Pprintf

This chapter describes "pprintf", a statement to pretty print data. It looks like the "sprintf" function of the OCaml library, and borrows some ideas of the Format OCaml library. Another statement, "lprintf", is a slightly different version of "pprintf" handling with locations.

  1. Syntax of the pprintf statement
  2. Semantics of pprintf
  3. Comparison with the OCaml modules Printf and Format
  4. Relation with the Camlp5 extensible printers
  5. The Pprintf module

Syntax of the pprintf statement

The "pprintf" statement is added by the parsing kit "pa_pprintf.cmo".

Notice that, in opposition to "printf", "fprintf", "sprintf", and all its variants, which are functions, this "pprintf" is a statement, not a function: "pprintf" is a keyword and the expander analyzes its string format parameter to generate specific statements. In particular, it cannot be used alone and has no type by itself.

        expression ::= pprintf-statement
 pprintf-statement ::= "pprintf" qualid format expressions
            qualid ::= qualid "." qualid
                     | uident
                     | lident
            format ::= string
       expressions ::= expression expressions
                     | <nothing>

Semantics of pprintf

The "pprintf" statement converts the format string into a string like the "sprintf" of the OCaml library "Printf" does (see the OCaml manual for details). The string format can accept new conversion specifications, "%p" and "%q", and some pretty printing annotations, starting with "@" like in the OCaml library "Format".

The "pprintf" statement takes as first parameter, a value of type "pr_context" defined below. Its second parameter is the extended format string. It can take other parameters, depending on the format, like "sprintf".

The result of "pprintf" is always a string. There is no versions applying to files or buffers.

The strings built by "pprintf" are concatened by the function "Pretty.sprintf" (see the chapter entitled "Pretty Print") which controls the line length and prevents overflowing.

Printing context

The "pprintf" statement takes, as first parameter, a printing context. It is a value of the following type:

  type pr_context =
    { ind : int;
      bef : string;
      aft : string;
      dang : string };

The fields are:

Basically, the "pprintf" statement concats the "bef" string, the formatted string and the "aft" string. The example:

  pprintf pc "hello world"

is equivalent to (and indeed generates):

  Pretty.sprintf "%shello world%s" pc.bef pc.aft

But if the format string contains conversion specifications "%p" or "%q", the "bef" and "aft" strings are actually transmitted to the corresponding functions:

  pprintf pc "hello %p world" f x

is equivalent to:

  f {(pc) with
     bef = Pretty.sprintf "%shello " pc.bef;
     aft = Pretty.sprintf " world%s" pc.aft}
    x

Thus, the decision of including the "bef" and the "aft" strings are delayed to the called function, allowing this function to possibly concatenate "bef" and "aft" to its own strings.

A typical case is, while printing programs, when an expression needs to be printed between parentheses. The code which does that looks like:

  pprintf pc "(%p)" expr e

The right parenthesis of this string format is included in the "aft" string transmitted to the function "expr". In a situation when several right parentheses are concatened this way, the fact that all these parentheses are grouped together allows the function which eventually print them to decide to break the line or not, these parentheses being taken into account in the line length.

For example, if the code contains a print of an program containing an application whose source is:

  myfunction myarg

and if the "aft" contains "))))))", the decision of printing in one line as:

  myfunction myarg))))))

or in two lines as:

  myfunction
    myarg))))))

is exact, the right parentheses being added to "myarg" to determine whether the line overflows or not.

Extended format

The extended format used by "pprintf" may contain any strings and conversion specifications allowed by the "sprintf" function (see module "Printf" of the OCaml library), plus:

The format string is applied like in the "sprintf" function. Specific actions are done for the extended features. The result is a string like for the "sprintf" function. The "string before" and "string after" defined by the fields "bef" and "aft" of the printing context are taken into account and it is not necessary to add them in the format.

Example:

  pprintf pc "hello, world"

generates:

  Pretty.sprintf "%shello, world%s" pc.bef pc.aft;

An empty format:

  pprintf pc "";

just prints the "before" and "after" strings:

  Pretty.sprintf "%s%s" pc.bef pc.aft;

Line length

The function "pprintf" uses the Camlp5 "Pretty" module. The line length can be set by changing the value of the reference "Pretty.line_length".

