erlfmt

https://github.com/whatsapp/erlfmt
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An opinionated Erlang code formatter that automatically formats code while preserving semantics and...
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License:Apache-2.0
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### Good application and module naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Example of correct naming for applications and modules.

```Erlang
%% Good
myapp/src/task_server.erl
```

--------------------------------

### Good variable naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of correct variable naming conventions.

```Erlang
%% Good
FileName
_File
StoredID
StoredId
```

--------------------------------

### Correct usage of () in macro names

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples demonstrating correct usage of parentheses in macro names.

```Erlang
%% Good
-define(NOT_CONSTANT(), application:get_env(myapp, key)).
-define(CONSTANT, 100).
```

--------------------------------

### Good macro definition naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of correct naming for macro definitions.

```Erlang
%% Good
-define(SOME_VALUE, 100).
-define(SOME_MACRO(), hidden:call()).
-define(ANOTHER_MACRO, "another macro").
```

--------------------------------

### Good predicate function naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Example of correct naming for a predicate function.

```Erlang
%% Good
is_even(Num) -> ...
```

--------------------------------

### Good function, attribute, and atom naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of correct naming for functions, attributes, and atoms.

```Erlang
%% Good
error
bad_return
read_file
```

--------------------------------

### CHANGELOG.md Entry Example

Source: https://github.com/whatsapp/erlfmt/blob/main/RELEASE.md

An example of how to format an entry in the CHANGELOG.md file for a new release.

```markdown
### v<major>.<minor>.<bugfix>

Document features and Bug fixes since last release.
See https://github.com/WhatsApp/erlfmt/blob/main/CHANGELOG.md for link to commits since last release.
Add issue numbers or pull requests numbers using hashtags to the fixes and features

If only bug fixes change the bug fix version
If features were added change the minor version
Major version changes should be a big conversation
```

--------------------------------

### Good variable naming and function prefixes

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of descriptive variable names and avoiding 'do_' prefixes.

```Erlang
%% Good
validate_all([Key | Keys]) ->
    validate(Key),
    validate_all(Keys).
```

--------------------------------

### Bad variable naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of incorrect variable naming conventions.

```Erlang
%% Bad
File_name
File_Name
_file
Stored_id
```

--------------------------------

### Good boolean operator usage with guards

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of correct usage of ',' and ';' within guards for boolean logic.

```Erlang
%% Also good
when is_atom(Name), Name =/= undefined.
when is_binary(Task); is_atom(Task).
```

--------------------------------

### Macro Handling Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

An example demonstrating various macros and how they are handled by Erlang formatters, with a focus on erlfmt's capabilities.

```erlang
-module(macros).

-include_lib("stdlib/include/assert.hrl").

-define(MACRO(), object).

argument(?MACRO()) -> ok.

match() ->
    ?assertMatch({ok, Num} when is_integer(Num), get_number()).

-define(IMPORTANT_PARENS(Expr), (Expr) * (Expr)).
-define(RECORD_NAME1, #foo).
-define(RECORD_NAME2, foo).

bar() ->
    ?IMPORTANT_PARENS(2 + 1),
    ?RECORD_NAME1{a = 1},
    #?RECORD_NAME2{a = 1}.

-define(NAME, name).

?NAME() -> ok.

-define(FALLBACK_CLAUSE(Name), Name(_) -> default_action()).

compute(X) when is_integer(X) -> ok;
?FALLBACK_CLAUSE(compute).

