io(3)

MODULE
io - Standard I/O Server Interface Functions

DESCRIPTION

This module provides an interface to standard Erlang IO servers. The output functions all return ok if they are successful, or exit if they are not. In the following description, a parameter within square brackets means that that parameter is optional. [IoDevice, ] is such an example. If included, it must be the Pid of a process which handles the IO protocols. This is often the IoDevice returned by file:open/2 (see file). For a description of the I/O protocols refer to Armstrong, Virding and Williams, 'Concurrent Programming in Erlang', Chapter 13.

EXPORTS

put_chars([IoDevice,] Chars)

    Writes the characters Chars to the standard output (IoDevice). Chars is a list of characters. The list is not necessarily flat.

nl([IoDevice])

    Writes new line to the standard output (IoDevice).

get_chars([IoDevice,] Prompt, Count)

    Gets Count characters from standard input (IoDevice), prompting it with Prompt. It returns:

        ListOfChars:
            Returns the input characters, if they are less thanCount.

        eof:
            End of file was encountered.

get_line([IoDevice,] Prompt)

    Gets a line from the standard input (IoDevice), prompting it with Prompt. It returns:

        ListOfChars:
            The characters in the line terminated by a LF unless the line read was the last line of the file and was not terminated by LF.

        eof:
            End of file was encountered.

write([IoDevice,] Term)

    Writes the term Term to the standard output (IoDevice).

read([IoDevice,] Prompt)

    Reads a term from the standard input (IoDevice), prompting it with Prompt. It returns:

        {ok, Term}:
            The parsing was successful.

        {error, ErrorInfo}:
            The parsing failed.

        eof:
            End of file was encountered.

fwrite(Format)
format(Format)

    Equivalent to fwrite(Format, []).

fwrite([IoDevice,] Format, Arguments)
format([IoDevice,] Format, Arguments)

    Writes the list of items in Arguments on the standard output (IoDevice) in accordance with Format. Format is a list of plain characters which are copied to the output device, and control sequences which cause the arguments to be printed. If Format is an atom, it is first converted to a list with the aid of atom_to_list/1.Arguments is the list of items to be printed.
    > io:fwrite("Hello world!~n", []).
    Hello world
    ok

    The general format of a control sequence is ~F.P.PadC. The character C determines the type of control sequence to be used, F and P are optional numeric arguments. If F, P, or Pad is *, the next argument in Arguments is used as the numeric value of F or P.

    F is the field width of the printed argument. A negative value means that the argument will be left justified within the field, otherwise it will be right justified. If no field width is specified, the required print width will be used. If the field width specified is too small, then the whole field will be filled with * characters.

    P is the precision of the printed argument. A default value is used if no precision is specified. The interpretation of precision depends on the control sequences. Unless otherwise specified, the argument within is used to determine print width.

    Pad is the padding character. This is the character used to pad the printed representation of the argument so that it conforms to the specified field width and precision. Only one padding character can be specified and, whenever applicable, it is used for both the field width and precision. The default padding character is ' ' (space).

    The following control sequences are available:

        ~:  The character ~ is written.

        c:  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 one. The following example illustrates:
            > io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).
            |     aaaaa|aaaaa     |ccccc|
            ok

        f:  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.

        e:  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.

        g:  The argument is a float which is written as f, if it is > 0.1, and < 10^4. Otherwise, it is written as e. The precision is the number of significant digits. It defaults to 6. There must always be a sufficient number of digits for printing a correct floating point representation of the argument.

        s:  Prints the argument with the string syntax. The argument is a list of character codes (possibly not a flat list), or an atom. The characters are printed without quotes. In this format, the printed argument is truncated to the given precision and field width.