The conversion specifications "p" and "q"

The "%p" conversion specification works like the "%a" of the printf statement. It takes two arguments and applies the first one to the printing context and to the second argument. The first argument must therefore have type "pr_context -> t -> unit" (for some type "t") and the second one "t".

Notice that this function can be called twice: one to test whether the resulting string holds in the line, and another one to possibly recall this function to print it in several lines. In the two cases, the printing context given as parameter is different.

It uses the functions defined in the "Pretty" module.

Example: the following statement:

  pprintf pc "hello, %p, world" f x

is equivalent to:

  f {(pc) with
     bef = Pretty.sprintf "%shello, " pc.bef;
     aft = Pretty.sprintf ", world%s" pc.aft}
    x

The "%q" conversion specification is like "%p" except that it takes a third argument which is the value of the "dang" field, useful when the syntax has "dangling" problems requiring parentheses. See chapter Extensions of printing for more explanations about dangling problems.

The same example with "%q":

  pprintf pc "hello, %q, world" f x "abc"

is equivalent to:

  f {(pc) with
     bef = Pretty.sprintf "%shello, " pc.bef;
     aft = Pretty.sprintf ", world%s" pc.aft;
     dang = "abc"}
    x

The pretty printing annotations

Breaks

The pretty printing annotations allow to indicate places where lines can be broken. They all start with the "at" sign "@". The main ones are called breaks and are:

Example - where "pc" is a variable of type "pr_context" (for example "Pprintf.empty_pc"):

  pprintf pc "hello,@;world"

builds the string, if it holds in the line:

  hello, world

and if it does not:

  hello,
    world

The second form:

  pprintf pc "hello,@ world"

is printed the same way, if it holds in the line, and if it does not, as:

  hello,
  world

The general form is:

The break "@;" is therefore equivalent to "@;<1 2>" and "" is equivalent to "@;<1 0>".

Parentheses

A second form of the pretty printing annotations is the parenthesization of format strings possibly containing other pretty printing annotations. They start with "@[" and end with "@]".

It allows to change the associativity of the breaks. For example:

  pprintf pc "@[the quick brown fox@;jumps@]@;over the lazy dog"

If the whole string holds on the line, it is printed:

  the quick brown fox jumps over the lazy dog

If the whole string does not hold on the line, but "the quick brow fox jumps" does, it is printed:

  the quick brown fox jumps
    over the lazy dog

If the string "the quick brown fox jumps" does not hold on the line, the whole string is printed:

  the quick brown fox
    jumps
    over the lazy dog

Conversely, if the code is right associated:

  pprintf pc "the quick brown fox@;@[jumps@;over the lazy dog@]"

It can be printed:

  the quick brown fox jumps over the lazy dog

or:

  the quick brown fox
    jumps over the lazy dog

or:

  the quick brown fox
    jumps
      over the lazy dog

The default is left associativity: without parentheses, it is printed like in the first example.

Incrementation of indentation

The open parenthesis of the parenthesized form, "@[" can be followed by "<n>" where "n" is a number. It increments the current indentation (for possible newlines in the parenthesized text) with this number.

Example:

  pprintf pc "@[<4>Incrementation@;actually of six characters@]"

makes the string (if not holding in the line):

  Incrementation
        actually of six characters

Break all or nothing

The open parenthesis of the parenthesized form, "@[" can be followed by "<a>". It specifies that if the string does not hold in the line, all breaks between the parentheses (at one only level) are printed in two lines, even if sub-strings could hold on the line. For example:

  pprintf pc "@[<a>the quick brown fox@;jumps@;over the lazy dog@]"

can be printed only as:

  the quick brown fox jumps over the lazy dog

or as:

  the quick brown fox
    jumps
    over the lazy dog

Break all

The open parenthesis of the parenthesized form, "@[" can be followed by "<b>". It specifies that all breaks are always printed in two lines. For example:

  pprintf pc "@[<b>the quick brown fox@;jumps@;over the lazy dog@]"

is printed in all circumstances:

  the quick brown fox
    jumps
    over the lazy dog

Break all if

The open parenthesis of the parenthesized form, "@[" can be followed by "<i>". Depending on the value of the boolean variable of the argument list, the breaks are all printed in two lines like with the "break all" option above, or not. For example:

  pprintf pc "%s@;@[<i>%s,@;%s@]" "good" True "morning" "everybody";
  pprintf pc "%s@;@[<i>%s,@;%s@]" "good" False "morning" "everybody";

are printed:

  good
    morning,
      everybody
  good morning, everybody

Parentheses not neighbours of breaks

In the examples above, we can remark that the left parentheses are always the begin of the string or are preceeded by a break, and that the right parentheses are always the end of the string or followed by a break.

