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# View Source io_lib (stdlib v6.2.2.3)

I/O library functions.

This module contains functions for converting to and from strings (lists of characters). They are used for implementing the functions in the `io` module. There is no guarantee that the character lists returned from some of the functions are flat, they can be deep lists. Function `lists:flatten/1` can be used for flattening deep lists.

# Summary

## Types

chars()

An possibly deep list containing only `char/0`s.

chars_limit()
continuation()

A continuation as returned by `fread/3`.

depth()
format_spec()

A map describing the contents of a format string.

fread_error()
fread_item()
latin1_string()

## Functions

build_text(FormatList)

For details, see `scan_format/2`.

char_list(Term)

Returns `true` if `Term` is a flat list of characters in the Unicode range, otherwise `false`.

deep_char_list(Term)

Returns `true` if `Term` is a, possibly deep, list of characters in the Unicode range, otherwise `false`.

deep_latin1_char_list(Term)

Returns `true` if `Term` is a, possibly deep, list of characters in the ISO Latin-1 range, otherwise `false`.

format(Format, Data)

Equivalent to `fwrite(Format, Data)`.

format(Format, Data, Options)

Equivalent to `fwrite(Format, Data, Options)`.

fread(Format, String)

Tries to read `String` in accordance with the control sequences in `Format`.

fread(Continuation, CharSpec, Format)

This is the re-entrant formatted reader. The continuation of the first call to the functions must be `[]`.

fwrite(Format, Data)

Returns a character list that represents `Data` formatted in accordance with `Format`.

fwrite(Format, Data, Options)

Returns a character list that represents `Data` formatted in accordance with `Format` in the same way as `fwrite/2` and `format/2`, but takes an extra argument, a list of options.

indentation(String, StartIndent)

Returns the indentation if `String` has been printed, starting at `StartIndent`.

latin1_char_list(Term)

Returns `true` if `Term` is a flat list of characters in the ISO Latin-1 range, otherwise `false`.

nl()

Returns a character list that represents a new line character.

print(Term)

Equivalent to `print(Term, 1, 80, -1)`.

print(Term, Column, LineLength, Depth)

Returns a list of characters that represents `Term`, but breaks representations longer than one line into many lines and indents each line sensibly.

printable_latin1_list(Term)

Returns `true` if `Term` is a flat list of printable ISO Latin-1 characters, otherwise `false`.

printable_list(Term)

Returns `true` if `Term` is a flat list of printable characters, otherwise `false`.

printable_unicode_list(Term)

Returns `true` if `Term` is a flat list of printable Unicode characters, otherwise `false`.

scan_format(Format, Data)

Returns a list corresponding to the specified format string, where control sequences have been replaced with corresponding tuples. This list can be passed to

unscan_format(FormatList)

For details, see `scan_format/2`.

write(Term)

Equivalent to `write(Term, -1)`.

write(Term, DepthOrOptions)

Returns a character list that represents `Term`. Option `Depth` controls the depth of the structures written.

write_atom(Atom)

Returns the list of characters needed to print atom `Atom`.

write_atom_as_latin1(Atom)

Returns the list of characters needed to print atom `Atom`. Non-Latin-1 characters are escaped.

write_char(Char)

Returns the list of characters needed to print a character constant in the Unicode character set.

write_char_as_latin1(Char)

Returns the list of characters needed to print a character constant in the Unicode character set. Non-Latin-1 characters are escaped.

write_latin1_char(Latin1Char)

Returns the list of characters needed to print a character constant in the ISO Latin-1 character set.

write_latin1_string(Latin1String)

Returns the list of characters needed to print `Latin1String` as a string.

write_string(String)

Returns the list of characters needed to print `String` as a string.

write_string_as_latin1(String)

Returns the list of characters needed to print `String` as a string. Non-Latin-1 characters are escaped.

# Types

# chars()

    -type chars() :: [char() | chars()].

An possibly deep list containing only `char/0`s.

# chars_limit()

    -type chars_limit() :: integer().

# continuation()

    -opaque continuation()

A continuation as returned by `fread/3`.

