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.
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:
- "ind" : the current indendation
- "bef" : what should be printed before, in the same line
- "aft" : what should be printed after, in the same line
- "dang" : the dangling token to know whether parentheses are necessary
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 conversion specifications: "%p" and "q",
- the pretty printing annotations introduced by, "@"
and followed by:
- the character ";" (semicolon), optionally followed by "<", two numbers and ">",
- the character " " (space),
- the character "[", optionally followed by the character
"<" and either:
- the character "a"
- the character "b"
- a number
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:
- "@;" specifying: write a space or 'a newline and an indentation incremented by 2 spaces'
- "@ " specifying: write a space or 'a newline and the indentation'
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:
- "@;<s o>", which is a break with "s" spaces if the string holds in the line, or an indentation offset (incrementation of the indentation) of "o" spaces if the string does not hold in the line.
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
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.
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".
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