Module `Fmt`

Format pretty-printer combinators.

Consult naming conventions for your pretty-printers.

References

The Format module documentation.
The required reading Format module tutorial.

Standard outputs

val stdout : Stdlib.Format.formatter: stdout is the standard output formatter.

val stderr : Stdlib.Format.formatter: stderr is the standard error formatter.

Formatting

val pf : Stdlib.Format.formatter -> ('a, Stdlib.Format.formatter, unit) Stdlib.format -> 'a: pf is Format.fprintf.

val pr : ('a, Stdlib.Format.formatter, unit) Stdlib.format -> 'a: pr is pf stdout.

val epr : ('a, Stdlib.Format.formatter, unit) Stdlib.format -> 'a: epr is pf stderr.

val str : ('a, Stdlib.Format.formatter, unit, string) Stdlib.format4 -> 'a

str is Format.asprintf.

Note. When using strf utf_8 and style_renderer are always respectively set to true and `None. See also str_like.

val kpf : (Stdlib.Format.formatter -> 'a) -> Stdlib.Format.formatter -> ('b, Stdlib.Format.formatter, unit, 'a) Stdlib.format4 -> 'b: kpf is Format.kfprintf.

val kstr : (string -> 'a) -> ('b, Stdlib.Format.formatter, unit, 'a) Stdlib.format4 -> 'b: kstr is like str but continuation based.

val str_like : Stdlib.Format.formatter -> ('a, Stdlib.Format.formatter, unit, string) Stdlib.format4 -> 'a: str_like ppf is like str except its utf_8 and style_renderer settings are those of ppf.

val with_buffer : ?⁠like:Stdlib.Format.formatter -> Stdlib.Buffer.t -> Stdlib.Format.formatter: with_buffer ~like b is a formatter whose utf_8 and style_renderer settings are copied from those of like (if provided).

val failwith : ('a, Stdlib.Format.formatter, unit, 'b) Stdlib.format4 -> 'a: failwith is kstr failwith, raises Stdlib.Failure with a pretty-printed string argument.

val failwith_notrace : ('a, Stdlib.Format.formatter, unit, 'b) Stdlib.format4 -> 'a: failwith_notrace is like failwith but raises with raise_notrace.

val invalid_arg : ('a, Stdlib.Format.formatter, unit, 'b) Stdlib.format4 -> 'a: invalid_arg is kstr invalid_arg, raises Stdlib.Invalid_argument with a pretty-printed string argument.

val error : ('b, Stdlib.Format.formatter, unit, ('a, string) Stdlib.result) Stdlib.format4 -> 'b: error fmt ... is kstr (fun s -> Error s) fmt ...

val error_msg : ('b, Stdlib.Format.formatter, unit, ('a, [ `Msg of string ]) Stdlib.result) Stdlib.format4 -> 'b: error_msg fmt ... is kstr (fun s -> Error (`Msg s)) fmt ...

Formatters

type 'a t = Stdlib.Format.formatter -> 'a -> unit: The type for formatters of values of type 'a.

val flush : 'a t: flush has the effect of Format.pp_print_flush

val nop : 'a t: nop formats nothing.

val any : (unit, Stdlib.Format.formatter, unit) Stdlib.format -> 'a t: any fmt ppf v formats any value with the constant format fmt.

val using : ('a -> 'b) -> 'b t -> 'a t: using f pp ppf v ppf ppf (f v).

val const : 'a t -> 'a -> 'b t: const pp_v v always formats v using pp_v.

val fmt : ('a, Stdlib.Format.formatter, unit) Stdlib.format -> Stdlib.Format.formatter -> 'a: fmt fmt ppf is pf ppf fmt. If fmt is used with a single non-constant formatting directive, generates a value of type t.

Separators

val cut : 'a t: cut has the effect of Format.pp_print_cut.

val sp : 'a t: sp has the effect of Format.pp_print_space.

val sps : int -> 'a t: sps n has the effect of Format.pp_print_break n 0.

val comma : 'a t: comma is Fmt.any ",@ ".

val semi : 'a t: semi is Fmt.any ";@ ".

