Module Scanf

module Scanf: sig .. end
Formatted input functions.


Functional input with format strings

The module Scanf provides formatted input functions or 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 scanning buffer and has type Scanf.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:

Hence, a typical call to the formatted input function Scanf.bscanf is bscanf ib fmt f, where:

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,

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.

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 assigment 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.
module Scanning: sig .. end
Scanning buffers

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 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.

Format string description


The space character in format strings

As mentioned above, a plain character in the format string is just matched with the characters of the input; however, one character is a special exception to this simple rule: the space character (ASCII code 32) does not match a single space character, but any amount of ``whitespace'' in the input. More precisely, a space inside the format string matches any number of tab, space, line feed and carriage return characters.

Matching 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) succeds and returns 1 when reading an input with various whitespace in it, such as Price = 1 $, Price = 1 $, or even Price=1$.

Conversion specifications in format strings


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 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.


Exceptions during scanning

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


Specialized 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 mixt 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
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.

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 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 does not have the same type as fmt.
val sscanf_format : string ->
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.