```

--------------------------------

### Bad variable naming and function prefixes

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of poor variable naming and use of 'do_' prefixes.

```Erlang
%% Bad
validate([X | Xs]) ->
    do_validate(X),
    validate(Xs).
```

--------------------------------

### New Release Draft Example

Source: https://github.com/whatsapp/erlfmt/blob/main/RELEASE.md

Example fields to fill when drafting a new release on GitHub.

```markdown
Tag Version: v<major>.<minor>.<bugfix>
Release title: Pick most significant feature
Description: Copy ChangeLog contents
```

--------------------------------

### Bad function, attribute, and atom naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of incorrect naming for functions, attributes, and atoms.

```Erlang
%% Bad
'Error'
badReturn
read_File
```

--------------------------------

### Bad macro definition naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of incorrect naming for macro definitions.

```Erlang
%% Bad
-define(some_value, 100).
-define(someMacro(), hidden:call()).
-define(AnotherMacro, "another macro").
```

--------------------------------

### Exported functions format

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Example of correctly formatting exported functions in Erlang, with each function occupying one line.

```erlang
%% Good
-export([
    api_function/1,
    api_another/2, api_another/3,
    api_third/4
]).
```

--------------------------------

### Suffix pattern example: starting with newline

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list with a trailing comma, where there is a newline after the opening bracket.

```erlang
[
  a,
...]
```

--------------------------------

### DSLish Lists - Unformatted Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionIgnore.md

An example of an unformatted DSLish list before erlfmt is applied.

```erlang
gen_part({constructed,bif},TypeName,{_Name,parts,Tag,_Type}) ->
    emit(["  case Data of",nl,
          "    L when is_list(L) ->",nl,
          "      'dec_",TypeName,"'(lists:map(fun(X) -> element(1, ",
          {call,ber,ber_decode_erlang,["X"]},") end, L),",{asis,Tag},");",nl,
          "    _ ->",nl,
          "      [Res] = 'dec_",TypeName,"'([Data],",{asis,Tag},"),",nl,
          "      Res",nl,
          "  end"])
```

--------------------------------

### Related elvis configuration for module naming (reiteration)

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Elvis configuration snippet for module naming conventions.

```Erlang
  {elvis_style, module_naming_convention, #{regex => "^([a-z][a-z0-9]*_?)*(_SUITE)?$"}}
```

--------------------------------

### Bad predicate function naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Example of incorrect naming for a predicate function.

```Erlang
%% Bad
evenp(Num) -> ...
```

--------------------------------

### Pipes as Prefix Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionPipes.md

Example of Erlang code formatting with pipes as a prefix.

```erlang
-spec foo() ->
    term()
    | [term()]
    | error.
```

--------------------------------

### Newline and single indentation example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, single-indented.

```erlang
bar(Some, Very, Long, Arguments)
    when is_tuple(X); is_list(X) ->
    body.
```

--------------------------------

### Comment Handling - Input Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

An Erlang code snippet demonstrating comment placement before formatting.

```erlang
-type foo() ::
    %% comment 1
    fun(
        (
            %% comment 2
            ...
        ) ->
            %% comment 3
            bar()
    ).

```

--------------------------------

### Bad boolean operator usage

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of incorrect usage of 'and' and 'or' operators.

```Erlang
%% Bad
is_atom(Name) and Name =/= undefined.
is_binary(Task) or is_atom(Task).
```

--------------------------------

### Comma-space formatting

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Demonstrates correct single space usage after a comma in Erlang code.

```erlang
%% Bad
application:get_env(kernel,start_pg2)
```

```erlang
%% Good
application:get_env(kernel, start_pg2)
```

--------------------------------

### Binary operator spacing

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Illustrates the requirement for single spaces before and after binary operators in Erlang.

```erlang
%% Bad
Values=[
    #{key=>value},
    #rec{key=value,key2=value2},
    120-90,
    Server!Message
]
```

```erlang
%% Good
Values = [
    #{key => value},
    #rec{key = value, key2 = value2},
    120 - 90,
    Server ! Message
]
```

--------------------------------

### Floating guards example 1

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, using floating guards.

```erlang
bar(X) when is_tuple(X);
            % some comment
            is_list(X) ->
    body.
```

--------------------------------

### Compressed formatting example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example of list comprehension formatting with the 'compressed' option.

```erlang
[function_with_long_name(A)
 || {A, B} <- Cs,
    filter(B)]
```

--------------------------------

### Pipes as Suffix Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionPipes.md

Example of Erlang code formatting with pipes as a suffix.

```erlang
-spec foo() ->
    term() |
    [term()] |
    error.
```

