Reference Manual


Table of Contents




Module Summary

The Erlang token scanner.


This module contains functions for tokenizing (scanning) characters into Erlang tokens.

Data Types

category() = atom()
option() =
    return | return_white_spaces | return_comments | text |
    {reserved_word_fun, resword_fun()}

symbol() = atom() | float() | integer() | string()
resword_fun() = fun((atom()) -> boolean())
token() =
    {category(), Anno :: erl_anno:anno(), symbol()} |
    {category(), Anno :: erl_anno:anno()}

tokens_result() =
    {ok, Tokens :: tokens(), EndLocation :: erl_anno:location()} |
    {eof, EndLocation :: erl_anno:location()} |
     ErrorInfo :: error_info(),
     EndLocation :: erl_anno:location()}

category(Token) -> category()
OTP 18.0


Token = token()

Returns the category of Token.

column(Token) -> erl_anno:column() | undefined
OTP 18.0


Token = token()

Returns the column of Token's collection of annotations.

end_location(Token) -> erl_anno:location() | undefined
OTP 18.0


Token = token()

Returns the end location of the text of Token's collection of annotations. If there is no text, undefined is returned.

format_error(ErrorDescriptor) -> string()


ErrorDescriptor = error_description()

Uses an ErrorDescriptor and returns a string that describes the error or warning. This function is usually called implicitly when an ErrorInfo structure is processed (see section Error Information).

line(Token) -> erl_anno:line()
OTP 18.0


Token = token()

Returns the line of Token's collection of annotations.

location(Token) -> erl_anno:location()
OTP 18.0


Token = token()

Returns the location of Token's collection of annotations.

reserved_word(Atom :: atom()) -> boolean()

Returns true if Atom is an Erlang reserved word, otherwise false.

string(String) -> Return
string(String, StartLocation) -> Return
string(String, StartLocation, Options) -> Return


String = string()
Options = options()
Return =
    {ok, Tokens :: tokens(), EndLocation} |
    {error, ErrorInfo :: error_info(), ErrorLocation}
StartLocation = EndLocation = ErrorLocation = erl_anno:location()

Takes the list of characters String and tries to scan (tokenize) them. Returns one of the following:

{ok, Tokens, EndLocation}

Tokens are the Erlang tokens from String. EndLocation is the first location after the last token.

{error, ErrorInfo, ErrorLocation}

An error occurred. ErrorLocation is the first location after the erroneous token.

string(String) is equivalent to string(String, 1), and string(String, StartLocation) is equivalent to string(String, StartLocation, []).

StartLocation indicates the initial location when scanning starts. If StartLocation is a line, Anno, EndLocation, and ErrorLocation are lines. If StartLocation is a pair of a line and a column, Anno takes the form of an opaque compound data type, and EndLocation and ErrorLocation are pairs of a line and a column. The token annotations contain information about the column and the line where the token begins, as well as the text of the token (if option text is specified), all of which can be accessed by calling column/1, line/1, location/1, and text/1.

A token is a tuple containing information about syntactic category, the token annotations, and the terminal symbol. For punctuation characters (such as ; and |) and reserved words, the category and the symbol coincide, and the token is represented by a two-tuple. Three-tuples have one of the following forms:

  • {atom, Anno, atom()}
  • {char, Anno, char()}
  • {comment, Anno, string()}
  • {float, Anno, float()}
  • {integer, Anno, integer()}
  • {var, Anno, atom()}
  • {white_space, Anno, string()}

Valid options:

{reserved_word_fun, reserved_word_fun()}

A callback function that is called when the scanner has found an unquoted atom. If the function returns true, the unquoted atom itself becomes the category of the token. If the function returns false, atom becomes the category of the unquoted atom.


Return comment tokens.


Return white space tokens. By convention, a newline character, if present, is always the first character of the text (there cannot be more than one newline in a white space token).


Short for [return_comments, return_white_spaces].


Include the token text in the token annotation. The text is the part of the input corresponding to the token.

symbol(Token) -> symbol()
OTP 18.0


Token = token()

Returns the symbol of Token.

text(Token) -> erl_anno:text() | undefined
OTP 18.0


Token = token()

Returns the text of Token's collection of annotations. If there is no text, undefined is returned.

tokens(Continuation, CharSpec, StartLocation) -> Return
tokens(Continuation, CharSpec, StartLocation, Options) -> Return


Continuation = return_cont() | []
CharSpec = char_spec()
StartLocation = erl_anno:location()
Options = options()
Return =
     Result :: tokens_result(),
     LeftOverChars :: char_spec()} |
    {more, Continuation1 :: return_cont()}
char_spec() = string() | eof
An opaque continuation.

This is the re-entrant scanner, which scans characters until either a dot ('.' followed by a white space) or eof is reached. It returns:

{done, Result, LeftOverChars}

Indicates that there is sufficient input data to get a result. Result is:

{ok, Tokens, EndLocation}

The scanning was successful. Tokens is the list of tokens including dot.

{eof, EndLocation}

End of file was encountered before any more tokens.

{error, ErrorInfo, EndLocation}

An error occurred. LeftOverChars is the remaining characters of the input data, starting from EndLocation.

{more, Continuation1}

More data is required for building a term. Continuation1 must be passed in a new call to tokens/3,4 when more data is available.

The CharSpec eof signals end of file. LeftOverChars then takes the value eof as well.

tokens(Continuation, CharSpec, StartLocation) is equivalent to tokens(Continuation, CharSpec, StartLocation, []).

For a description of the options, see string/3.

Error Information

ErrorInfo is the standard ErrorInfo structure that is returned from all I/O modules. The format is as follows:

{ErrorLocation, Module, ErrorDescriptor}

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



The continuation of the first call to the re-entrant input functions must be []. For a complete description of how the re-entrant input scheme works, see Armstrong, Virding and Williams: 'Concurrent Programming in Erlang', Chapter 13.