Sequencing

val append : 'a t -> 'a t -> 'a t: append pp_v0 pp_v1 ppf v is pp_v0 ppf v; pp_v1 ppf v.

val (++) : 'a t -> 'a t -> 'a t: ( ++ ) is append.

val concat : ?⁠sep:unit t -> 'a t list -> 'a t: concat ~sep pps formats a value using the formaters pps and separting each format with sep (defaults to cut).

val iter : ?⁠sep:unit t -> (('a -> unit) -> 'b -> unit) -> 'a t -> 'b t: iter ~sep iter pp_elt formats the iterations of iter over a value using pp_elt. Iterations are separated by sep (defaults to cut).

val iter_bindings : ?⁠sep:unit t -> (('a -> 'b -> unit) -> 'c -> unit) -> ('a * 'b) t -> 'c t: iter_bindings ~sep iter pp_binding formats the iterations of iter over a value using pp_binding. Iterations are separated by sep (defaults to cut).

Boxes

val box : ?⁠indent:int -> 'a t -> 'a t: box ~indent pp ppf wraps pp in a pretty-printing box. The box tries to print as much as possible on every line, while emphasizing the box structure (see Format.pp_open_box). Break hints that lead to a new line add indent to the current indentation (defaults to 0).

val hbox : 'a t -> 'a t: hbox is like box but is a horizontal box: the line is not split in this box (but may be in sub-boxes). See Format.pp_open_hbox.

val vbox : ?⁠indent:int -> 'a t -> 'a t: vbox is like box but is a vertical box: every break hint leads to a new line which adds indent to the current indentation (defaults to 0). See Format.pp_open_vbox.

val hvbox : ?⁠indent:int -> 'a t -> 'a t: hvbox is like hbox if it fits on a single line, or like vbox otherwise. See Format.pp_open_hvbox.

val hovbox : ?⁠indent:int -> 'a t -> 'a t: hovbox is a condensed box. See Format.pp_open_hovbox.

Brackets

val parens : 'a t -> 'a t: parens pp_v ppf is pf "@[<1>(%a)@]" pp_v.

val brackets : 'a t -> 'a t: brackets pp_v ppf is pf "@[<1>[%a]@]" pp_v.

val braces : 'a t -> 'a t: braces pp_v ppf is pf "@[<1>{%a}@]" pp_v.

val quote : ?⁠mark:string -> 'a t -> 'a t: quote ~mark pp_v ppf is pf "@[<1>@<1>%s%a@<1>%s@]" mark pp_v mark, mark defaults to "\"", it is always counted as spanning as single column (this allows for UTF-8 encoded marks).

Records

val id : 'a -> 'a: id is Fun.id.

val field : ?⁠label:string t -> ?⁠sep:unit t -> string -> ('b -> 'a) -> 'a t -> 'b t: field ~label ~sep l prj pp_v pretty prints a labelled field value as pf "@[<1>%a%a%a@]" label l sep () (using prj pp_v). label defaults to styled `Yellow string and sep to any ":@ ".

val record : ?⁠sep:unit t -> 'a t list -> 'a t: record ~sep fields pretty-prints a value using the concatenation of fields, separated by sep (defaults to cut) and framed in a vertical box.

Stdlib types

Formatters for structures give full control to the client over the formatting process and do not wrap the formatted structures with boxes. Use the Dump module to quickly format values for inspection.

val bool : bool t: bool is Format.pp_print_bool.

val int : int t: int is pf ppf "%d".

val nativeint : nativeint t: nativeint ppf is pf ppf "%nd".

val int32 : int32 t: int32 ppf is pf ppf "%ld".

val int64 : int64 t: int64 ppf is pf ppf "%Ld".

val uint : int t: uint ppf is pf ppf "%u".

val unativeint : nativeint t: unativeint ppf is pf ppf "%nu".

val uint32 : int32 t: uint32 ppf is pf ppf "%lu".

val uint64 : int64 t: uint64 ppf is pf ppf "%Lu".

val float : float t: float ppf is pf ppf "%g".

val float_dfrac : int -> float t: float_dfrac d rounds the float to the dth decimal fractional digit and formats the result with "%g". Ties are rounded towards positive infinity. The result is only defined for 0 <= d <= 16.

val float_dsig : int -> float t

float_dsig d rounds the normalized decimal significand of the float to the dth decimal fractional digit and formats the result with "%g". Ties are rounded towards positive infinity. The result is NaN on infinities and only defined for 0 <= d <= 16.