--------------------------------

### Newline and half indentation example 1

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, half-indented.

```erlang
bar(X)
  when is_tuple(X);
       % some comment
       is_list(X) ->
    body.
```

--------------------------------

### Good boolean operator usage with andalso/orelse

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples of correct usage of 'andalso' and 'orelse' for boolean logic.

```Erlang
%% Good
is_atom(Name) andalso Name =/= undefined.
is_binary(Task) orelse is_atom(Task).
```

--------------------------------

### Related elvis configuration for macro names

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Elvis configuration snippet for enforcing macro naming conventions.

```Erlang
  {elvis_style, macro_names}
```

--------------------------------

### Macro with Magic Name Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionIgnore.md

An example demonstrating the use of a macro with a magic name to influence formatting, specifically within a list processing function.

```erlang
-define(IDERLFMT(ID), ID).

gen_part({constructed, bif}, TypeName, {_Name, parts, Tag, _Type}) ->
    ?IDERLFMT(emit(["  case Data of",nl,
                    "    L when is_list(L) ->",nl,
                    "      'dec_",TypeName,"'(lists:map(fun(X) -> element(1, ",
                    {call,ber,ber_decode_erlang,["X"]},") end, L),",{asis,Tag},");",nl,
                    "    _ ->",nl,
                    "      [Res] = 'dec_",TypeName,"'([Data],",{asis,Tag},"),",nl,
                    "      Res",nl,
                    "  end"])).
```

--------------------------------

### Related elvis configuration for module and function naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Elvis configuration snippets for enforcing module and function naming conventions.

```Erlang
  {elvis_style, module_naming_convention, #{regex => "^([a-z][a-z0-9]*_?)*(_SUITE)?$"}},
  {elvis_style, function_naming_convention, #{regex => "^([a-z]*_?)*$"}}
```

--------------------------------

### Newline and no indentation example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, not indented.

```erlang
bar(X)
when is_tuple(X);
     % some comment
     is_list(X) ->
    body.
```

--------------------------------

### Newline and double indentation example 1

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, doubly-indented.

```erlang
bar(X)
        when is_tuple(X);
             % some comment
             is_list(X) ->
    body.
```

--------------------------------

### Preserving Representation - erl_tidy Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

Erlang code showing how erl_tidy simplifies all examples (lists, atoms, strings, numbers).

```erlang
1000000,

"abc",

"abcABC",

[1, 2, 3],

[1, 2, 3 | improper_list],

{undefined, undefined}.

```

--------------------------------

### Newline and half indentation example 2

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, half-indented, for a function with many arguments.

```erlang
insert_nested({Field, Meta, Key0, Value0}, Comments0)
  when Field =:= map_field_assoc;
       Field =:= map_field_exact;
       Field =:= record_field;
       Field =:= generate;
       Field =:= b_generate ->
    body.
```

--------------------------------

### Multiline string conversion

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Demonstrates how to handle multiline strings by concatenating them with newline characters in Erlang.

```erlang
%% Bad
Message = "erlfmt is a code formatter for Erlang.

Arguments:

  files -- files to format
..."
```

```erlang
%% Good
Message =
    "erlfmt is a code formatter for Erlang.\n"
    "\n"
    "Arguments:\n"
    "\n"
    "  files -- files to format\n"
    "..."
```

```erlang
%% Also good
Message =
    "erlfmt is a code formatter for Erlang.\n\n"
    "Arguments:\n\n"
    "  files -- files to format\n"
    "..."
```

--------------------------------

### Newline and double indentation example 2

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, doubly-indented, for a function with many arguments.

```erlang
insert_nested({Field, Meta, Key0, Value0}, Comments0)
        when Field =:= map_field_assoc;
             Field =:= map_field_exact;
             Field =:= record_field;
             Field =:= generate;
             Field =:= b_generate ->
    body.
```

