path: root/stdlib/scanf.mli

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(*                                                                     *)
(*                           Objective Caml                            *)
(*                                                                     *)
(*            Pierre Weis, projet Cristal, INRIA Rocquencourt          *)
(*                                                                     *)
(*  Copyright 2002 Institut National de Recherche en Informatique et   *)
(*  en Automatique.  All rights reserved.  This file is distributed    *)
(*  under the terms of the GNU Library General Public License, with    *)
(*  the special exception on linking described in file ../LICENSE.     *)
(*                                                                     *)
(***********************************************************************)

(* $Id$ *)

(** Formatted input functions. *)

(** {6 Functional input with format strings.} *)

(** The formatted input functions provided by module [Scanf] are functionals
    that apply their function argument to the values they read in the input.
    The specification of the values to read is simply given by a format string
    (the same format strings as those used to print material using module
    {!Printf} or module {!Format}).

    As an example, consider the formatted input function [scanf] that reads
    from standard input; a typical call to [scanf] is simply [scanf fmt f],
    meaning that [f] should be applied to the arguments read according to the
    format string [fmt]. For instance, if [f] is defined as [let f x = x + 1],
    then [scanf "%d" f] will read a decimal integer [i] from [stdin] and return
    [f i]; thus, if we enter [41] at the keyboard, [scanf "%d" f] evaluates to
    [42].

    This module provides general formatted input functions that read from any
    kind of input, including strings, files, or anything that can return
    characters. The more general source of characters is named a [scanbuf], and
    it is the first argument of scanning functions.
    Hence, a typical call to a formatted input function [bscanf] is
    [bscanf ib fmt f], meaning that [f] should be applied to the arguments
    read from input [ib], according to the format string [fmt].

    The Caml scanning facility is reminiscent of the corresponding C feature.
    However, it is also largely different, simpler, and yet more powerful: the
    formatted input functions are higher-order functionals and the parameter
    passing mechanism is simply the regular function application not the
    variable assigment based mechanism which is typical of formatted input in
    imperative languages; the format strings also feature useful additions to
    easily define complex tokens; as expected of a functional programming
    language feature, the formatted input functions support polymorphism, in
    particular arbitrary interaction with polymorphic user-defined scanners.
    Furthermore, the Caml formatted input facility is fully type-checked at
    compile time. *)

(** {6 Scanning buffers} *)
module Scanning : sig

type scanbuf;;

(** The type of scanning buffers. A scanning buffer is the source from which a
    formatted input function gets characters. The scanning buffer holds the
    current state of the scan, plus a function to get the next char from the
    input, and a token buffer to store the string matched so far.

    Note: a scan may often require to examine one character in advance;
    when this ``lookahead'' character does not belong to the token read,
    it is stored back in the scanning buffer and becomes the next
    character read. *)

val stdib : scanbuf;;
(** The scanning buffer reading from [stdin].
    [stdib] is equivalent to [Scanning.from_channel stdin].

    Note: when input is read interactively from [stdin], the newline character
    that triggers the evaluation is incorporated in the input; thus, scanning
    specifications must properly skip this character (simply add a ['\n']
    as the last character of the format string). *)

val from_string : string -> scanbuf;;
(** [Scanning.from_string s] returns a scanning buffer which reads from the
    given string.
    Reading starts from the first character in the string.
    The end-of-input condition is set when the end of the string is reached. *)

val from_file : string -> scanbuf;;
(** Bufferized file reading in text mode. The efficient and usual
    way to scan text mode files (in effect, [from_file] returns a
    scanning buffer that reads characters in large chunks, rather than one
    character at a time as buffers returned by [from_channel] do).
    [Scanning.from_file fname] returns a scanning buffer which reads
    from the given file [fname] in text mode. *)

val from_file_bin : string -> scanbuf;;
(** Bufferized file reading in binary mode. *)

val from_function : (unit -> char) -> scanbuf;;
(** [Scanning.from_function f] returns a scanning buffer with the given
    function as its reading method.

    When scanning needs one more character, the given function is called.

