io

If set in binary mode (binary or {binary, true}), the I/O server sends binary data (encoded in UTF-8) as answers to the get_line, get_chars and, if possible, get_until requests (see the I/O protocol description in STDLIB User's Guide for details). The immediate effect is that get_chars/2,3 and get_line/1,2 return UTF-8 binaries instead of lists of chars for the affected IO device.

By default, all IO devices in OTP are set in list mode, but the I/O functions can handle any of these modes and so should other, user written, modules behaving as clients to I/O-servers.

This option is supported by the standard shell (group.erl), the 'oldshell' (user.erl) and the file I/O servers.

Denotes if the terminal should echo input. Only supported for the standard shell I/O-server (group.erl)

Provide a function for tab-completion (expansion) like the Erlang shell. This function is called when the user presses the TAB key. The expansion is active when calling line-reading functions such as get_line/1,2.

The function is called with the current line, upto the cursor, as a reversed string. It should return a three-tuple: {yes|no, string(), [string(), ...]}. The first element gives a beep if no, otherwise the expansion is silent, the second is a string that will be entered at the cursor position, and the third is a list of possible expansions. If this list is non-empty, the list will be printed and the current input line will be written once again.

Trivial example (beep on anything except empty line, which is expanded to "quit"):

 fun("") -> {yes, "quit", []};
    (_) -> {no, "", ["quit"]} end

This option is supported by the standard shell only (group.erl).

Specifies how characters are input or output from or to the actual IO device, implying that i.e. a terminal is set to handle Unicode input and output or a file is set to handle UTF-8 data encoding.

The option does not affect how data is returned from the I/O functions or how it is sent in the I/O-protocol, it only affects how the IO device is to handle Unicode characters towards the "physical" device.

The standard shell will be set for either Unicode or latin1 encoding when the system is started. The actual encoding is set with the help of the LANG or LC_CTYPE environment variables on Unix-like system or by other means on other systems. The bottom line is that the user can input Unicode characters and the IO device will be in {encoding, unicode} mode if the IO device supports it. The mode can be changed, if the assumption of the runtime system is wrong, by setting this option.

The IO device used when Erlang is started with the "-oldshell" or "-noshell" flags is by default set to latin1 encoding, meaning that any characters beyond codepoint 255 will be escaped and that input is expected to be plain 8-bit ISO-latin-1. If the encoding is changed to Unicode, input and output from the standard file descriptors will be in UTF-8 (regardless of operating system).

Files can also be set in {encoding, unicode}, meaning that data is written and read as UTF-8. More encodings are possible for files, see below.

{encoding, unicode | latin1} is supported by both the standard shell (group.erl including werl on Windows®), the 'oldshell' (user.erl) and the file I/O servers.

For disk files, the encoding can be set to various UTF variants. This will have the effect that data is expected to be read as the specified encoding from the file and the data will be written in the specified encoding to the disk file.

{encoding, utf8} will have the same effect as {encoding, unicode} on files.

The extended encodings are only supported on disk files (opened by the file:open/2 function)

The character ~ is written.

The argument is a number that will be interpreted as an ASCII code. The precision is the number of times the character is printed and it defaults to the field width, which in turn defaults to 1. The following example illustrates:

1> io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).
|     aaaaa|bbbbb     |ccccc|
ok

If the Unicode translation modifier (t) is in effect, the integer argument can be any number representing a valid Unicode codepoint, otherwise it should be an integer less than or equal to 255, otherwise it is masked with 16#FF:

2> io:fwrite("~tc~n",[1024]).
\x{400}
ok
3> io:fwrite("~c~n",[1024]).
^@
ok

The argument is a float which is written as [-]ddd.ddd, where the precision is the number of digits after the decimal point. The default precision is 6 and it cannot be less than 1.

The argument is a float which is written as [-]d.ddde+-ddd, where the precision is the number of digits written. The default precision is 6 and it cannot be less than 2.

The argument is a float which is written as f, if it is >= 0.1 and < 10000.0. Otherwise, it is written in the e format. The precision is the number of significant digits. It defaults to 6 and should not be less than 2. If the absolute value of the float does not allow it to be written in the f format with the desired number of significant digits, it is also written in the e format.

Prints the argument with the string syntax. The argument is, if no Unicode translation modifier is present, an iolist(), a binary(), or an atom(). If the Unicode translation modifier (t) is in effect, the argument is unicode:chardata(), meaning that binaries are in UTF-8. The characters are printed without quotes. The string is first truncated by the given precision and then padded and justified to the given field width. The default precision is the field width.

