425 lines
20 KiB
OCaml
425 lines
20 KiB
OCaml
(***********************************************************************)
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(* *)
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(* Objective Caml *)
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(* *)
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(* Pierre Weis, projet Cristal, INRIA Rocquencourt *)
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(* *)
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(* Copyright 2002 Institut National de Recherche en Informatique et *)
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(* en Automatique. All rights reserved. This file is distributed *)
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(* under the terms of the GNU Library General Public License, with *)
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(* the special exception on linking described in file ../LICENSE. *)
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(* *)
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(***********************************************************************)
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(* $Id$ *)
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(** Formatted input functions. *)
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(** {6 Introduction} *)
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(** {7 Functional input with format strings} *)
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(** The module [Scanf] provides formatted input functions or {e scanners}.
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The formatted input functions can read from any kind of input, including
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strings, files, or anything that can return characters. The more general
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source of characters is named a {e scanning buffer} and has type
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{!Scanning.scanbuf}. The more general formatted input function reads from
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any scanning buffer and is named [bscanf].
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Generally speaking, the formatted input functions have 3 arguments:
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- the first argument is a source of characters for the input,
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- the second argument is a format string that specifies the values to
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read,
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- the third argument is a {e receiver function} that is applied to the
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values read.
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Hence, a typical call to the formatted input function {!Scanf.bscanf} is
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[bscanf ib fmt f], where:
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- [ib] is a source of characters (typically a {e
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scanning buffer} with type {!Scanning.scanbuf}),
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- [fmt] is a format string (the same format strings as those used to print
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material with module {!Printf} or {!Format}),
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- [f] is a function that has as many arguments as the number of values to
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read in the input. *)
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(** {7 A simple example} *)
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(** As suggested above, the expression [bscanf ib "%d" f] reads a decimal
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integer [n] from the source of characters [ib] and returns [f n].
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For instance,
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- if we use [stdib] as the source of characters ({!Scanning.stdib} is
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the predefined input buffer that reads from standard input),
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- if we define the receiver [f] as [let f x = x + 1],
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then [bscanf stdib "%d" f] reads an integer [n] from the standard input
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and returns [f n] (that is [n + 1]). Thus, if we evaluate [bscanf stdib
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"%d" f], and then enter [41] at the keyboard, we get [42] as the final
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result. *)
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(** {7 Formatted input as a functional feature} *)
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(** The Caml scanning facility is reminiscent of the corresponding C feature.
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However, it is also largely different, simpler, and yet more powerful:
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the formatted input functions are higher-order functionals and the
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parameter passing mechanism is just the regular function application not
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the variable assigment based mechanism which is typical for formatted
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input in imperative languages; the Caml format strings also feature
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useful additions to easily define complex tokens; as expected within a
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functional programming language, the formatted input functions also
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support polymorphism, in particular arbitrary interaction with
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polymorphic user-defined scanners. Furthermore, the Caml formatted input
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facility is fully type-checked at compile time. *)
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(** {6 Scanning buffers} *)
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module Scanning : sig
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type input_channel;;
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(* The notion of input channel for the [Scanf] module.
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The type [scanbuf] is an alias for [input_channel]. *)
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type scanbuf = input_channel;;
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(** The type of scanning buffers. A scanning buffer is the source from which a
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formatted input function gets characters. The scanning buffer holds the
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current state of the scan, plus a function to get the next char from the
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input, and a token buffer to store the string matched so far.
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Note: a scan may often require to examine one character in advance;
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when this ``lookahead'' character does not belong to the token read,
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it is stored back in the scanning buffer and becomes the next
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character read. *)
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val stdin : scanbuf;;
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(** The standard input notion for the module [Scanf].
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[stdin] is equivalent to [Scanning.from_channel Pervasives.stdin].
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Note: when input is read interactively from [stdin], the newline character
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that triggers the evaluation is incorporated in the input; thus, scanning
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specifications must properly skip this character (simply add a ['\n']
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as the last character of the format string). *)
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val stdib : scanbuf;;
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(** An alias for [Scanf.stdin], the scanning buffer reading from
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[Pervasives.stdin]. *)
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val from_string : string -> scanbuf;;
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(** [Scanning.from_string s] returns a scanning buffer which reads from the
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given string.
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Reading starts from the first character in the string.
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The end-of-input condition is set when the end of the string is reached. *)
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val from_file : string -> scanbuf;;
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(** Bufferized file reading in text mode. The efficient and usual
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way to scan text mode files (in effect, [from_file] returns a
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scanning buffer that reads characters in large chunks, rather than one
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character at a time as buffers returned by [from_channel] do).