--------------------------------

### Aligned comment example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/Readme.md

Example of aligned comments in Erlang.

```erlang
#{
    A => a,                   % aligned comment
    B => bb,                  % another aligned comment
    C => ccc                  % and yet another aligned comment
}
```

--------------------------------

### Compressed with 4 spaces formatting example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example of list comprehension formatting with 'compressed' and 4-space indentation.

```erlang
[function_with_long_name(A)
    || A <- a_generator(),
        some_check(A)]
```

--------------------------------

### Clause style consistency

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Illustrates consistent formatting for clauses within case, if, try, receive, and functions in Erlang.

```erlang
%% Bad
lookup(1) -> one;
lookup(2) ->
    two.

reverse_lookup(one) ->
    1;
reverse_lookup(two) -> 2.
```

```erlang
%% Good
lookup(1) -> one;
lookup(2) -> two.

reverse_lookup(one) ->
    1;
reverse_lookup(two) ->
    2.
```

--------------------------------

### Directly following comment example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/Readme.md

Example of directly following comments in Erlang.

```erlang
#{
    A => a, % directly following comment
    B => bbbbb, % also directly following comment
    C => cccccc% also directly following if there is no space
}
```

--------------------------------

### Multi-line expression assignment

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Shows how to begin the expression on a new line when assigning the result of a multi-line expression in Erlang.

```erlang
%% Bad
Prefix = case Base of
             binary -> "2#";
             octal -> "8#";
             hex -> "16#"
         end.
```

```erlang
%% Good
Prefix =
    case Base of
        binary -> "2#";
        octal -> "8#";
        hex -> "16#"
    end.
```

--------------------------------

### All comments always on a newline - Example with pipe operator

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments.md

An example demonstrating the 'all comments on a newline' style with the pipe operator.

```erlang
X =
    % comment
    A |
    % another comment
    BB |
    % that comment
    CCC.

```

--------------------------------

### DSLish Lists - Formatted Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionIgnore.md

Demonstrates how erlfmt formats lists that do not fit on a single line, assigning each element its own line.

```erlang
[
    a_long_expression,
    b_long_expression,
    c_long_expression
]
```

--------------------------------

### Floating guards example 3 (diff impact)

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example illustrating diff impact with floating guards when changing argument names.

```erlang
insert_nested(KeyValue, Comments0) when Field =:= map_field_assoc;
                                        Field =:= map_field_exact;
                                        Field =:= record_field;
                                        Field =:= generate;
                                        Field =:= b_generate ->
    body.
```

--------------------------------

### Related elvis configuration for variable naming

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Elvis configuration snippet for enforcing variable naming conventions.

```Erlang
  {elvis_style, variable_naming_convention, #{regex => "^([A-Z][0-9a-zA-Z]*)$"}}
```

--------------------------------

### Prefix pattern example: ending with newline

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list with a leading comma, where there is a newline before the closing bracket.

```erlang
... 
  , a
]
```

--------------------------------

### Before erlfmt

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

Example of Erlang code before formatting by erlfmt.

```erl
what_is(Erlang) ->
case Erlang of movie->[hello(mike,joe,robert),credits]; language->formatting_arguments end
.
```

--------------------------------

### Floating guards example 2 (unformatted)

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionWhenMultilineGuards.md

Example of `when` with multiline guards, using floating guards for a function with many arguments. Marked as unformatted.

```erlang
insert_nested({Field, Meta, Key0, Value0}, Comments0) when Field =:= map_field_assoc;
                                                           Field =:= map_field_exact;
                                                           Field =:= record_field;
                                                           Field =:= generate;
                                                           Field =:= b_generate ->
    body.
```

--------------------------------

### Dedent double pipes, just a little - Basic example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

The chosen formatting by erlfmt, with double pipes dedented slightly.

```erlang
[    function_with_long_name(A)
 || {A, B} <- Cs, filter(B)
]
```

--------------------------------

### Configure erlfmt options in rebar.config

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/RebarUsage.md

Example of configuring erlfmt command-line options with defaults in rebar.config.

```erlang
{erlfmt, [
    write,
    {files, "{src,include,test}/*.{hrl,erl}}"
]}.
```