# depth()

(not exported)

    -type depth() :: -1 | non_neg_integer().

# format_spec()

    -type format_spec() ::
              #{control_char := char(),
                args := [any()],
                width := none | integer(),
                adjust := left | right,
                precision := none | integer(),
                pad_char := char(),
                encoding := unicode | latin1,
                strings := boolean(),
                maps_order => maps:iterator_order()}.

A map describing the contents of a format string.

- `control_char` is the type of control sequence: `$P`, `$w`, and so on.
- `args` is a list of the arguments used by the control sequence, or an empty list if the control sequence does not take any arguments.
- `width` is the field width.
- `adjust` is the adjustment.
- `precision` is the precision of the printed argument.
- `pad_char` is the padding character.
- `encoding` is set to `true` if translation modifier `t` is present.
- `strings` is set to `false` if modifier `l` is present.
- `maps_order` is set to `undefined` by default, `ordered` if modifier `k` is present, or `reversed` or `CmpFun` if modifier `K` is present.

# fread_error()

    -type fread_error() :: atom | based | character | float | format | input | integer | string | unsigned.

# fread_item()

    -type fread_item() :: string() | atom() | integer() | float().

# latin1_string()

    -type latin1_string() :: [unicode:latin1_char()].

# Functions

# build_text(FormatList)

(since OTP 18.0)

    -spec build_text(FormatList) -> chars() when FormatList :: [char() | format_spec()].

For details, see `scan_format/2`.

# char_list(Term)

    -spec char_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a flat list of characters in the Unicode range, otherwise `false`.

# deep_char_list(Term)

    -spec deep_char_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a, possibly deep, list of characters in the Unicode range, otherwise `false`.

# deep_latin1_char_list(Term)

(since OTP R16B)

    -spec deep_latin1_char_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a, possibly deep, list of characters in the ISO Latin-1 range, otherwise `false`.

# format(Format, Data)

    -spec format(Format, Data) -> chars() when Format :: io:format(), Data :: [term()].

Equivalent to `fwrite(Format, Data)`.

# format(Format, Data, Options)

(since OTP 21.0)

    -spec format(Format, Data, Options) -> chars()
                    when
                        Format :: io:format(),
                        Data :: [term()],
                        Options :: [Option],
                        Option :: {chars_limit, CharsLimit},
                        CharsLimit :: chars_limit().

Equivalent to `fwrite(Format, Data, Options)`.

# fread(Format, String)

    -spec fread(Format, String) -> Result
                   when
                       Format :: string(),
                       String :: string(),
                       Result ::
                           {ok, InputList :: [fread_item()], LeftOverChars :: string()} |
                           {more,
                            RestFormat :: string(),
                            Nchars :: non_neg_integer(),
                            InputStack :: chars()} |
                           {error, {fread, What :: fread_error()}}.

Tries to read `String` in accordance with the control sequences in `Format`.

For a detailed description of the available formatting options, see `io:fread/3`. It is assumed that `String` contains whole lines.

The function returns:

- `{ok, InputList, LeftOverChars}` - The string was read. `InputList` is the list of successfully matched and read items, and `LeftOverChars` are the input characters not used.

- `{more, RestFormat, Nchars, InputStack}` - The string was read, but more input is needed to complete the original format string. `RestFormat` is the remaining format string, `Nchars` is the number of characters scanned, and `InputStack` is the reversed list of inputs matched up to that point.

- `{error, What}` - The read operation failed and parameter `What` gives a hint about the error.

Example:

    3> io_lib:fread("~f~f~f", "15.6 17.3e-6 24.5").
    {ok,[15.6,1.73e-5,24.5],[]}

# fread(Continuation, CharSpec, Format)

    -spec fread(Continuation, CharSpec, Format) -> Return
                   when
                       Continuation :: continuation() | [],
                       CharSpec :: string() | eof,
                       Format :: string(),
                       Return ::
                           {more, Continuation1 :: continuation()} |
                           {done, Result, LeftOverChars :: string()},
                       Result ::
                           {ok, InputList :: [fread_item()]} | eof | {error, {fread, What :: fread_error()}}.