Warning. The current implementation overflows on large d and floats.

val char : char t: char is Format.pp_print_char.

val string : string t: string is Format.pp_print_string.

val buffer : Stdlib.Buffer.t t: buffer formats a Buffer.t value's current contents.

val exn : exn t: exn formats an exception.

val exn_backtrace : (exn * Stdlib.Printexc.raw_backtrace) t: exn_backtrace formats an exception backtrace.

val pair : ?⁠sep:unit t -> 'a t -> 'b t -> ('a * 'b) t: pair ~sep pp_fst pp_snd formats a pair. The first and second projection are formatted using pp_fst and pp_snd and are separated by sep (defaults to cut).

val option : ?⁠none:unit t -> 'a t -> 'a option t: option ~none pp_v formats an optional value. The Some case uses pp_v and None uses none (defaults to nop).

val result : ok:'a t -> error:'b t -> ('a, 'b) Stdlib.result t: result ~ok ~error formats a result value using ok for the Ok case and error for the Error case.

val list : ?⁠sep:unit t -> 'a t -> 'a list t: list sep pp_v formats list elements. Each element of the list is formatted in order with pp_v. Elements are separated by sep (defaults to cut). If the list is empty, this is nop.

val array : ?⁠sep:unit t -> 'a t -> 'a array t: array sep pp_v formats array elements. Each element of the array is formatted in order with pp_v. Elements are separated by sep (defaults to cut). If the array is empty, this is nop.

val seq : ?⁠sep:unit t -> 'a t -> 'a Stdlib.Seq.t t: seq sep pp_v formats sequence elements. Each element of the sequence is formatted in order with pp_v. Elements are separated by sep (defaults to cut). If the sequence is empty, this is nop.

val hashtbl : ?⁠sep:unit t -> ('a * 'b) t -> ('a, 'b) Stdlib.Hashtbl.t t: hashtbl ~sep pp_binding formats the bindings of a hash table. Each binding is formatted with pp_binding and bindings are separated by sep (defaults to cut). If the hash table has multiple bindings for a given key, all bindings are formatted, with the most recent binding first. If the hash table is empty, this is nop.

val queue : ?⁠sep:unit t -> 'a t -> 'a Stdlib.Queue.t t: queue ~sep pp_v formats queue elements. Each element of the queue is formatted in least recently added order with pp_v. Elements are separated by sep (defaults to cut). If the queue is empty, this is nop.

val stack : ?⁠sep:unit t -> 'a t -> 'a Stdlib.Stack.t t: stack ~sep pp_v formats stack elements. Each element of the stack is formatted from top to bottom order with pp_v. Elements are separated by sep (defaults to cut). If the stack is empty, this is nop.

module Dump : sig ... end: Formatters for inspecting OCaml values.

Magnitudes

val si_size : scale:int -> string -> int t

si_size ~scale unit formats a non negative integer representing unit unit at scale 10^{scale * 3}, depending on its magnitude, using power of 3 SI prefixes (i.e. all of them except deca, hector, deci and centi). Only US-ASCII characters are used, µ (10^-6) is written using u.

scale indicates the scale 10^{scale * 3} an integer represents, for example -1 for munit (10^-3), 0 for unit (10⁰), 1 for kunit (10³); it must be in the range [-8;8] or Invalid_argument is raised.