    When the function has no more character to provide, it must signal an
    end-of-input condition by raising the exception [End_of_file]. *)

val from_channel : in_channel -> scanbuf;;
(** [Scanning.from_channel ic] returns a scanning buffer which reads
    one character at a time from the input channel [ic], starting at the
    current reading position. *)

val end_of_input : scanbuf -> bool;;
(** [Scanning.end_of_input ib] tests the end-of-input condition of the given
    scanning buffer. *)

val beginning_of_input : scanbuf -> bool;;
(** [Scanning.beginning_of_input ib] tests the beginning of input condition of
    the given scanning buffer. *)

val name_of_input : scanbuf -> string;;
(** [Scanning.file_name_of_input ib] returns the name of the character source
    for the scanning buffer [ib]. *)

end;;

(** {6 Type of formatted input functions} *)

type ('a, 'b, 'c, 'd) scanner =
     ('a, Scanning.scanbuf, 'b, 'c, 'a -> 'd, 'd) format6 -> 'c;;
(** The type of formatted input scanners: [('a, 'b, 'c, 'd) scanner] is the
    type of a formatted input function that reads from some scanning buffer
    according to some format string; more precisely, if [scan] is some
    formatted input function, then [scan ib fmt f] applies [f] to the arguments
    specified by the format string [fmt], when [scan] has read those arguments
    from the scanning buffer [ib].

    For instance, the [scanf] function below has type [('a, 'b, 'c, 'd)
    scanner], since it is a formatted input function that reads from [stdib]:
    [scanf fmt f] applies [f] to the arguments specified by [fmt], reading
    those arguments from [stdin] as expected.

    If the format [fmt] has some [%r] indications, the corresponding input
    functions must be provided before the [f] argument. For instance, if
    [read_elem] is an input function for values of type [t], then [bscanf ib
    "%r;" read_elem f] reads a value of type [t] followed by a [';']
    character. *)

exception Scan_failure of string;;
(** The exception that formatted input functions raise when the input cannot be
    read according to the given format. *)

(** {6 Formatted input functions} *)

val bscanf : Scanning.scanbuf -> ('a, 'b, 'c, 'd) scanner;;
(** [bscanf ib fmt r1 ... rN f] reads arguments for the function [f] from the
   scanning buffer [ib] according to the format string [fmt], and applies [f]
   to these values.
   The result of this call to [f] is returned as the result of [bscanf].
   For instance, if [f] is the function [fun s i -> i + 1], then
   [Scanf.sscanf "x = 1" "%s = %i" f] returns [2].

   Arguments [r1] to [rN] are user-defined input functions that read the
   argument corresponding to a [%r] conversion.

   The format is a character string which contains three types of
   objects:
   - plain characters, which are simply matched with the characters of the
   input,
   - conversion specifications, each of which causes reading and conversion of
   one argument for the function [f],
   - scanning indications to specify boundaries of tokens.

   Among plain characters the space character (ASCII code 32) has a
   special meaning: it matches ``whitespace'', that is any number of tab,
   space, line feed and carriage return characters. Hence, a space in the format
   matches any amount of whitespace in the input.

   Conversion specifications consist in the [%] character, followed by
   an optional flag, an optional field width, and followed by one or
   two conversion characters. The conversion characters and their
   meanings are:

   - [d]: reads an optionally signed decimal integer.
   - [i]: reads an optionally signed integer
     (usual input formats for hexadecimal ([0x[d]+] and [0X[d]+]),
      octal ([0o[d]+]), and binary [0b[d]+] notations are understood).
   - [u]: reads an unsigned decimal integer.
   - [x] or [X]: reads an unsigned hexadecimal integer.
   - [o]: reads an unsigned octal integer.
   - [s]: reads a string argument that spreads as much as possible, until the
     following bounding condition holds: a whitespace has been found, a
     scanning indication has been encountered, or the end-of-input has been
     reached.
     Hence, this conversion always succeeds: it returns an empty
     string if the bounding condition holds when the scan begins.
   - [S]: reads a delimited string argument (delimiters and special
     escaped characters follow the lexical conventions of Caml).
   - [c]: reads a single character. To test the current input character
     without reading it, specify a null field width, i.e. use
     specification [%0c]. Raise [Invalid_argument], if the field width
     specification is greater than 1.
   - [C]: reads a single delimited character (delimiters and special
     escaped characters follow the lexical conventions of Caml).
   - [f], [e], [E], [g], [G]: reads an optionally signed
     floating-point number in decimal notation, in the style [dddd.ddd
     e/E+-dd].
   - [F]: reads a floating point number according to the lexical
     conventions of Caml (hence the decimal point is mandatory if the
     exponent part is not mentioned).
   - [B]: reads a boolean argument ([true] or [false]).
   - [b]: reads a boolean argument (for backward compatibility; do not use
     in new programs).
   - [ld], [li], [lu], [lx], [lX], [lo]: reads an [int32] argument to
     the format specified by the second letter (decimal, hexadecimal, etc).
   - [nd], [ni], [nu], [nx], [nX], [no]: reads a [nativeint] argument to
     the format specified by the second letter.
   - [Ld], [Li], [Lu], [Lx], [LX], [Lo]: reads an [int64] argument to
     the format specified by the second letter.
   - [\[ range \]]: reads characters that matches one of the characters
     mentioned in the range of characters [range] (or not mentioned in
     it, if the range starts with [^]). Reads a [string] that can be
     empty, if the next input character does not match the range. The set of
     characters from [c1] to [c2] (inclusively) is denoted by [c1-c2].
     Hence, [%\[0-9\]] returns a string representing a decimal number
     or an empty string if no decimal digit is found; similarly,
     [%\[\\048-\\057\\065-\\070\]] returns a string of hexadecimal digits.
     If a closing bracket appears in a range, it must occur as the
     first character of the range (or just after the [^] in case of
     range negation); hence [\[\]\]] matches a [\]] character and
     [\[^\]\]] matches any character that is not [\]].
   - [r]: user-defined reader. Takes the next [ri] formatted input function and
     applies it to the scanning buffer [ib] to read the next argument. The
     input function [ri] must therefore have type [Scanning.scanbuf -> 'a] and
     the argument read has type ['a].
   - [\{ fmt %\}]: reads a format string argument.
     The format string read must have the same type as the format string
     specification [fmt].
     For instance, ["%\{%i%\}"] reads any format string that can read a value of
     type [int]; hence [Scanf.sscanf "fmt:\\\"number is %u\\\"" "fmt:%\{%i%\}"]
     succeeds and returns the format string ["number is %u"].
   - [\( fmt %\)]: scanning format substitution.
     Reads a format string to replace [fmt].
     The format string read must have the same type as the format string
     specification [fmt].
     For instance, ["%\( %i% \)"] reads any format string that can read a value
     of type [int]; hence [Scanf.sscanf "\\\"%4d\\\"1234.00" "%\(%i%\)"]
     is equivalent to [Scanf.sscanf "1234.00" "%4d"].
   - [l]: returns the number of lines read so far.
   - [n]: returns the number of characters read so far.
   - [N] or [L]: returns the number of tokens read so far.
   - [!]: matches the end of input condition.
   - [%]: matches one [%] character in the input.

   Following the [%] character that introduces a conversion, there may be
   the special flag [_]: the conversion that follows occurs as usual,
   but the resulting value is discarded.
   For instance, if [f] is the function [fun i -> i + 1], then
   [Scanf.sscanf "x = 1" "%_s = %i" f] returns [2].

   The field width is composed of an optional integer literal
   indicating the maximal width of the token to read.
   For instance, [%6d] reads an integer, having at most 6 decimal digits;
   [%4f] reads a float with at most 4 characters; and [%8\[\\000-\\255\]]
   returns the next 8 characters (or all the characters still available,
   if fewer than 8 characters are available in the input).

   Scanning indications appear just after the string conversions [%s]
   and [%\[ range \]] to delimit the end of the token. A scanning
   indication is introduced by a [@] character, followed by some
   constant character [c]. It means that the string token should end
   just before the next matching [c] (which is skipped). If no [c]
   character is encountered, the string token spreads as much as
   possible. For instance, ["%s@\t"] reads a string up to the next
   tab character or to the end of input. If a scanning
   indication [\@c] does not follow a string conversion, it is treated
   as a plain [c] character.

