Formatted output functions (also known as unparsing).
General overview
The functions of this module produce output according to a Pervasives.format, as described below. Some functions write to the standard output (i.e. the screen), some to error channels, some to strings or to buffers, or some to abstract outputs.
Note The types used in this module are confusing at first. If you are a beginner, you should probably ignore them in a first time and concentrate on formats.
For a first explanation, we will concentrate on function printf. As all the functions in this module, the behavior of printf is dictated by a Format. This format is a string, composed of regular text and directives, and which dictates how to interpret the other arguments passed to the function. Every directive starts with character %. The most common directive is %s, which serves to display a string, something quite useful for pretty-printing or translation. Anther common directive is %i, which serves to display an integer.
For instance, "foobar" is a format with no directive. Calling printf "foobar" prints "foobar" on the screen and returns (). On the other hand, "%s" is a format with one directive for printing strings. printf "%s" does nothing yet but returns a function with type string -> unit. In turn, printf "%s"
"foobar" prints "foobar" on the screen and returns (). The main interest of this module is that directives may be combined together and with text, to allow more complex printing. For instance printf "(%s)\n" is a function with type string -> unit which, when passed string "foobar" prints "(foobar)" and ends the line. Similarly, printf "Here's the result: %s.\n\tComputation
took %i seconds.\n" "foobar" 5 prints
Here's the result: foobar
Computation took 5 seconds.
Note that \n (the newline character) and \t (the tabulation) are not specific to this module but rather part of the conventions on characters strings in OCaml.
Other directives and functions make this module extremely useful for printing, pretty-printing and translation of messages to the user's language. For more information, see the documentation of Format and the various functions.
type ('a, 'b, 'c) t = ('a, 'b, 'c) Stdlib.Pervasives.formatThe format to use for displaying the various arguments passed to the function.
Syntactically, the format is a character string which contains two types of objects: plain characters, which are simply copied, and directives, each of which causes the conversion and printing of arguments.
Simple directives
All directives start with the % character. In their simplest form, a directive is % followed by exactly one character:
%d, %i, %n, %l, %L, or %N: convert an integer argument to signed decimal.%u: convert an integer argument to unsigned decimal.%x: convert an integer argument to unsigned hexadecimal, using lowercase letters.%X: convert an integer argument to unsigned hexadecimal, using uppercase letters.%o: convert an integer argument to unsigned octal.%s: insert a string argument.%S: insert a string argument in OCaml syntax (double quotes, escapes).%c: insert a character argument.%C: insert a character argument in OCaml syntax (single quotes, escapes).%f: convert a floating-point argument to decimal notation, in the style dddd.ddd.%F: convert a floating-point argument to OCaml syntax (dddd. or dddd.ddd or d.ddd e+-dd).%e or %E: convert a floating-point argument to decimal notation, in the style d.ddd e+-dd (mantissa and exponent).%g or %G: convert a floating-point argument to decimal notation, in style %f or %e, E (whichever is more compact).%B: convert a boolean argument to the string true or false%b: convert a boolean argument (for backward compatibility; do not use in new programs).%ld, %li, %lu, %lx, %lX, %lo: convert an int32 argument to the format specified by the second letter (decimal, hexadecimal, etc).%nd, %ni, %nu, %nx, %nX, %no: convert a nativeint argument to the format specified by the second letter.%Ld, %Li, %Lu, %Lx, %LX, %Lo: convert an int64 argument to the format specified by the second letter.!: take no argument and flush the output.%: take no argument and output one % character.,: the no-op delimiter for conversion specifications
Unparsers
%a: user-defined printer. Typically, this printer corresponds to two arguments: a printing function f, with type 'a output -> 'c -> unit and the item x you want to print, with type 'c. Item x will be printing by calling f out x, where out is the output you are currently using -- if you are calling printf, this output is the standard output (i.e. the screen), if you are calling eprintf, this will be the error channel, if you are calling fprintf, this will be the output you provided yourself, etc. More generally, if your Format has type ('a, 'b, 'd) format or ('a, 'b, 'd, 'e) format4, the printing function f must have type 'b -> 'c -> 'd, where x has type 'd.%t: same as %a but takes only a printing function f, without an item. If your Format has type ('a, 'b, 'd) format or ('a, 'b, 'd, 'e) format4, function f must have type 'b -> 'd.
Formatting formats
%\{ fmt %\}: convert a Format to a string. The format argument must have the same type as the internal format string fmt. In other words, printf "%\{ %s %\}" accepts an argument whose type must be the same as that of format "%s", and prints that format argument as if it were a character string.%( fmt %): format string substitution. Takes a format string argument and substitutes it to the internal format string fmt to print following arguments. The argument must have the same type as fmt. printf "%\{ %s %\}" accepts an argument whose type must be the same as that of format "%s", and uses that argument to print the following arguments.
Additional options
The general format of directives is
% [flags] [width] [.precision] type
type is one of d, i, n, l, L, N, u, x ..., ( fmt %) and behaves as explained above.