This format can be used for printing any object and truncating the output so it fits a specified field:

1> io:fwrite("|~10w|~n", [{hey, hey, hey}]).
|**********|
ok
2> io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).
|{hey,hey,h|
3> io:fwrite("|~-10.8s|~n", [io_lib:write({hey, hey, hey})]).
|{hey,hey  |
ok

A list with integers larger than 255 is considered an error if the Unicode translation modifier is not given:

4> io:fwrite("~ts~n",[[1024]]).
\x{400}
ok
5> io:fwrite("~s~n",[[1024]]).
** exception exit: {badarg,[{io,format,[<0.26.0>,"~s~n",[[1024]]]},
   ...

Writes data with the standard syntax. This is used to output Erlang terms. Atoms are printed within quotes if they contain embedded non-printable characters, and floats are printed accurately as the shortest, correctly rounded string.

Writes the data with standard syntax in the same way as ~w, but breaks terms whose printed representation is longer than one line into many lines and indents each line sensibly. It also tries to detect lists of printable characters and to output these as strings. The Unicode translation modifier is used for determining what characters are printable. For example:

1> T = [{attributes,[[{id,age,1.50000},{mode,explicit},
{typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},
{typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].
...
2> io:fwrite("~w~n", [T]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,
[73,78,84,69,71,69,82]}],[{id,cho},{mode,explicit},{typena
me,'Cho'}]]},{typename,'Person'},{tag,{'PRIVATE',3}},{mode
,implicit}]
ok
3> io:fwrite("~62p~n", [T]).
[{attributes,[[{id,age,1.5},
               {mode,explicit},
               {typename,"INTEGER"}],
              [{id,cho},{mode,explicit},{typename,'Cho'}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok

The field width specifies the maximum line length. It defaults to 80. The precision specifies the initial indentation of the term. It defaults to the number of characters printed on this line in the same call to io:fwrite or io:format. For example, using T above:

4> io:fwrite("Here T = ~62p~n", [T]).
Here T = [{attributes,[[{id,age,1.5},
                        {mode,explicit},
                        {typename,"INTEGER"}],
                       [{id,cho},
                        {mode,explicit},
                        {typename,'Cho'}]]},
          {typename,'Person'},
          {tag,{'PRIVATE',3}},
          {mode,implicit}]
ok

When the modifier l is given no detection of printable character lists will take place. For example:

5> S = [{a,"a"}, {b, "b"}].
6> io:fwrite("~15p~n", [S]).
[{a,"a"},
 {b,"b"}]
ok
7> io:fwrite("~15lp~n", [S]).
[{a,[97]},
 {b,[98]}]
ok

Binaries that look like UTF-8 encoded strings will be output with the string syntax if the Unicode translation modifier is given:

9> io:fwrite("~p~n",[[1024]]).
[1024]
10> io:fwrite("~tp~n",[[1024]]).
"\x{400}"
11> io:fwrite("~tp~n", [<<128,128>>]).
<<128,128>>
12> io:fwrite("~tp~n", [<<208,128>>]).
<<"\x{400}"/utf8>>
ok

Writes data in the same way as ~w, but takes an extra argument which is the maximum depth to which terms are printed. Anything below this depth is replaced with .... For example, using T above:

8> io:fwrite("~W~n", [T,9]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
[{id,cho},{mode,...},{...}]]},{typename,'Person'},
{tag,{'PRIVATE',3}},{mode,implicit}]
ok

If the maximum depth has been reached, then it is impossible to read in the resultant output. Also, the ,... form in a tuple denotes that there are more elements in the tuple but these are below the print depth.

Writes data in the same way as ~p, but takes an extra argument which is the maximum depth to which terms are printed. Anything below this depth is replaced with .... For example:

9> io:fwrite("~62P~n", [T,9]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
              [{id,cho},{mode,...},{...}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok

Writes an integer in base 2..36, the default base is 10. A leading dash is printed for negative integers.

The precision field selects base. For example:

1> io:fwrite("~.16B~n", [31]).
1F
ok
2> io:fwrite("~.2B~n", [-19]).
-10011
ok
3> io:fwrite("~.36B~n", [5*36+35]).
5Z
ok

Like B, but takes an extra argument that is a prefix to insert before the number, but after the leading dash, if any.

The prefix can be a possibly deep list of characters or an atom.

1> io:fwrite("~X~n", [31,"10#"]).
10#31
ok
2> io:fwrite("~.16X~n", [-31,"0x"]).
-0x1F
ok

Like B, but prints the number with an Erlang style #-separated base prefix.

1> io:fwrite("~.10#~n", [31]).
10#31
ok
2> io:fwrite("~.16#~n", [-31]).
-16#1F
ok

Like B, but prints lowercase letters.

Like X, but prints lowercase letters.

Like #, but prints lowercase letters.

Writes a new line.

Ignores the next term.

The formatting succeeded.