   Raise [Scanf.Scan_failure] if the input does not match the format.

   Raise [Failure] if a conversion to a number is not possible.

   Raise [End_of_file] if the end of input is encountered while some more
   characters are needed to read the current conversion specification.
   As a consequence, scanning a [%s] conversion never raises exception
   [End_of_file]: if the end of input is reached the conversion succeeds and
   simply returns the characters read so far, or [""] if none were read.

   Raise [Invalid_argument] if the format string is invalid.

   Notes:

   - the scanning indications introduce slight differences in the
   syntax of [Scanf] format strings compared to those used by the
   [Printf] module. However, the scanning indications are similar
   to those used in the [Format] module; hence, when producing
   formatted text to be scanned by [!Scanf.bscanf], it is wise
   to use printing functions from [Format] (or, if you need to
   use functions from [Printf], banish or carefully double check
   the format strings that contain ['\@'] characters).

   - in addition to relevant digits, ['_'] characters may appear
   inside numbers (this is reminiscent to the usual Caml lexical
   conventions). If stricter scanning is desired, use the range
   conversion facility instead of the number conversions.

   - the [scanf] facility is not intended for heavy duty lexical
   analysis and parsing. If it appears not expressive enough for your
   needs, several alternative exists: regular expressions (module
   [Str]), stream parsers, [ocamllex]-generated lexers,
   [ocamlyacc]-generated parsers.
*)

val fscanf : in_channel -> ('a, 'b, 'c, 'd) scanner;;
(** Same as {!Scanf.bscanf}, but reads from the given channel.

    Warning: since all formatted input functions operate from a scanning
    buffer, be aware that each [fscanf] invocation must allocate a new
    fresh scanning buffer (unless you make careful use of partial
    application). Hence, there are chances that some characters seem
    to be skipped (in fact they are pending in the previously used
    scanning buffer). This happens in particular when calling [fscanf] again
    after a scan involving a format that necessitated some look ahead
    (such as a format that ends by skipping whitespace in the input).

    To avoid confusion, consider using [bscanf] with an explicitly
    created scanning buffer. Use for instance [Scanning.from_file f]
    to allocate the scanning buffer reading from file [f].

    This method is not only clearer it is also faster, since scanning
    buffers to files are optimized for fast buffered reading. *)

val sscanf : string -> ('a, 'b, 'c, 'd) scanner;;
(** Same as {!Scanf.bscanf}, but reads from the given string. *)

val scanf : ('a, 'b, 'c, 'd) scanner;;
(** Same as {!Scanf.bscanf}, but reads from the predefined scanning
    buffer {!Scanf.Scanning.stdib} that is connected to [stdin]. *)

val kscanf :
  Scanning.scanbuf -> (Scanning.scanbuf -> exn -> 'd) ->
    ('a, 'b, 'c, 'd) scanner;;
(** Same as {!Scanf.bscanf}, but takes an additional function argument
  [ef] that is called in case of error: if the scanning process or
  some conversion fails, the scanning function aborts and calls the
  error handling function [ef] with the scanning buffer and the
  exception that aborted the scanning process. *)

(** {6 Reading format strings} *)

val bscanf_format :
  Scanning.scanbuf -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
    (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g;;
(** [bscanf_format ib fmt f] reads a format string token from the scannning
  buffer [ib], according to the given format string [fmt], and applies [f] to
  the resulting format string value.
  Raise [Scan_failure] if the format string value read doesn't have the
  same type as [fmt]. *)

val sscanf_format :
  string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
    (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g;;
(** Same as {!Scanf.bscanf_format}, but reads from the given string. *)

val format_from_string :
  string ->
    ('a, 'b, 'c, 'd, 'e, 'f) format6 -> ('a, 'b, 'c, 'd, 'e, 'f) format6;;
(** [format_from_string s fmt] converts a string argument to a format string,
  according to the given format string [fmt].
  Raise [Scan_failure] if [s], considered as a format string, doesn't
  have the same type as [fmt]. *)