The optional flags are:
-: left-justify the output (default is right justification).0: for numerical conversions, pad with zeroes instead of spaces.+: for numerical conversions, prefix number with a + sign if positive.- space: for numerical conversions, prefix number with a space if positive.
#: request an alternate formatting style for numbers.
The optional width is an integer indicating the minimal width of the result. For instance, %6d prints an integer, prefixing it with spaces to fill at least 6 characters.
The optional precision is a dot . followed by an integer indicating how many digits follow the decimal point in the %f, %e, and %E conversions. For instance, %.4f prints a float with 4 fractional digits.
The integer in a width or precision can also be specified as *, in which case an extra integer argument is taken to specify the corresponding width or precision. This integer argument precedes immediately the argument to print. For instance, %.*f prints a float with as many fractional digits as the value of the argument given before the float.
Common functions
The usual printf function, prints to the standard output stdout, i.e. normally to the screen. If you are lost, this is probably the function you're looking for.
The usual eprintf function, prints to the standard error output stderr, used to display warnings and errors. Otherwise identical to printf.
val sprintf : ('a, unit, string) t -> 'aA function which doesn't print its result but returns it as a string. Useful for building messages, for translation purposes or for display in a window, for instance.
While this function is quite convenient, don't abuse it to create very large strings such as files, that's not its role. For this kind of usage, prefer the more modular and usually faster fprintf.
Note that any function called with %a should return strings, i.e. should have type unit -> string.
A function which doesn't print its result but returns it as a string. Useful for building messages, for translation purposes or for display in a window, for instance.
While this function is quite convenient, don't abuse it to create very large strings such as files, that's not its role. For this kind of usage, prefer the more modular and usually faster fprintf. Note that any function called with %a should be able to print its result, i.e. should have type 'b output -> unit.
Warning: a partial application of this function can only be used once, because the BatInnerIO.output that it uses is closed afterwards. Example: let f = sprintf2 "%a" Int.print in [f 1; f 2] will fail.
General functions
General function. This function prints to any output. Typically, if you are attempting to build a large output such as a file, this is probably the function you are looking for. If you are writing a pretty-printer, this is probably the function you are looking for. If you are you are looking for a function to use for argument %a with printf, eprintf, sprintf2, ifprintf, bprintf2, kfprintf, ksprintf2, kbprintf2 or any other function with type (_, _ output, unit) format or (_, _ output, unit, _) format4, this is also probably the function you are looking for.
As fprintf but doesn't actually print anything. Sometimes useful for debugging.
val bprintf : Stdlib.Buffer.t -> ('a, Stdlib.Buffer.t, unit) t -> 'aAs fprintf, but with buffers instead of outputs. In particular, any unparser called with %a should write to a buffer rather than to an output
As printf but writes to a buffer instead of printing to the output. By opposition to bprintf, only the result is changed with respect to printf, not the inner workings.
Functions with continuations
Same as fprintf, but instead of returning immediately, passes the output to its first argument at the end of printing.
val ksprintf : (string -> 'a) -> ('b, unit, string, 'a) Stdlib.format4 -> 'bSame as sprintf above, but instead of returning the string, passes it to the first argument.
val ksprintf2 :
(string -> 'b) ->
('c, 'a BatInnerIO.output, unit, 'b) Stdlib.format4 ->
'cSame as sprintf2 above, but instead of returning the string, passes it to the first argument.
val kbprintf :
(Stdlib.Buffer.t -> 'a) ->
Stdlib.Buffer.t ->
('b, Stdlib.Buffer.t, unit, 'a) Stdlib.format4 ->
'bSame as bprintf, but instead of returning immediately, passes the buffer to its first argument at the end of printing.
val kbprintf2 :
(Stdlib.Buffer.t -> 'b) ->
Stdlib.Buffer.t ->
('c, 'a BatInnerIO.output, unit, 'b) Stdlib.format4 ->
'cSame as bprintf2, but instead of returning immediately, passes the buffer to its first argument at the end of printing.
val kprintf : (string -> 'a) -> ('b, unit, string, 'a) Stdlib.format4 -> 'bYou only need to read this if you intend to create your new printf-like functions, which happens generally by toying with mkprintf.
('a, 'b, 'c, 'd) format4 is the type of arguments for printf-style functions such that
'a is the type of arguments, with a return type of 'd
- if your format looks like
"%s", 'a is string -> 'd - if your format looks like
"%s%s", 'a is string -> string -> 'd - ...
'b is the type of the first argument given to unparsers (i.e. functions introduced with %a or %t)
- if your unparsers take a
unit argument, 'b should be unit - if your unparsers take a
string output, 'b should be string output - ...
'c is the final return type of unparsers
- if you have an unparser introduced with
%t and its result has type unit, 'c should be unit - if you have an unparser introduced with
%a and its type is string output -> string -> unit, 'c should be unit - ...
'd is the final return value of the function once all arguments have been printed
('a, 'b, 'c) format or ('a, 'b, 'c) t is just a shortcut for ('a, 'b, 'c, 'c) format4.
Important
Note that Obj.magic is involved behind this, so be careful.