Except at the maximal yotta scale always tries to show three digits of data with trailing fractional zeros omited. Rounds towards positive infinity (over approximates).

val byte_size : int t: byte_size is si_size ~scale:0 "B".

val bi_byte_size : int t: bi_byte_size formats a byte size according to its magnitude using binary prefixes up to pebi bytes (2¹⁵).

val uint64_ns_span : int64 t: uint64_ns_span formats an unsigned nanosecond time span according to its magnitude using SI prefixes on seconds and accepted non-SI units. Years are counted in Julian years (365.25 SI-accepted days) as defined by the International Astronomical Union (IAU). Only US-ASCII characters are used (us is used for µs).

Binary data

type 'a vec = int * (int -> 'a): The type for random addressable, sized sequences. Each (n, f) represents the sequence f 0, ..., f (n - 1).

val on_bytes : char vec t -> bytes t: on_bytes pp is pp adapted to format (entire) bytes.

val on_string : char vec t -> string t: on_string pp is pp adapted to format (entire) strings.

val ascii : ?⁠w:int -> ?⁠subst:unit t -> unit -> char vec t: ascii ~w ~subst () formats character sequences by printing characters in the printable US-ASCII range ([0x20];[0x7E]) as is, and replacing the rest with subst (defaults to fmt "."). w causes the output to be right padded to the size of formatting at least w sequence elements (defaults to 0).

val octets : ?⁠w:int -> ?⁠sep:unit t -> unit -> char vec t: octets ~w ~sep () formats character sequences as hexadecimal digits. It prints groups of successive characters of unspecified length together, separated by sep (defaults to sp). w causes the output to be right padded to the size of formatting at least w sequence elements (defaults to 0).

val addresses : ?⁠addr:int t -> ?⁠w:int -> 'a vec t -> 'a vec t: addresses pp formats sequences by applying pp to consecutive subsequences of length w (defaults to 16). addr formats subsequence offsets (defaults to an unspecified hexadecimal format).

val hex : ?⁠w:int -> unit -> char vec t: hex ~w () formats character sequences as traditional hex dumps, matching the output of xxd and forcing line breaks after every w characters (defaults to 16).

Words, paragraphs, text and lines

Note. These functions only work on US-ASCII strings and/or with newlines ('\n'). If you are dealing with UTF-8 strings or different kinds of line endings you should use the pretty-printers from Uuseg_string.

White space. White space is one of the following US-ASCII characters: space ' ' (0x20), tab '\t' (0x09), newline '\n' (0x0A), vertical tab (0x0B), form feed (0x0C), carriage return '\r' (0x0D).

val words : string t: words formats words by suppressing initial and trailing white space and replacing consecutive white space with a single Format.pp_print_space.

val paragraphs : string t: paragraphs formats paragraphs by suppressing initial and trailing spaces and newlines, replacing blank lines (a line made only of white space) by a two Format.pp_force_newline and remaining consecutive white space with a single Format.pp_print_space.

val text : string t: text formats text by respectively replacing spaces and newlines in the string with Format.pp_print_space and Format.pp_force_newline.

val lines : string t: lines formats lines by replacing newlines ('\n') in the string with calls to Format.pp_force_newline.

val truncated : max:int -> string t: truncated ~max formats a string using at most max characters. If the string doesn't fit, it is truncated and ended with three consecutive dots which do count towards max.

val text_loc : ((int * int) * (int * int)) t: text_loc formats a line-column text range according to GNU conventions.

HCI fragments

val one_of : ?⁠empty:unit t -> 'a t -> 'a list t

one_of ~empty pp_v ppf l formats according to the length of l

0, formats empty (defaults to nop).
1, formats the element with pp_v.
2, formats "either %a or %a" with the list elements
n, formats "one of %a, ... or %a" with the list elements

val did_you_mean : ?⁠pre:unit t -> ?⁠post:unit t -> kind:string -> 'a t -> ('a * 'a list) t

did_you_mean ~pre kind ~post pp_v formats a faulty value v of kind kind and a list of hints that v could have been mistaken for.

pre defaults to unit "Unknown", post to nop they surround the faulty value before the "did you mean" part as follows "%a %s %a%a." pre () kind pp_v v post (). If hints is empty no "did you mean" part is printed.