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[Scanning.from_file fname] returns a scanning buffer which reads
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from the given file [fname] in text mode. *)
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val from_file_bin : string -> scanbuf;;
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(** Bufferized file reading in binary mode. *)
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val from_function : (unit -> char) -> scanbuf;;
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(** [Scanning.from_function f] returns a scanning buffer with the given
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function as its reading method.
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When scanning needs one more character, the given function is called.
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When the function has no more character to provide, it must signal an
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end-of-input condition by raising the exception [End_of_file]. *)
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val from_channel : in_channel -> scanbuf;;
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(** [Scanning.from_channel ic] returns a scanning buffer which reads from the
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input channel [ic], starting at the current reading position. *)
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val end_of_input : scanbuf -> bool;;
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(** [Scanning.end_of_input ib] tests the end-of-input condition of the given
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scanning buffer. *)
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val beginning_of_input : scanbuf -> bool;;
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(** [Scanning.beginning_of_input ib] tests the beginning of input condition of
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the given scanning buffer. *)
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val name_of_input : scanbuf -> string;;
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(** [Scanning.file_name_of_input ib] returns the name of the character source
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for the scanning buffer [ib]. *)
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end;;
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(** {6 Type of formatted input functions} *)
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type ('a, 'b, 'c, 'd) scanner =
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('a, Scanning.scanbuf, 'b, 'c, 'a -> 'd, 'd) format6 -> 'c;;
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(** The type of formatted input scanners: [('a, 'b, 'c, 'd) scanner] is the
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type of a formatted input function that reads from some scanning buffer
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according to some format string; more precisely, if [scan] is some
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formatted input function, then [scan ib fmt f] applies [f] to the arguments
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specified by the format string [fmt], when [scan] has read those arguments
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from the scanning input buffer [ib].
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For instance, the [scanf] function below has type [('a, 'b, 'c, 'd)
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scanner], since it is a formatted input function that reads from [stdib]:
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[scanf fmt f] applies [f] to the arguments specified by [fmt], reading
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those arguments from [stdin] as expected.
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If the format [fmt] has some [%r] indications, the corresponding input
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functions must be provided before the receiver [f] argument. For
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instance, if [read_elem] is an input function for values of type [t],
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then [bscanf ib "%r;" read_elem f] reads a value [v] of type [t] followed
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by a [';'] character, and returns [f v]. *)
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exception Scan_failure of string;;
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(** The exception that formatted input functions raise when the input cannot be
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read according to the given format. *)
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(** {6 The general formatted input function} *)
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val bscanf : Scanning.scanbuf -> ('a, 'b, 'c, 'd) scanner;;
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(** [bscanf ib fmt r1 ... rN f] reads arguments for the function [f], from the
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scanning buffer [ib], according to the format string [fmt], and applies [f]
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to these values.
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The result of this call to [f] is returned as the result of the entire
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[bscanf] call.
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For instance, if [f] is the function [fun s i -> i + 1], then
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[Scanf.sscanf "x= 1" "%s = %i" f] returns [2].
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Arguments [r1] to [rN] are user-defined input functions that read the
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argument corresponding to a [%r] conversion. *)
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(** {6 Format string description} *)
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(** The format is a character string which contains three types of
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objects:
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- plain characters, which are simply matched with the characters of the
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input,
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- conversion specifications, each of which causes reading and conversion of
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one argument for the function [f],
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- scanning indications to specify boundaries of tokens. *)
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(** {7 The space character in format strings} *)
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(** As mentioned above, a plain character in the format string is just
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matched with the characters of the input; however, one character is a
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special exception to this simple rule: the space character (ASCII code
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32) does not match a single space character, but any amount of
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``whitespace'' in the input. More precisely, a space inside the format
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string matches {e any number} of tab, space, line feed and carriage
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return characters.
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Matching {e any} amount of whitespace, a space in the format string
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also matches no amount of whitespace at all; hence, the call [bscanf ib
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"Price = %d $" (fun p -> p)] succeds and returns [1] when reading an
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input with various whitespace in it, such as [Price = 1 $],
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[Price = 1 $], or even [Price=1$]. *)
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(** {7 Conversion specifications in format strings} *)
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(** Conversion specifications consist in the [%] character, followed by
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an optional flag, an optional field width, and followed by one or
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two conversion characters. The conversion characters and their
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meanings are:
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- [d]: reads an optionally signed decimal integer.
