf.printf "\n*** %s ***\n%!" s;

let print_tuple fmt l => Pretty.to_formatter fmt (format_tuple format_int l);

let print_heterogenous_list fmt wrap => Pretty.to_formatter fmt (make_heterogenous_list wrap);

let print_list_in_labels fmt wrap => Pretty.to_formatter fmt (make_list_in_labels wrap);

let print_sum wrap::wrap=? fmt l => Pretty.to_formatter fmt (format_sum wrap::?wrap l);

let print_matrix
    align_closing1::align_closing1=?
    align_closing2::align_closing2=?
    wrap1::wrap1=?
    wrap2::wrap2=?
    m
    fmt
    () =>
  Pretty.to_formatter
    fmt
    (
      format_matrix
        align_closing1::?align_closing1
        align_closing2::?align_closing2
        wrap1::?wrap1
        wrap2::?wrap2
        m
    );

let print_function_definition style name param fmt body =>
  Pretty.to_formatter fmt (format_function_definition style name param body);

let () = {
  let ints = Array.to_list (Array.init 10 (fun i => i));
  let program html margin style =>
for i in 67 to 69 {
    print_int i;
    print_string "\n";
        with_margin
          html::html
          i
          (print_function_definition begin_style "hello" ["arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg1", "arg2", "arg3"])

          ["print \"hello\"", "return (1 < 2)"]};
  program false 599 begin_style;
};