This is the re-entrant formatted reader. The continuation of the first call to the functions must be `[]`.

For a complete description of how the re-entrant input scheme works, see Armstrong, Virding, Williams: 'Concurrent Programming in Erlang', Chapter 13.

The function returns:

- `{done, Result, LeftOverChars}` - The input is complete. The result is one of the following:

  - `{ok, InputList}` - The string was read. `InputList` is the list of successfully matched and read items, and `LeftOverChars` are the remaining characters.

  - `eof` - End of file was encountered. `LeftOverChars` are the input characters not used.

  - `{error, What}` - An error occurred and parameter `What` gives a hint about the error.

- `{more, Continuation}` - More data is required to build a term. `Continuation` must be passed to `fread/3` when more data becomes available.

# fwrite(Format, Data)

    -spec fwrite(Format, Data) -> chars() when Format :: io:format(), Data :: [term()].

Returns a character list that represents `Data` formatted in accordance with `Format`.

For a detailed description of the available formatting options, see `io:fwrite/1,2,3`. If the format string or argument list contains an error, a fault is generated.

If and only if the Unicode translation modifier is used in the format string (that is, `~ts` or `~tc`), the resulting list can contain characters beyond the ISO Latin-1 character range (that is, numbers > 255). If so, the result is still an ordinary Erlang `string/0`, and can well be used in any context where Unicode data is allowed.

# fwrite(Format, Data, Options)

(since OTP 21.0)

    -spec fwrite(Format, Data, Options) -> chars()
                    when
                        Format :: io:format(),
                        Data :: [term()],
                        Options :: [Option],
                        Option :: {chars_limit, CharsLimit},
                        CharsLimit :: chars_limit().

Returns a character list that represents `Data` formatted in accordance with `Format` in the same way as `fwrite/2` and `format/2`, but takes an extra argument, a list of options.

Valid option:

- `{chars_limit, CharsLimit}` - A soft limit on the number of characters returned. When the number of characters is reached, remaining structures are replaced by "`...`". `CharsLimit` defaults to -1, which means no limit on the number of characters returned.

# indentation(String, StartIndent)

    -spec indentation(String, StartIndent) -> integer() when String :: string(), StartIndent :: integer().

Returns the indentation if `String` has been printed, starting at `StartIndent`.

# latin1_char_list(Term)

(since OTP R16B)

    -spec latin1_char_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a flat list of characters in the ISO Latin-1 range, otherwise `false`.

# nl()

    -spec nl() -> string().

Returns a character list that represents a new line character.

# print(Term)

    -spec print(Term) -> chars() when Term :: term().

Equivalent to `print(Term, 1, 80, -1)`.

# print(Term, Column, LineLength, Depth)

    -spec print(Term, Column, LineLength, Depth) -> chars()
                   when
                       Term :: term(),
                       Column :: non_neg_integer(),
                       LineLength :: non_neg_integer(),
                       Depth :: depth().

Returns a list of characters that represents `Term`, but breaks representations longer than one line into many lines and indents each line sensibly.

Also tries to detect and output lists of printable characters as strings.

- `Column` is the starting column; defaults to 1.
- `LineLength` is the maximum line length; defaults to 80.
- `Depth` is the maximum print depth; defaults to -1, which means no limitation.

# printable_latin1_list(Term)

(since OTP R16B)

    -spec printable_latin1_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a flat list of printable ISO Latin-1 characters, otherwise `false`.

# printable_list(Term)

    -spec printable_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a flat list of printable characters, otherwise `false`.

What is a printable character in this case is determined by startup flag `+pc` to the Erlang VM; see `io:printable_range/0` and `erl(1)`.

# printable_unicode_list(Term)

(since OTP R16B)

    -spec printable_unicode_list(Term) -> boolean() when Term :: term().

Returns `true` if `Term` is a flat list of printable Unicode characters, otherwise `false`.