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- [i]: reads an optionally signed integer
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(usual input formats for hexadecimal ([0x[d]+] and [0X[d]+]),
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octal ([0o[d]+]), and binary [0b[d]+] notations are understood).
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- [u]: reads an unsigned decimal integer.
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- [x] or [X]: reads an unsigned hexadecimal integer.
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- [o]: reads an unsigned octal integer.
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- [s]: reads a string argument that spreads as much as possible, until the
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following bounding condition holds: a whitespace has been found, a
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scanning indication has been encountered, or the end-of-input has been
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reached.
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Hence, this conversion always succeeds: it returns an empty
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string, if the bounding condition holds when the scan begins.
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- [S]: reads a delimited string argument (delimiters and special
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escaped characters follow the lexical conventions of Caml).
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- [c]: reads a single character. To test the current input character
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without reading it, specify a null field width, i.e. use
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specification [%0c]. Raise [Invalid_argument], if the field width
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specification is greater than 1.
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- [C]: reads a single delimited character (delimiters and special
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escaped characters follow the lexical conventions of Caml).
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- [f], [e], [E], [g], [G]: reads an optionally signed
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floating-point number in decimal notation, in the style [dddd.ddd
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e/E+-dd].
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- [F]: reads a floating point number according to the lexical
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conventions of Caml (hence the decimal point is mandatory if the
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exponent part is not mentioned).
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- [B]: reads a boolean argument ([true] or [false]).
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- [b]: reads a boolean argument (for backward compatibility; do not use
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in new programs).
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- [ld], [li], [lu], [lx], [lX], [lo]: reads an [int32] argument to
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the format specified by the second letter (decimal, hexadecimal, etc).
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- [nd], [ni], [nu], [nx], [nX], [no]: reads a [nativeint] argument to
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the format specified by the second letter.
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- [Ld], [Li], [Lu], [Lx], [LX], [Lo]: reads an [int64] argument to
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the format specified by the second letter.
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- [\[ range \]]: reads characters that matches one of the characters
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mentioned in the range of characters [range] (or not mentioned in
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it, if the range starts with [^]). Reads a [string] that can be
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empty, if the next input character does not match the range. The set of
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characters from [c1] to [c2] (inclusively) is denoted by [c1-c2].
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Hence, [%\[0-9\]] returns a string representing a decimal number
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or an empty string if no decimal digit is found; similarly,
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[%\[\\048-\\057\\065-\\070\]] returns a string of hexadecimal digits.
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If a closing bracket appears in a range, it must occur as the
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first character of the range (or just after the [^] in case of
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range negation); hence [\[\]\]] matches a [\]] character and
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[\[^\]\]] matches any character that is not [\]].
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- [r]: user-defined reader. Takes the next [ri] formatted input function and
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applies it to the scanning buffer [ib] to read the next argument. The
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input function [ri] must therefore have type [Scanning.scanbuf -> 'a] and
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the argument read has type ['a].
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- [\{ fmt %\}]: reads a format string argument.
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The format string read must have the same type as the format string
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specification [fmt].
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For instance, ["%\{%i%\}"] reads any format string that can read a value of
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type [int]; hence [Scanf.sscanf "fmt:\\\"number is %u\\\"" "fmt:%\{%i%\}"]
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succeeds and returns the format string ["number is %u"].
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- [\( fmt %\)]: scanning format substitution.
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Reads a format string to replace [fmt].
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The format string read must have the same type as the format string
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specification [fmt].
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For instance, ["%\( %i% \)"] reads any format string that can read a value
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of type [int]; hence [Scanf.sscanf "\\\"%4d\\\"1234.00" "%\(%i%\)"]
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is equivalent to [Scanf.sscanf "1234.00" "%4d"].
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- [l]: returns the number of lines read so far.
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- [n]: returns the number of characters read so far.
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- [N] or [L]: returns the number of tokens read so far.
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- [!]: matches the end of input condition.
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- [%]: matches one [%] character in the input.
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Following the [%] character that introduces a conversion, there may be
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the special flag [_]: the conversion that follows occurs as usual,
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but the resulting value is discarded.
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For instance, if [f] is the function [fun i -> i + 1], then
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[Scanf.sscanf "x = 1" "%_s = %i" f] returns [2].
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The field width is composed of an optional integer literal
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indicating the maximal width of the token to read.
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For instance, [%6d] reads an integer, having at most 6 decimal digits;
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[%4f] reads a float with at most 4 characters; and [%8\[\\000-\\255\]]
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returns the next 8 characters (or all the characters still available,
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if fewer than 8 characters are available in the input).
