# 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:

- 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 `f`,
- scanning indications to specify boundaries of tokens.

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:

- `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 next white space, the next scanning indication, or the end-of-input is 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 to the format specified by the internal format `fmt`. The format string to be read must have the same type as the internal format `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 `fmt`.
- `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).

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.

Notes:

- the scanning indications introduce slight differences in the syntax of `Scanf` format strings compared to those used by the `Printf` module. However, scanning indications are similar to those of the `Format` module; hence, when producing formatted text to be scanned by `!Scanf.bscanf`, it is wise to use printing functions from `Format` (or, if you need to use functions from `Printf`, banish or carefully double check the format strings that contain `'@'` characters).

- in addition to 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.

    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`.