Module Scanf

module Scanf: sig .. end
Formatted input functions.

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. Hence, a typical call to a formatted input function bscan is bscan 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.

module Scanning: sig .. end
Scanning buffers
exception Scan_failure of string
The exception raised by formatted input functions when the input cannot be read according to the given format.
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 some 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.

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:

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:

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.


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 -> 'a) -> ('b, 'c, 'd, 'a) 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.
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.