Module Pretty

module Pretty: sig .. end

Utility functions for pretty-printing. The major features provided by this module are

An fprintf-style interface with support for user-defined printers
The printout is fit to a width by selecting some of the optional newlines
Constructs for alignment and indentation
Print ellipsis starting at a certain nesting depth
Constructs for printing lists and arrays

Pretty-printing occurs in two stages:

Construct a Pretty.doc object that encodes all of the elements to be printed along with alignment specifiers and optional and mandatory newlines
Format the Pretty.doc to a certain width and emit it as a string, to an output stream or pass it to a user-defined function

The formatting algorithm is not optimal but it does a pretty good job while still operating in linear time. The original version was based on a pretty printer by Philip Wadler which turned out to not scale to large jobs.

API

type doc

The type of unformated documents. Elements of this type can be constructed in two ways. Either with a number of constructor shown below, or using the Pretty.dprintf function with a printf-like interface. The Pretty.dprintf method is slightly slower so we do not use it for large jobs such as the output routines for a compiler. But we use it for small jobs such as logging and error messages.

Constructors for the doc type.

val nil : doc

Constructs an empty document

val (++) : doc -> doc -> doc

Concatenates two documents. This is an infix operator that associates to the left.

val concat : doc -> doc -> doc

val text : string -> doc

A document that prints the given string

val num : int -> doc

A document that prints an integer in decimal form

val num64 : int64 -> doc

A document that prints a 64-bit int in decimal form

val real : float -> doc

A document that prints a real number

val chr : char -> doc

A document that prints a character. This is just like Pretty.text with a one-character string.

val line : doc

A document that consists of a mandatory newline. This is just like

(text
    "\n")

. The new line will be indented to the current indentation level, unless you use Pretty.leftflush right after this.

val leftflush : doc

Use after a Pretty.line to prevent the indentation. Whatever follows next will be flushed left. Indentation resumes on the next line.

val break : doc

A document that consists of either a space or a line break. Also called an optional line break. Such a break will be taken only if necessary to fit the document in a given width. If the break is not taken a space is printed instead.

val align : doc

Mark the current column as the current indentation level. Does not print anything. All taken line breaks will align to this column. The previous alignment level is saved on a stack.

val unalign : doc

Reverts to the last saved indentation level.

val mark : doc

Mark the beginning of a markup section. The width of a markup section is considered 0 for the purpose of computing identation

val unmark : doc

The end of a markup section

Syntactic sugar

val indent : int -> doc -> doc

Indents the document. Same as ((text " ") ++ align ++ doc ++ unalign), with the specified number of spaces.

val markup : doc -> doc

Prints a document as markup. The marked document cannot contain line breaks or alignment constructs.

val seq : sep:doc -> doit:('a -> doc) -> elements:'a list -> doc

Formats a sequence. sep is a separator, doit is a function that converts an element to a document.

val docList : ?sep:doc -> ('a -> doc) -> unit -> 'a list -> doc

An alternative function for printing a list. The unit argument is there to make this function more easily usable with the Pretty.dprintf interface. The first argument is a separator, by default a comma.

val d_list : string -> (unit -> 'a -> doc) -> unit -> 'a list -> doc

sm: Yet another list printer. This one accepts the same kind of printing function that Pretty.dprintf does, and itself works in the dprintf context. Also accepts a string as the separator since that's by far the most common.

val docArray : ?sep:doc ->
       (int -> 'a -> doc) -> unit -> 'a array -> doc

Formats an array. A separator and a function that prints an array element. The default separator is a comma.

val docOpt : ('a -> doc) -> unit -> 'a option -> doc

Prints an 'a option with None or Some

val d_int32 : int32 -> doc

Print an int32

val f_int32 : unit -> int32 -> doc

val d_int64 : int64 -> doc

val f_int64 : unit -> int64 -> doc

module MakeMapPrinter: functor (Map : sigtype key 

type 'a t 

val fold : (key -> 'a -> 'b -> 'b) ->
       'a t -> 'b -> 'b
end) -> sig .. end

Format maps.

module MakeSetPrinter: functor (Set : sigtype elt 

type t 

val fold : (elt -> 'a -> 'a) ->
       t -> 'a -> 'a
end) -> sig .. end

Format sets.

val insert : unit -> doc -> doc

A function that is useful with the printf-like interface

val dprintf : ('a, unit, doc, doc) format4 -> 'a

This function provides an alternative method for constructing doc objects. The first argument for this function is a format string argument (of type ('a, unit, doc) format; if you insist on understanding what that means see the module Printf). The format string is like that for the printf function in C, except that it understands a few more formatting controls, all starting with the @ character.

See the gprintf function if you want to pipe the result of dprintf into some other functions.

The following special formatting characters are understood (these do not correspond to arguments of the function):

@[ Inserts an Pretty.align. Every format string must have matching Pretty.align and Pretty.unalign.
@] Inserts an Pretty.unalign.
@! Inserts a Pretty.line. Just like "\n"
@? Inserts a Pretty.break.
@< Inserts a Pretty.mark.
@> Inserts a Pretty.unmark.
@^ Inserts a Pretty.leftflush Should be used immediately after @! or "\n".
@@ : inserts a @ character

In addition to the usual printf % formatting characters the following two new characters are supported:

%t Corresponds to an argument of type unit -> doc. This argument is invoked to produce a document
%a Corresponds to two arguments. The first of type unit -> 'a -> doc and the second of type 'a. (The extra unit is do to the peculiarities of the built-in support for format strings in Ocaml. It turns out that it is not a major problem.) Here is an example of how you use this:

dprintf "Name=%s, SSN=%7d, Children=@[%a@]\n"
             pers.name pers.ssn (docList (chr ',' ++ break) text)
             pers.children

The result of dprintf is a Pretty.doc. You can format the document and emit it using the functions Pretty.fprint and Pretty.sprint.

val gprintf : (doc -> 'a) -> ('b, unit, doc, 'a) format4 -> 'b

Like Pretty.dprintf but more general. It also takes a function that is invoked on the constructed document but before any formatting is done. The type of the format argument means that 'a is the type of the parameters of this function, unit is the type of the first argument to %a and %t formats, doc is the type of the intermediate result, and 'b is the type of the result of gprintf.

val fprint : out_channel -> width:int -> doc -> unit

Format the document to the given width and emit it to the given channel

val sprint : width:int -> doc -> string

Format the document to the given width and emit it as a string

val fprintf : out_channel -> ('a, unit, doc) format -> 'a

Like Pretty.dprintf followed by Pretty.fprint

val printf : ('a, unit, doc) format -> 'a

Like Pretty.fprintf applied to stdout

val eprintf : ('a, unit, doc) format -> 'a

Like Pretty.fprintf applied to stderr

val withPrintDepth : int -> (unit -> unit) -> unit

Invokes a thunk, with printDepth temporarily set to the specified value

The following variables can be used to control the operation of the printer

val printDepth : int ref

Specifies the nesting depth of the align/unalign pairs at which everything is replaced with ellipsis

val printIndent : bool ref

If false then does not indent

val fastMode : bool ref

If set to true then optional breaks are taken only when the document has exceeded the given width. This means that the printout will looked more ragged but it will be faster

val flushOften : bool ref

If true the it flushes after every print

val flattenBeforePrint : bool ref

Whether to rebalance doc before printing it to avoid stack-overflows

val countNewLines : int ref

Keep a running count of the taken newlines. You can read and write this from the client code if you want

val auto_printer : string -> 'a

A function that when used at top-level in a module will direct the pa_prtype module generate automatically the printing functions for a type