erl_scan(3)

📦

version 29.0.6 · 3 versions

🎀 erl_scan — bestie, it's the Erlang token scanner 💅

Okay so, real talk: this is a page from the Erlang Module Definition section of the manual, and it's all about erl_scan, the module that turns your messy little characters into tidy Erlang tokens. Sit down, hydrate, let's go. ✨

✨ NAME ✨

erl_scan — The Erlang token scanner. 👑

🌸 DESCRIPTION 🌸

This module contains functions for tokenizing (scanning) characters into Erlang tokens. That's the whole vibe. Characters in, tokens out. 🦋

💖 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:

Module:format_error(ErrorDescriptor)

🫶 Notes 🫶

The continuation of the first call to the re-entrant input functions must be []. Not optional, bestie — [] or nothing. 😭

For a complete description of how the re-entrant input scheme works, see Armstrong, Virding and Williams: 'Concurrent Programming in Erlang', Chapter 13.

👀 See Also 👀

erl_anno, erl_parse, io

🌈 DATA TYPES 🌈

Lowkey the driest part but it's giving essential, so here we go:

category() :: atom().

char_spec() :: string() | eof.

error_description() :: term().

error_info() ::
          {erl_anno:location(), module(), error_description()}.

option() ::
          return | return_white_spaces | return_comments | text |
          {reserved_word_fun, resword_fun()} |
          {text_fun, text_fun()} |
          {compiler_internal, [term()]}.

options() :: option() | [option()].

resword_fun() :: fun((atom()) -> boolean()).

-opaque return_cont()

symbol() :: atom() | float() | integer() | string().

text_fun() :: fun((atom(), string()) -> boolean()).

token() ::
          {category(), Anno :: erl_anno:anno(), symbol()} |
          {category(), Anno :: erl_anno:anno()}.

tokens() :: [token()].

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

🔥 FUNCTIONS 🔥

🌟 `category(Token) -> category() when Token :: token().`

Returns the category of Token. Simple. Iconic. No notes. 💅

🌟 `column(Token) -> erl_anno:column() | undefined when Token :: token().`

Returns the column of Token's collection of annotations.

🌟 `end_location(Token) -> erl_anno:location() | undefined when Token :: token().`

Returns the end location of the text of Token's collection of annotations. If there is no text, undefined is returned. So like, if there's nothing there, you get nothing. 🫠

🌟 `format_error(ErrorDescriptor) -> string() when 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() when Token :: token().`

Returns the line of Token's collection of annotations.

🌟 `location(Token) -> erl_anno:location() when 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. Binary behaviour, honestly.

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

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

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_comments — Return comment tokens.
return_white_spaces — 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).
return — Short for [return_comments, return_white_spaces].
text — Include the token text in the token annotation. The text is the part of the input corresponding to the token. See also text_fun.
{text_fun, text_fun()} — A callback function used to determine whether the full text for the token shall be included in the token annotation. Arguments of the function are the category of the token and the full token string. This is only used when text is not present. If neither are present the text will not be saved in the token annotation.
{compiler_internal, term()} — Pass compiler-internal options to the scanner. The set of internal options understood by the scanner should be considered experimental and can thus be changed at any time without prior warning. ⚠️

The following options are currently understood:

ssa_checks — Tokenizes source code annotations used for encoding tests on the BEAM SSA code produced by the compiler.

🌟 `symbol(Token) -> symbol() when Token :: token().`

Returns the symbol of Token.

🌟 `text(Token) -> erl_anno:text() | undefined when Token :: token().`

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

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

when
    Continuation :: return_cont() | [],
    CharSpec :: char_spec(),
    StartLocation :: erl_anno:location(),
    Options :: options(),
    Return ::
        {done,
         Result :: tokens_result(),
         LeftOverChars :: char_spec()} |
        {more, Continuation1 :: return_cont()}.

This is the re-entrant scanner, which scans characters until either a dot ('.' followed by a white space) or eof is reached. The dot really said "that's a wrap". ✨

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.

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

🫶 Quick vibe check, bestie: that {compiler_internal, term()} situation is experimental, so don't build your whole personality on it. It can change without warning. Periodt.

And that's the whole page, no cap. This little bundle of joy comes to you courtesy of Ericsson AB, running with stdlib 8.0.4 — my beloved erl_scan(3), scanned, tokenized and slayed. 💖✨👑

📦 where this page comes from

ubuntu stonking: erlang-doc 1:29.0.6+dfsg-1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms

erl_scan(3)

🎀 erl_scan — bestie, it's the Erlang token scanner 💅

✨ NAME ✨

🌸 DESCRIPTION 🌸

💖 Error Information 💖

🫶 Notes 🫶

👀 See Also 👀

🌈 DATA TYPES 🌈

🔥 FUNCTIONS 🔥

🌟 category(Token) -> category() when Token :: token().

🌟 column(Token) -> erl_anno:column() | undefined when Token :: token().

🌟 end_location(Token) -> erl_anno:location() | undefined when Token :: token().

🌟 format_error(ErrorDescriptor) -> string() when ErrorDescriptor :: error_description().

🌟 line(Token) -> erl_anno:line() when Token :: token().

🌟 location(Token) -> erl_anno:location() when Token :: token().

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

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

🌟 symbol(Token) -> symbol() when Token :: token().

🌟 text(Token) -> erl_anno:text() | undefined when Token :: token().

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