            This format can be used for printing any object and truncating the output so it fits a specified field:
            > io:fwrite("|~10w|~n", [{hey, hey, hey}]).
            |**********|
            ok
            > io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).
            |{hey, hey, h|
            ok

        w:  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 in the default g format.

        p:  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. For example:
            > T = [{attributes,[[{id,age,1.50000},{mode,explicit},
                   {typename,"INTEGER"}],
                   [{id,cho},{mode,explicit},{typename,'Cho'}]]},
                   {typename,'Person'},{tag,{'PRIVATE',3}},
                   {mode,implicit}].
            ...
            > io:fwrite("~w~n", [T]).
            [{attributes,[[{id,age,1.50000},{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
            > io:fwrite("~p~n", [T]).
            [{attributes,[[{id,age,1.50000},
                           {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 orio:format. For example, using T above:
            > io:fwrite("Here T = ~p~n", [T]).
            Here T = [{attributes,[[{id,age,1.50000},
                                    {mode,explicit},
                                    {typename,"INTEGER"}],
                                   [{id,cho},{mode,explicit},
                                    {typename,'Cho'}]]},
                      {typename,'Person'},
                      {tag,{'PRIVATE',3}},
                      {mode,implicit}]
            ok

        W:  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:
            > io:fwrite("~W~n", [T,9]).
            [{attributes,[[{id,age,1.50000},{mode,explicit},{typename|
            ...}],[{id,cho},{mode|...},{...}]]},{typename,'Person'},{t
            ag,{'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.

        P:  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:
            > io:fwrite("~P~n", [T,9]).
            [{attributes,[[{id,age,1.50000},{mode,explicit},
                           {typename|...}],
                          [{id,cho},{mode|...},{...}]]},
             {typename,'Person'},
             {tag,{'PRIVATE',3}},
             {mode,implicit}]
            ok

        n:  Writes a new line.

        i:  Ignores the next term.

    Returns:

        ok: The formatting succeeded.

    If an error occurs, there is no output. For example:
    > io:fwrite("~s ~w ~i ~w ~c ~n",['abc def', 'abc def',
                                     {foo, 1},{foo, 1}, 65]).
    abc def 'abc def'  {foo, 1} A
    ok
    > io:fwrite("~s", [65]).
    ** exited: {badarg,[{io,format,[<0.21.0>,"~s","A"]},
                        {erl_eval,expr,3},
                        {erl_eval,exprs,4},
                        {shell,eval_loop,2}]} **

    In this example, an attempt was made to output the single character '65' with the aid of the string formatting directive "~s".

    The two functions fwrite and format are identical. The old name format has been retained for backwards compatibility, while the new name fwrite has been added as a logical complement to fread.

fread([IoDevice,] Prompt, Format)

    Reads characters from the standard input (IoDevice), prompting it with Prompt. Interprets the characters in accordance with Format. Format is a list of control sequences which directs the interpretation of the input.

    Format may contain:

        *   White space characters (SPACE, TAB and NEWLINE) which cause input to be read to the next non-white space character.

        *   Ordinary characters which must match the next input character.

        *   Control sequences, which have the general format~*FC. The character * is an optional return suppression character. It provides a method to specify a field which is to be omitted. F is the field width of the input field and C determines the type of control sequence.

            Unless otherwise specified, leading white-space is ignored for all control sequences. An input field cannot be more than one line wide. The following control sequences are available:

                ~:  A single ~ is expected in the input.

                d:  A decimal integer is expected.

                f:  A floating point number is expected. It must follow the Erlang floating point number syntax.

                s:  A string of non-white-space characters is read. If a field width has been specified, this number of characters are read and all trailing white-space characters are stripped. An Erlang string (list of characters) is returned.

                a:  Similar to s, but the resulting string is converted into an atom.

                c:  The number of characters equal to the field width are read (default is 1) and returned as an Erlang string. However, leading and trailing white-space characters are not omitted as they are with s. All characters are returned.

                l:  Returns the number of characters which have been scanned up to that point, including white-space characters.

            It returns:

                {ok, InputList}:
                    The read was successful and InputList is the list of successfully matched and read items.

                {error, What}:
                    The read operation failed and the parameterWhat can be used as argument toreport_error/1 to produce an error message.

                eof:
                    End of file was encountered.