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Notes:
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- as mentioned above, a [%s] convertion always succeeds, even if there is
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nothing to read in the input: it simply returns [""].
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- in addition to the relevant digits, ['_'] characters may appear
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inside numbers (this is reminiscent to the usual Caml lexical
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conventions). If stricter scanning is desired, use the range
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conversion facility instead of the number conversions.
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- the [scanf] facility is not intended for heavy duty lexical
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analysis and parsing. If it appears not expressive enough for your
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needs, several alternative exists: regular expressions (module
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[Str]), stream parsers, [ocamllex]-generated lexers,
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[ocamlyacc]-generated parsers. *)
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(** {7 Scanning indications in format strings} *)
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(** Scanning indications appear just after the string conversions [%s]
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and [%\[ range \]] to delimit the end of the token. A scanning
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indication is introduced by a [@] character, followed by some
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constant character [c]. It means that the string token should end
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just before the next matching [c] (which is skipped). If no [c]
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character is encountered, the string token spreads as much as
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possible. For instance, ["%s@\t"] reads a string up to the next
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tab character or to the end of input. If a scanning
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indication [\@c] does not follow a string conversion, it is treated
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as a plain [c] character.
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Note:
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- the scanning indications introduce slight differences in the syntax of
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[Scanf] format strings, compared to those used for the [Printf]
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module. However, the scanning indications are similar to those used in
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the [Format] module; hence, when producing formatted text to be scanned
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by [!Scanf.bscanf], it is wise to use printing functions from the
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[Format] module (or, if you need to use functions from [Printf], banish
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or carefully double check the format strings that contain ['\@']
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characters). *)
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(** {7 Exceptions during scanning} *)
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(** Scanners may raise the following exceptions when the input cannot be read
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according to the format string:
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- Raise [Scanf.Scan_failure] if the input does not match the format.
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- Raise [Failure] if a conversion to a number is not possible.
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- Raise [End_of_file] if the end of input is encountered while some more
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characters are needed to read the current conversion specification.
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- Raise [Invalid_argument] if the format string is invalid.
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Note:
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- as a consequence, scanning a [%s] conversion never raises exception
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[End_of_file]: if the end of input is reached the conversion succeeds and
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simply returns the characters read so far, or [""] if none were ever read. *)
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(** {6 Specialized formatted input functions} *)
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val fscanf : in_channel -> ('a, 'b, 'c, 'd) scanner;;
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(** Same as {!Scanf.bscanf}, but reads from the given channel.
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Warning: since all formatted input functions operate from a scanning
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buffer, be aware that each [fscanf] invocation will operate with a
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scanning buffer reading from the given channel. This extra level of
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bufferization can lead to strange scanning behaviour if you use low level
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primitives on the channel (reading characters, seeking the reading
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position, and so on).
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As a consequence, never mixt direct low level reading and high level
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scanning from the same input channel. *)
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val sscanf : string -> ('a, 'b, 'c, 'd) scanner;;
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(** Same as {!Scanf.bscanf}, but reads from the given string. *)
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val scanf : ('a, 'b, 'c, 'd) scanner;;
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(** Same as {!Scanf.bscanf}, but reads from the predefined scanning
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buffer {!Scanf.Scanning.stdib} that is connected to [stdin]. *)
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val kscanf :
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Scanning.scanbuf -> (Scanning.scanbuf -> exn -> 'd) ->
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('a, 'b, 'c, 'd) scanner;;
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(** Same as {!Scanf.bscanf}, but takes an additional function argument
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[ef] that is called in case of error: if the scanning process or
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some conversion fails, the scanning function aborts and calls the
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error handling function [ef] with the scanning buffer and the
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exception that aborted the scanning process as arguments. *)
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(** {6 Reading format strings from input} *)
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val bscanf_format :
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Scanning.scanbuf -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
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(('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g;;
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(** [bscanf_format ib fmt f] reads a format string token from the scannning
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buffer [ib], according to the given format string [fmt], and applies [f] to
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the resulting format string value.
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Raise [Scan_failure] if the format string value read does not have the
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same type as [fmt]. *)
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val sscanf_format :
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string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
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(('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g;;
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(** Same as {!Scanf.bscanf_format}, but reads from the given string. *)
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val format_from_string :
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string ->
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('a, 'b, 'c, 'd, 'e, 'f) format6 -> ('a, 'b, 'c, 'd, 'e, 'f) format6;;
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(** [format_from_string s fmt] converts a string argument to a format string,
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according to the given format string [fmt].
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Raise [Scan_failure] if [s], considered as a format string, does not
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have the same type as [fmt]. *)
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