When the parentheses "@[" and "@]" are not preceeded or followed by the string begin nor end, nor preceeded or followed by breaks, they are considered as the "bef" or "aft" part of the neighbour string. For example, the following forms:

  pprintf pc "the quick brown fox@[ jumps over@]"

and:

  pprintf pc "@[the quick brown fox @]jumps over"

are respectively equivalent to:

  let pc = {(pc) with aft = sprintf " jumps over%s" pc.aft} in
  Pretty.sprintf "%sthe quick brown fox%s" pc.bef pc.aft

and:

  let pc = {(pc) with bef = sprintf "%sthe quick brown fox" pc.bef} in
  Pretty.sprintf "%sjumps over%s" pc.bef pc.aft

In these examples, the results are identical, but it can be important if the non-parenthesized part contain one or several "%p". In this case, the corresponding function receives the "bef" or "aft" part in its pr_context variable and can take it into account when printing its data.

Lprintf

"Lprintf" is like "pprintf" with the same parameters. It is equivalent to an call to the function "expand_lprintf":

   lprintf pc "..."

is equivalent to:

   expand_lprintf pc loc (fun pc -< pprintf pc "...")

The function "expand_lprintf" and the variable "loc" must be defined by the user in the environment where "lprintf" is used.

"Lprintf" is used in predefined printers "pr_r.ml" and "pr_o.ml" to allow optional insertions of location comments in the output.

Comparison with the OCaml modules Printf and Format

Pprintf and Printf

The statement "pprintf" acts like the function "Printf.sprintf". But since it requires this extra parameter of type "pr_context" and needs the "%p" and "%q" conversions specifications (which do not exist in "Printf"), it was not possible to use the "Printf" machinery directly and a new statement had to be added.

The principle of "pprintf" and "sprintf" are the same. However, "pprintf" is a syntax extension and has no type by itself. It cannot be used alone or without all its required parameters.

Pprintf and Format

The pretty printing annotations look like the ones of the OCaml module Format. Actually, they have different semantics. They do not use boxes like "Format" does. In "pprintf" statement, the machinery acts only on indentations.

Notice that, with "pprintf", it is always possible to know the current indentation (it is the field "ind" of the "pr_context" variable) and it is therefore possible to take decisions before printing.

For example, it is possible, in a printer of OCaml statements, to decide to print all match cases symmetrically, i.e. all with one line for each case or all with newlines after the patterns.

It is what is done in the option "-flag E" added by the pretty printing kits "pr_r.cmo" (pretty print in revised syntax) and "pr_o.cmo" (pretty print in normal syntax). See chapter Commands and Files or type "camlp5 pr_r.cmo -help" or "camlp5 pr_o.cmo -help".

Another difference is that the internal behaviour of this printing system is accessible, and it is always possible to use the basic functions of the "Pretty" module ("horiz_vertic" and "sprintf") if the behaviour of "pprintf" is not what is desired by the programmer.

Relation with the Camlp5 extensible printers

The extensible printers of Camlp5 (see its corresponding chapter) use the type "pr_context" of "pprintf". It is therefore possible to use "pprintf" in the semantic actions of the extensible printers. But it is not mandatory. An extensible printer can just use the "Pretty" module or even neither "pprintf" nor "Pretty".

The printing kits "pr_r.ml" and "pr_o.ml" (respectively pretty print in revised and in normal syntax) and some other related to them, are examples of usage of the "pprintf" statement.

The Pprintf module

See its section in the chapter "Library".


Copyright 2007-2014 Daniel de Rauglaudre (INRIA)

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