    Examples:
    > io:fread('enter>', "~f~f~f").
    enter>1.9 35.5e3 15.0
    {ok, [1.90000, 3.55000e+4, 15.0000]}
    > io:fread('enter>', "~10f~d").
    enter>     5.67899
    {ok, [5.67800, 99]}
    > io:fread('enter>', ":~10s:~10c:").
    enter>:   alan  :   joe   :
    {ok, ["alan", "   joe    "]}

scan_erl_exprs(Prompt)
scan_erl_exprs([IoDevice,] Prompt, StartLine)

    Reads data from the standard input (IoDevice), prompting it with Prompt. Reading starts at line number StartLine (1). The data is tokenized as if it were a sequence of Erlang expressions until a final '.' is reached. This token is also returned. It returns:

        {ok, Tokens, EndLine}:
            The tokenization succeeded.

        {error, ErrorInfo, EndLine}:
            An error occurred.

        {eof, EndLine}:
            End of file was encountered.

    Example:
    > io:scan_erl_exprs('enter>').
    enter>abc(), "hey".
    {ok,[{atom, 1, abc},{'(', 1}, {')', 1}, {', ', 1},
        {string, 1, "hey"}, {dot, 1}], 2}
    > io:scan_erl_exprs('enter>').
    enter>1.0er.
    {error, {1, erl_scan, float}, 2}

scan_erl_form(Prompt)
scan_erl_form(IoDevice, Prompt[, StartLine])

    Reads data from the standard input (IoDevice), prompting it with Prompt. Starts reading at line number StartLine (1). The data is tokenized as if it were an Erlang form - one of the valid Erlang expressions in an Erlang source file - until a final '.' is reached. This last token is also returned. The return values are the same as for scan_erl_exprs.

parse_erl_exprs(Prompt)
parse_erl_exprs(IoDevice, Prompt[, StartLine])

    Reads data from the standard input (IoDevice), prompting it with Prompt. Starts reading at line number StartLine (1). The data is tokenized and parsed as if it were a sequence of Erlang expressions until a final '.' is reached. It returns:

        {ok, ExpressionList, EndLine}:
            The parsing was successful.

        {error, ErrorInfo, EndLine}:
            An error occurred.

        {eof, EndLine}:
            End of file was encountered.

    Example:
    > io:parse_erl_exprs('enter>').
    enter>abc(), "hey".
    {ok, [{call, 1, [], abc, []}, {string, 1, "hey"}], 2}
    > io:parse_erl_exprs ('enter>').
    enter>abc("hey".
    {error, {1, erl_parse, {before, {terminator,') '}, {dot, 1}}}, 2}

parse_erl_form(Prompt)
parse_erl_form(IoDevice, Prompt[, StartLine])

    Reads data from the standard input (IoDevice), prompting it with Prompt Starts reading at line number StartLine (1). The data is tokenized and parsed as if it were an Erlang form - one of the valid Erlang expressions in an Erlang source file - until a final '.' is reached. It returns:

        {ok, Form, EndLine}:
            The parsing was successful.

        {error, ErrorInfo, EndLine}:
            An error occurred.

        {eof, EndLine}:
            End of file was encountered.

Standard Input/Output

All Erlang processes have a default standard IO device. This device is used when no IoDevice argument is specified in the IO calls. However, it is sometimes desirable to use an explicit IoDevice argument which refers to the default IO device. This is the case with functions that can access either a file or the default IO device. The atom standard_io has this special meaning. The following example illustrates this:
  > io:read('enter>').
  enter>foo.
  {term, foo}
  > io:read(standard_io, 'enter>').
  enter>bar.
  {term, bar}

There is always a process registered under the name of user. This can be used for sending output to the user.

Error Information

The ErrorInfo mentioned above is the standard ErrorInfo structure which is returned from all IO modules. It has the following format:
    {ErrorLine, Module, ErrorDescriptor}

A string which describes the error is obtained with the following call:
apply(Module, format_error, ErrorDescriptor)

AUTHOR
Robert Virding - support@erlang.ericsson.se