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# erl_scan

### MODULE

erl_scan

### MODULE SUMMARY

The Erlang Token Scanner

### DESCRIPTION

This module contains functions for tokenizing characters into Erlang tokens.

### DATA TYPES

    category() = atom()
    column() = integer() > 0
    line() = integer()
    location() = line() | {line(), column()}
    reserved_word_fun() -> fun(atom()) -> bool()
    set_attribute_fun() -> fun(term()) -> term()
    symbol() = atom() | float() | integer() | string()
    token() = {category(), attributes()} | {category(), attributes(), symbol()}
    attributes() = line() | list() | tuple()

### EXPORTS

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

Types:

String = string()
Return = {ok, Tokens, EndLocation} | Error
Tokens = [token()]
Error = {error, ErrorInfo, EndLocation}
StartLocation = EndLocation = location()
Options = Option | [Option]
Option = {reserved_word_fun,reserved_word_fun()} | return_comments | return_white_spaces | return | text

Takes the list of characters String and tries to scan (tokenize) them. Returns {ok, Tokens, EndLocation}, where Tokens are the Erlang tokens from String. EndLocation is the first location after the last token.

{error, ErrorInfo, EndLocation} is returned if an error occurs. EndLocation 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 attributes() as well as EndLocation and ErrorLocation will be lines. If StartLocation is a pair of a line and a column attributes() takes the form of an opaque compound data type, and EndLocation and ErrorLocation will be pairs of a line and a column. The token attributes contain information about the column and the line where the token begins, as well as the text of the token (if the text option is given), all of which can be accessed by calling token_info/1,2 or attributes_info/1,2.

A token is a tuple containing information about syntactic category, the token attributes, and the actual terminal symbol. For punctuation characters (e.g. ;, |) 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, Info, atom()}, {char, Info, integer()}, {comment, Info, string()}, {float, Info, float()}, {integer, Info, integer()}, {var, Info, atom()}, and {white_space, Info, string()}.

The valid options are:

{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 will be the category of the token; if the function returns false, atom will be the category of the unquoted atom.

return_comments

  Return comment tokens.

return_white_spaces

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

return

  Short for [return_comments, return_white_spaces].

text

  Include the token's text in the token attributes. The text is the part of the input corresponding to the token.

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

Types:

Continuation = [] | Continuation1
Return = {done, Result, LeftOverChars} | {more, Continuation1}
LeftOverChars = CharSpec
CharSpec = string() | eof
Continuation1 = tuple()
Result = {ok, Tokens, EndLocation} | {eof, EndLocation} | Error
Tokens = [token()]
Error = {error, ErrorInfo, EndLocation}
StartLocation = EndLocation = location()
Options = Option | [Option]
Option = {reserved_word_fun,reserved_word_fun()} | return_comments | return_white_spaces | return

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

{done, Result, LeftOverChars}

  This return 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 will then take the value eof as well.

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

See string/3 for a description of the various options.

reserved_word(Atom) -> bool()

Types:

Atom = atom()

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

token_info(Token) -> TokenInfo

Types:

Token = token()
TokenInfo = [TokenInfoTuple]
TokenInfoTuple = {TokenItem, Info}
TokenItem = atom()
Info = term()

Returns a list containing information about the token Token. The order of the TokenInfoTuples is not defined. The following TokenItems are returned: category, column, length, line, symbol, and text. See token_info/2 for information about specific TokenInfoTuples.

Note that if token_info(Token, TokenItem) returns undefined for some TokenItem in the list above, the item is not included in TokenInfo.

token_info(Token, TokenItemSpec) -> TokenInfo

Types:

Token = token()
TokenItemSpec = TokenItem | [TokenItem]
TokenInfo = TokenInfoTuple | undefined | [TokenInfoTuple]
TokenInfoTuple = {TokenItem, Info}
TokenItem = atom()
Info = term()

Returns a list containing information about the token Token. If TokenItemSpec is a single TokenItem, the returned value is the corresponding TokenInfoTuple, or undefined if the TokenItem has no value. If TokenItemSpec is a list of TokenItem, the result is a list of TokenInfoTuple. The TokenInfoTuples will appear with the corresponding TokenItems in the same order as the TokenItems appeared in the list of TokenItems. TokenItems with no value are not included in the list of TokenInfoTuple.

The following TokenInfoTuples with corresponding TokenItems are valid:

{category, category()}

  The category of the token.

{column, column()}

  The column where the token begins.

{length, integer() > 0}

  The length of the token's text.

{line, line()}

  The line where the token begins.

{location, location()}

  The line and column where the token begins, or just the line if the column unknown.

{symbol, symbol()}

  The token's symbol.

{text, string()}

  The token's text..

attributes_info(Attributes) -> AttributesInfo

Types:

Attributes = attributes()
AttributesInfo = [AttributeInfoTuple]
AttributeInfoTuple = {AttributeItem, Info}
AttributeItem = atom()
Info = term()

Returns a list containing information about the token attributes Attributes. The order of the AttributeInfoTuples is not defined. The following AttributeItems are returned: column, length, line, and text. See attributes_info/2 for information about specific AttributeInfoTuples.

Note that if attributes_info(Token, AttributeItem) returns undefined for some AttributeItem in the list above, the item is not included in AttributesInfo.

attributes_info(Attributes, AttributeItemSpec) -> AttributesInfo

Types:

Attributes = attributes()
AttributeItemSpec = AttributeItem | [AttributeItem]
AttributesInfo = AttributeInfoTuple | undefined | [AttributeInfoTuple]
AttributeInfoTuple = {AttributeItem, Info}
AttributeItem = atom()
Info = term()

Returns a list containing information about the token attributes Attributes. If AttributeItemSpec is a single AttributeItem, the returned value is the corresponding AttributeInfoTuple, or undefined if the AttributeItem has no value. If AttributeItemSpec is a list of AttributeItem, the result is a list of AttributeInfoTuple. The AttributeInfoTuples will appear with the corresponding AttributeItems in the same order as the AttributeItems appeared in the list of AttributeItems. AttributeItems with no value are not included in the list of AttributeInfoTuple.

The following AttributeInfoTuples with corresponding AttributeItems are valid:

{column, column()}

  The column where the token begins.

{length, integer() > 0}

  The length of the token's text.

{line, line()}

  The line where the token begins.

{location, location()}

  The line and column where the token begins, or just the line if the column unknown.

{text, string()}

  The token's text..

set_attribute(AttributeItem, Attributes, SetAttributeFun) -> AttributesInfo

Types:

AttributeItem = line
Attributes = attributes()
SetAttributeFun = set_attribute_fun()

Sets the value of the line attribute of the token attributes Attributes.

The SetAttributeFun is called with the value of the line attribute, and is to return the new value of the line attribute.

format_error(ErrorDescriptor) -> string()

Types:

ErrorDescriptor = errordesc()

Takes an ErrorDescriptor and returns a string which describes the error or warning. This function is usually called implicitly when processing an ErrorInfo structure (see below).

### Error Information

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

    {ErrorLocation, Module, ErrorDescriptor}

A string which describes the error is obtained with the following call:

    Module:format_error(ErrorDescriptor)

### Notes

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

### See Also

io(3), erl_parse(3)