--------------------------------

### Map with single-line comments

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments.md

Example of a map with single-line comments on each key-value pair.

```erlang
#{
    A => a, % comment
    B => bb, % another comment
    C => ccc % that comment
}
```

--------------------------------

### After erlfmt

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

Example of Erlang code after formatting by erlfmt, demonstrating improved readability.

```erlang formatted demo2
what_is(Erlang) ->
    case Erlang of
        movie -> [hello(mike, joe, robert), credits];
        language -> no_more_formatting_arguments
    end.
```

--------------------------------

### Preserving Representation - rebar3_format Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

Erlang code showing how rebar3_format simplifies lists, atoms, and strings.

```erlang
1_000_000,

"\x61\x62\x63",

"abcABC",

[1, 2, 3],

[1, 2, 3 | improper_list],

{undefined, undefined}.

```

--------------------------------

### Two spaces after double pipes - Basic example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

An alternative formatting with two spaces after the double pipes for alignment.

```erlang
[    function_with_long_name(A)
||  {A, B} <- Cs, filter(B)
]
```

--------------------------------

### Compressed formatting with multiline expressions

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example showing inconsistent handling of multiline expressions with the 'compressed' option.

```erlang
[function_with_long_name(
    A,
    AlongArgument
 )
 || {A, B} <- Cs,
    filter(B)]
```

--------------------------------

### Preserving Representation - erlfmt Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

Erlang code demonstrating how erlfmt preserves the exact representation of strings, atoms, and integers.

```erlang
1_000_000,

"\x61\x62\x63",

"\x61\x62\x63" "\x41\x42\x43",

[1 | [2 | [3 | []]]],

[1 | [2 | [3 | improper_list]]],

{undefined, 'undefined'}.

```

--------------------------------

### Dedent double pipes, just a little - Multiline argument

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example of the chosen formatting with a multiline function argument.

```erlang
[    function_with_long_name(
        A,
        ALongArgument
    )
 || {A, B} <- Cs,
    filter(B)
]
```

--------------------------------

### Parentheses, braces, and brackets spacing

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Enforces no spaces around parentheses, braces, or brackets for function/record/maps/lists in Erlang.

```erlang
%% Bad
function_definition( { ok, Argument } ) ->
    mod:function_call(Argument).

sort( [ 1,2,3 ] ).
```

```erlang
%% Good
function_definition({ok, Argument}) ->
    mod:function_call(Argument).

sort([1, 2, 3]).
```

--------------------------------

### Dedent double pipes - Filter on same line

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example with dedented double pipes and the filter on the same line.

```erlang
[    function_with_long_name(A)
|| {A, B} <- Cs, filter(B)
]
```

--------------------------------

### Preserving Representation - steamroller Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

Erlang code showing how steamroller preserves lists but simplifies strings, atoms, and numbers.

```erlang
1000000,
"abc",
"abc" "ABC",
[1 | [2 | [3 | []]]],
[1 | [2 | [3 | improper_list]]],
{undefined, undefined}.

```

--------------------------------

### Incorrect omission of () in macro names

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Examples where parentheses should not be omitted in macro names.

```Erlang
%% Bad
-define(NOT_CONSTANT, application:get_env(myapp, key)).
-define(CONSTANT(), 100).
```

--------------------------------

### Dedent double pipes - Basic example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

A list comprehension where the double pipes are dedented to the left.

```erlang
[    function_with_long_name(A)
|| {A, B} <- Cs,
    filter(B)
]
```

--------------------------------

### Binary operator indentation

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Demonstrates indenting the right-hand side of a binary operator if it's on a new line in Erlang.

```erlang
%% Bad
f(Op, Atom) ->
    is_atom(Op) andalso
    Atom =/= '!' andalso
    Atom =/= '='.
```

```erlang
%% Good
f(Op, Atom) ->
    is_atom(Op) andalso
        Atom =/= '!' andalso
        Atom =/= '='.
```

