package batteries

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Formatted input functions.

  • author Pierre Weis (Base module)
  • author David Teller
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 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 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 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.
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.

Formatted input as a functional feature

The OCaml 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 OCaml 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 OCaml formatted input facility is fully type-checked at compile time.

module Scanning : sig ... end
Type of formatted input functions
type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.scanbuf, 'b, 'c, 'a -> 'd, 'd) Pervasives.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.

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 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.
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) succeeds 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

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 OCaml).
  • 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 OCaml).
  • 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 OCaml (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).


  • 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 OCaml 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.
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.


  • 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).
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.


  • 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.
Specialized formatted input functions
val fscanf : Pervasives.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

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.

Reading format strings from input
val bscanf_format : Scanning.scanbuf -> ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6 -> (('a, 'b, 'c, 'd, 'e, 'f) Pervasives.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.

  • raises 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) Pervasives.format6 -> (('a, 'b, 'c, 'd, 'e, 'f) Pervasives.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) Pervasives.format6 -> ('a, 'b, 'c, 'd, 'e, 'f) Pervasives.format6

format_from_string s fmt converts a string argument to a format string, according to the given format string fmt.

  • raises Scan_failure

    if s, considered as a format string, does not have the same type as fmt.


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