# scan_format(Format, Data)

(since OTP 18.0)

    -spec scan_format(Format, Data) -> FormatList
                         when
                             Format :: io:format(), Data :: [term()], FormatList :: [char() | format_spec()].

Returns a list corresponding to the specified format string, where control sequences have been replaced with corresponding tuples. This list can be passed to:

- `build_text/1` to have the same effect as `format(Format, Args)`
- `unscan_format/1` to get the corresponding pair of `Format` and `Args` (with every `*` and corresponding argument expanded to numeric values)

A typical use of this function is to replace unbounded-size control sequences like `~w` and `~p` with the depth-limited variants `~W` and `~P` before formatting to text in, for example, a logger.

# unscan_format(FormatList)

(since OTP 18.0)

    -spec unscan_format(FormatList) -> {Format, Data}
                           when
                               FormatList :: [char() | format_spec()],
                               Format :: io:format(),
                               Data :: [term()].

For details, see `scan_format/2`.

# write(Term)

    -spec write(Term) -> chars() when Term :: term().

Equivalent to `write(Term, -1)`.

# write(Term, DepthOrOptions)

    -spec write(Term, Depth) -> chars() when Term :: term(), Depth :: depth();
               (Term, Options) -> chars()
                   when
                       Term :: term(),
                       Options :: [Option],
                       Option ::
                           {chars_limit, CharsLimit} | {depth, Depth} | {encoding, latin1 | utf8 | unicode},
                       CharsLimit :: chars_limit(),
                       Depth :: depth().

Returns a character list that represents `Term`. Option `Depth` controls the depth of the structures written.

When the specified depth is reached, everything below this level is replaced by "`...`".

`Depth` defaults to -1, which means no limitation. Option `CharsLimit` puts a soft limit on the number of characters returned. When the number of characters is reached, remaining structures are replaced by "`...`". `CharsLimit` defaults to -1, which means no limit on the number of characters returned.

Example:

    1> lists:flatten(io_lib:write({1,[2],[3],[4,5],6,7,8,9})).
    "{1,[2],[3],[4,5],6,7,8,9}"
    2> lists:flatten(io_lib:write({1,[2],[3],[4,5],6,7,8,9}, 5)).
    "{1,[2],[3],[...],...}"
    3> lists:flatten(io_lib:write({[1,2,3],[4,5],6,7,8,9}, [{chars_limit,20}])).
    "{[1,2|...],[4|...],...}"

# write_atom(Atom)

    -spec write_atom(Atom) -> chars() when Atom :: atom().

Returns the list of characters needed to print atom `Atom`.

# write_atom_as_latin1(Atom)

(since OTP 20.0)

    -spec write_atom_as_latin1(Atom) -> latin1_string() when Atom :: atom().

Returns the list of characters needed to print atom `Atom`. Non-Latin-1 characters are escaped.

# write_char(Char)

    -spec write_char(Char) -> chars() when Char :: char().

Returns the list of characters needed to print a character constant in the Unicode character set.

# write_char_as_latin1(Char)

(since OTP R16B)

    -spec write_char_as_latin1(Char) -> latin1_string() when Char :: char().

Returns the list of characters needed to print a character constant in the Unicode character set. Non-Latin-1 characters are escaped.

# write_latin1_char(Latin1Char)

(since OTP R16B)

    -spec write_latin1_char(Latin1Char) -> latin1_string() when Latin1Char :: unicode:latin1_char().

Returns the list of characters needed to print a character constant in the ISO Latin-1 character set.

# write_latin1_string(Latin1String)

(since OTP R16B)

    -spec write_latin1_string(Latin1String) -> latin1_string() when Latin1String :: latin1_string().

Returns the list of characters needed to print `Latin1String` as a string.

# write_string(String)

    -spec write_string(String) -> chars() when String :: string().

Returns the list of characters needed to print `String` as a string.

# write_string_as_latin1(String)

(since OTP R16B)

    -spec write_string_as_latin1(String) -> latin1_string() when String :: string().

Returns the list of characters needed to print `String` as a string. Non-Latin-1 characters are escaped.