--------------------------------

### Aligned Binary Strings with Ignore Comment

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionIgnore.md

Example of aligned binary strings with comments, demonstrating the use of erlfmt-ignore for readability.

```erlang
%% erlfmt-ignore
<<
% Phase 0     Phase 1     Phase 2     Phase 3     Phase 4     Phase 5     Phase 6
% A B C D     A B C D     A B C D     A B C D     A B C D     A B C D     A B C D
2#01001000, 2#10100000, 2#00010000, 2#00010000, 2#00010011, 2#00000000, 2#00000000,  % LUT0 - Black
2#01001000, 2#10100000, 2#10000000, 2#00000000, 2#00000011, 2#00000000, 2#00000000,  % LUTT1 - White
>>
```

--------------------------------

### Double pipes on their own line - Filter on same line

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example with double pipes on their own line and the filter on the same line.

```erlang
[    function_with_long_name(A)
||
    {A, B} <- Cs, filter(B)
]
```

--------------------------------

### erlfmt line break hints example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

Demonstrates how erlfmt preserves user-introduced line breaks and empty lines in lists and export statements.

```erlang
-export([
    %% public functions
    foo/1, foo/2,
    bar/1,

    %% testing helpers
    baz/5
]).

x() ->
    Foo = [
        short,
        list
    ].
```

--------------------------------

### Double pipes on their own line - Basic example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

A list comprehension format where the double pipes are placed on their own line.

```erlang
[    function_with_long_name(A)
||
    {A, B} <- Cs,
    filter(B)
]
```

--------------------------------

### Comment Handling - erlfmt Trailing Comment Example Input

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

An Erlang code snippet with trailing comments before erlfmt formatting.

```erlang
%% Constants
-define(VERSION_CHECK_INTERVAL_MILLIS_DEFAULT, 10000). % Minimum interval between health checks
-define(MAX_WRITE_FAILURES, 3).

```

--------------------------------

### View rebar3 help for all options

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/RebarUsage.md

Command to display help for rebar3, including erlfmt options.

```sh
$ rebar3 --help
```

--------------------------------

### Standalone comment example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/Readme.md

Example of a standalone comment in Erlang.

```erlang
   %% My standalone comment
   case IAmOnTheNextLine of
   ...
```

--------------------------------

### Rebar3 Configuration for Hex Plugin

Source: https://github.com/whatsapp/erlfmt/blob/main/RELEASE.md

Command to create the rebar3 configuration file to include the hex plugin.

```bash
$ mkdir -p ~/.config/rebar3/ && echo "{plugins, [rebar3_hex]}.". >> ~/.config/rebar3/rebar.config
```

--------------------------------

### Hex User Authentication

Source: https://github.com/whatsapp/erlfmt/blob/main/RELEASE.md

Command to authenticate the Hex user.

```bash
$ rebar3 hex user auth
```

--------------------------------

### Comment Handling - steamroller Reordering Example Input

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/ErlangFormatterComparison.md

An Erlang code snippet with nested comments for testing steamroller's reordering.

```erlang
comprehension() ->
    [
        [
            %% comment 1
            X
        ]
        ||
           X <-
               %% comment 3
               [
                   %% comment 4
               ]
           %% comment 5
    ].

```

--------------------------------

### Prefix pattern example: no newline before closing bracket

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list with a leading comma, where there is no newline immediately before the closing bracket.

```erlang
... 
, a/1 ]
```

--------------------------------

### Build & Test Commands

Source: https://github.com/whatsapp/erlfmt/blob/main/CLAUDE.md

Commands for compiling, testing, static analysis, and full project checks.

```bash
rebar3 compile              # Compile
rebar3 ct                   # Run all tests (Common Test)
rebar3 ct --suite=test/erlfmt_SUITE --case=<test_name>  # Run a single test
rebar3 dialyzer             # Static type analysis
make check                  # Full check: compile, test, dialyzer, format check
```