Conditional UTF-8 formatting

Note. Since Format is not UTF-8 aware using UTF-8 output may derail the pretty printing process. Use the pretty-printers from Uuseg_string if you are serious about UTF-8 formatting.

val if_utf_8 : 'a t -> 'a t -> 'a t

if_utf_8 pp_u pp ppf v is:

pp_u ppf v if utf_8 ppf is true.
pp ppf v otherwise.

val utf_8 : Stdlib.Format.formatter -> bool: utf_8 ppf is true if UTF-8 output is enabled on ppf. If set_utf_8 hasn't been called on ppf this is true.

val set_utf_8 : Stdlib.Format.formatter -> bool -> unit

set_utf_8 ppf b enables or disables conditional UTF-8 formatting on ppf.

raises Invalid_argument: if ppf is Format.str_formatter: it is is always UTF-8 enabled.

Styled formatting

type color = [ | `Black | `Blue | `Cyan | `Green | `Magenta | `Red | `White | `Yellow ]: The type for colors.

The type for styles:

`None resets the styling.
`Bold, `Faint, `Italic, `Underline and `Reverse are display attributes.
`Fg _ is the foreground color or high-intensity color on `Hi _.
`Bg _ is the foreground color or high-intensity color on `Hi _.
#color is the foreground colour, deprecated use `Fg #color instead.

val styled : style -> 'a t -> 'a t: styled s pp formats like pp but styled with s.

Style rendering control

type style_renderer = [ | `Ansi_tty | `None ]

The type for style renderers.

`Ansi_tty, renders styles using ANSI escape sequences.
`None, styled rendering has no effect.

val style_renderer : Stdlib.Format.formatter -> style_renderer: style_renderer ppf is the style renderer used by ppf. If set_style_renderer has never been called on ppf this is `None.

val set_style_renderer : Stdlib.Format.formatter -> style_renderer -> unit

set_style_renderer ppf r sets the style renderer of ppf to r.

raises Invalid_argument: if ppf is Format.str_formatter: its renderer is always `None.

Converting with string value converters

val of_to_string : ('a -> string) -> 'a t: of_to_string f ppf v is string ppf (f v).

val to_to_string : 'a t -> 'a -> string: to_to_string pp_v v is strf "%a" pp_v v.

Deprecated

val strf : ('a, Stdlib.Format.formatter, unit, string) Stdlib.format4 -> 'a

deprecated: use str instead.

val kstrf : (string -> 'a) -> ('b, Stdlib.Format.formatter, unit, 'a) Stdlib.format4 -> 'b

deprecated: use kstr instead.

val strf_like : Stdlib.Format.formatter -> ('a, Stdlib.Format.formatter, unit, string) Stdlib.format4 -> 'a

deprecated: use str_like instead.

val always : (unit, Stdlib.Format.formatter, unit) Stdlib.format -> 'a t

deprecated: use any instead.

val unit : (unit, Stdlib.Format.formatter, unit) Stdlib.format -> unit t

deprecated: use any.

val prefix : unit t -> 'a t -> 'a t

deprecated: use ( ++ ).

val suffix : unit t -> 'a t -> 'a t

deprecated: use ( ++ ).

val styled_unit : style -> (unit, Stdlib.Format.formatter, unit) Stdlib.format -> unit t

deprecated: , use styled s (any fmt) instead

Naming conventions

Given a type ty use:

pp_ty for a pretty printer that provides full control to the client and does not wrap the formatted value in an enclosing box. See these examples.
pp_dump_ty for a pretty printer that provides little control over the pretty-printing process, wraps the rendering in an enclosing box and tries as much as possible to respect the OCaml syntax. These pretty-printers should make it easy to inspect and understand values of the given type, they are mainly used for quick printf debugging and/or toplevel interaction. See these examples.

If you are in a situation where making a difference between dump_ty and pp_ty doesn't make sense then use pp_ty.

For a type ty that is the main type of the module (the "M.t" convention) drop the suffix, that is simply use M.pp and M.pp_dump.