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.     *)
(*                                                                     *)
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(* $Id$ *)

(** Formatted input functions. *)

(** {6 Introduction} *)

(** {7 Functional input with format strings} *)

(** The module [Scanf] provides formatted input functions or {e scanners}.

    The formatted input functions can read from any kind of input, including
    strings, files, or anything that can return characters. The more general
    source of characters is named a {e scanning buffer} and has type
    {!Scanning.scanbuf}. The more general formatted input function reads from
    any scanning buffer and is named [bscanf].

    Generally speaking, the formatted input functions have 3 arguments:
    - the first argument is a source of characters for the input,
    - the second argument is a format string that specifies the values to
      read,
    - the third argument is a {e receiver function} that is applied to the
      values read.

    Hence, a typical call to the formatted input function {!Scanf.bscanf} is
    [bscanf ib fmt f], where:

    - [ib] is a source of characters (typically a {e
    scanning buffer} with type {!Scanning.scanbuf}),

    - [fmt] is a format string (the same format strings as those used to print
    material with module {!Printf} or {!Format}),

    - [f] is a function that has as many arguments as the number of values to
    read in the input. *)

(** {7 A simple example} *)

(** As suggested above, the expression [bscanf ib "%d" f] reads a decimal
    integer [n] from the source of characters [ib] and returns [f n].

    For instance,

    - if we use [stdib] as the source of characters ({!Scanning.stdib} is
    the predefined input buffer that reads from standard input),

    - if we define the receiver [f] as [let f x = x + 1],

    then [bscanf stdib "%d" f] reads an integer [n] from the standard input
    and returns [f n] (that is [n + 1]). Thus, if we evaluate [bscanf stdib
    "%d" f], and then enter [41] at the keyboard, we get [42] as the final
    result. *)

(** {7 Formatted input as a functional feature} *)

(** 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 just the regular function application not
    the variable assignment based mechanism which is typical for formatted
    input in imperative languages; the Caml format strings also feature
    useful additions to easily define complex tokens; as expected within a
    functional programming language, the formatted input functions also
    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 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 input 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 receiver [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 [v] of type [t] followed
    by a [';'] character, and returns [f v]. *)

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

(** {6 The general formatted input function} *)

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 the entire
    [bscanf] call.
    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. *)

(** {6 Format string description} *)

(** 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. *)

(** {7 The space character in format strings} *)

(** As mentioned above, a plain character in the format string is just
    matched with the next character of the input; however, two characters are
    special exceptions to this rule: the space character ([' '] or ASCII code
    32) and the line feed character (['\n'] or ASCII code 10).
    A space does not match a single space character, but any amount of
    ``whitespace'' in the input. More precisely, a space inside the format
    string matches {e any number} of tab, space, line feed and carriage
    return characters. Similarly, a line feed character in the format string
    matches either a single line feed or a carriage return followed by a line
    feed.

    Matching {e any} amount of whitespace, a space in the format string
    also matches no amount of whitespace at all; hence, the call [bscanf ib
    "Price = %d $" (fun p -> p)] succeeds and returns [1] when reading an
    input with various whitespace in it, such as [Price = 1 $],
    [Price  =  1    $], or even [Price=1$]. *)

(** {7 Conversion specifications in format strings} *)

(** 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.
    - [,]: the no-op delimiter for conversion specifications.

    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).

    Notes:

    - as mentioned above, a [%s] conversion always succeeds, even if there is
      nothing to read in the input: it simply returns [""].

    - in addition to the 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. *)

(** {7 Scanning indications in format strings} *)

(** 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.

    Note:

    - the scanning indications introduce slight differences in the syntax of
    [Scanf] format strings, compared to those used for 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 the
    [Format] module (or, if you need to use functions from [Printf], banish
    or carefully double check the format strings that contain ['\@']
    characters). *)

(** {7 Exceptions during scanning} *)

(** Scanners may raise the following exceptions when the input cannot be read
    according to the format string:

    - 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.

    - Raise [Invalid_argument] if the format string is invalid.

    Note:

    - 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. *)

(** {6 Specialised formatted input functions} *)

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 will operate with a
    scanning buffer reading from the given channel. This extra level of
    bufferization can lead to strange scanning behaviour if you use low level
    primitives on the channel (reading characters, seeking the reading
    position, and so on).

    As a consequence, never mix direct low level reading and high level
    scanning from the same input channel. *)

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 from input} *)

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 scanning
    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 does not 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, does not
    have the same type as [fmt]. *)