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% lfe_io(3) % Robert Virding % 2008-2019


lfe_io - Lisp Flavoured Erlang (LFE) io functions


This module provides a standard set of io functions for LFE. In the following description, many functions have an optional parameter IoDevice. If included, it must be the pid of a process which handles the IO protocols such as the IoDevice returned by file:open/2.

Two functions in this module are used to generate aesthetically attractive representations of abstract forms, which are suitable for printing. These functions return (possibly deep) lists of characters and generate an error if the form is wrong.


chars() = [char() | chars()]

filesexpr() = {Sexpr,Line}

This is the format returned by lfe_io:parse_file/1 and is used by the compiler to give better error information.


read([[IoDevice,] Prompt]) -> {ok,Sexpr} | {error,ErrorInfo} | eof

Read an s-expr from the standard input (IoDevice) with a prompt (Prompt). Note that this is not line-oriented in that it stops as soon as it has consumed enough characters.

read_line([[IoDevice,] Prompt]) -> {ok,Sexpr} | {error,ErrorInfo} | eof

Read an s-expr from the standard input (IoDevice) with a prompt (Prompt). Note that this is line-oriented in that it reads whole lines discarding left-over characters in the last line.

read_string(String) -> {ok,Sexpr} | {error,ErrorInfo}

Read an s-expr from String. Note that this only reads from String discarding left-over characters at the end of the string.

print([IoDevice,] Sexpr) -> ok

Print the s-expr Sexpr to the standard output (IoDevice).

print1(Sexpr) -> DeepCharList

Return the list of characters which represent the s-expr Sexpr.

prettyprint([IoDevice,] Sexpr) -> ok

Pretty print the s-expr Sexpr to the standard output (IoDevice).

prettyprint1(Sexpr) -> DeepCharList

prettyprint1(Sexpr, Depth) -> DeepCharList

prettyprint1(Sexpr, Depth, Indentation) -> DeepCharList

prettyprint1(Sexpr, Depth, Indentation, LineLength) -> DeepCharList

Return the list of characters which represents the prettyprinted s-expr Sexpr. Default values for Depth is 30, Indentation is 0 and LineLength is 80.

format([IoDevice,] Format, Args) -> ok

fwrite([IoDevice,] Format, Args) -> ok

format1(Format, Args) -> DeepCharList

fwrite1(Format, Args) -> DeepCharList

Print formatted output. The following commands are valid in the format string:

  • ~w, ~W - print LFE terms
  • ~p, ~P - prettyprint LFE terms
  • ~s - print a string
  • ~e, ~f, ~g - print floats
  • ~b, ~B - based integers
  • ~x, ~X - based integers with a prefix
  • ~+, ~# - based integers in vanilla erlang format
  • ~c, ~n, ~i

Currently they behave as for vanilla erlang except that ~w, ~W, ~p, ~P print the terms as LFE sexprs.

read_file(FileName) -> {ok,[Sexpr]} | {error,ErrorInfo}

Read the file Filename returning a list of s-exprs (as it should be).

parse_file(FileName) -> {ok,[FileSexpr]} | {error,ErrorInfo}


FileSexpr = filesexpr()

Read the file Filename returning a list of pairs containing s-expr and line number of the start of the s-expr.

scan_sexpr(Cont, Chars [,Line]) -> {done,Ret,RestChars}|{more,Cont1}

This is a re-entrant call which scans tokens from the input and returns a parsed sepxr. If there are enough characters to parse a sexpr or it detects and error then it returns {done,...} otherwise it returns {more,Cont} where Cont is used in the next call to scan_sexpr with more characters to try and parse a sexpr. This is continued until a sexpr has been parsed. Cont is initially [].

It is not designed to be called directly by an application but used through the i/o system where it can typically be called in an application by:

io:request(In, {get_until,unicode,Prompt,Module,scan_sexpr,[Line]})


The ErrorInfo mentioned above is the standard ErrorInfo structure which is returned from all IO modules. It has the following format:


A string describing the error is obtained with the following call:

apply(Module, format_error, ErrorDescriptor)