--------------------------------

### Suffix pattern example: no newline after opening bracket

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list with a trailing comma, where there is no newline immediately after the opening bracket.

```erlang
[ a/1,
...]
```

--------------------------------

### Plotting Comment Distribution by Column

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/columns.html

JavaScript code to plot the distribution of comments by their starting column number using Plotly.js. It visualizes the percentage of comments starting at different column positions for various Erlang projects.

```javascript
var data = [
  {
    x: [17, 19, 20, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 56, 76],
    y: [1.580135440180587, 1.3544018058690745, 3.6681715575620766, 2.0316027088036117, 1.580135440180587, 1.18510158013544, 1.7494356659142212, 1.693002257336343, 1.9751693002257336, 2.4266365688487586, 7.618510158013544, 2.6523702031602707, 1.580135440180587, 2.539503386004515, 2.4266365688487586, 1.1286681715575622, 1.4108352144469527, 5.756207674943567, 5.586907449209932, 2.200902934537246, 2.2573363431151243, 1.4108352144469527, 1.580135440180587, 2.144469525959368, 2.144469525959368, 1.4672686230248306, 2.37020316027088, 17.099322799097067, 1.4108352144469527, 1.2415349887133182, 2.0316027088036117, 1.0158013544018059],
    mode: 'markers',
    type: 'scatter',
    name: 'OTP'
  },
  {
    x: [68, 72],
    y: [28.0, 20.0],
    mode: 'markers',
    type: 'scatter',
    name: 'kazoo'
  },
  {
    x: [],
    y: [],
    mode: 'markers',
    type: 'scatter',
    name: 'Inaka'
  },
  {
    x: [],
    y: [],
    mode: 'markers',
    type: 'scatter',
    name: 'MongooseIM'
  },
  {
    x: [],
    y: [],
    mode: 'markers',
    type: 'scatter',
    name: 'ejabberd'
  },
  {
    x: [9, 11, 17, 18, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 56, 58, 59, 60, 61, 68, 92],
    y: [2.593192868719611, 1.7017828200972447, 1.4181523500810373, 1.8638573743922204, 1.9448946515397083, 1.2155591572123177, 1.4181523500810373, 1.6207455429497568, 1.4586709886547813, 2.1880064829821717, 2.1880064829821717, 2.674230145867099, 2.3095623987034037, 1.2155591572123177, 1.0940032414910859, 4.416531604538087, 1.8233387358184765, 1.5802269043760129, 1.8233387358184765, 2.674230145867099, 2.512155591572123, 2.025931928687196, 1.8233387358184765, 3.9708265802269045, 1.7828200972447326, 3.282009724473258, 1.2965964343598055, 1.7017828200972447, 1.6207455429497568, 1.2560777957860616, 1.539708265802269, 1.9448946515397083, 1.1345218800648298, 1.539708265802269, 1.3371150729335495, 1.2965964343598055, 1.1750405186385737, 1.1345218800648298, 2.552674230145867, 2.025931928687196, 1.6612641815235007, 1.2560777957860616, 1.5802269043760129],
    mode: 'markers',
    type: 'scatter',
    name: 'WhatsApp'
  }
];
var layout = {
  title: 'Percentage of comments at column number',
  xaxis: {
    title: 'comment starts at column number'
  },
  yaxis: {
    title: 'percentage of comments'
  }
};
Plotly.newPlot('myPlot', data, layout);
```

--------------------------------

### Unformatted Scenario

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

An example of a line that exceeds the length limit.

```erlang
scenario(dial_phone_number(),  ring(), hello(mike),hello(joe), hello(robert),   system_working(), seems_to_be())
```

--------------------------------

### Command to reproduce analysis

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/Readme.md

Command to run the analysis after setting up the repositories and editing the paths.

```sh
$ make
```

--------------------------------

### Right Associative Operator Example

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionAssociative.md

Demonstrates how right-associative binary operators like '++' are parsed and how erlfmt's greedy layout algorithm breaks them.

```erlang
A ++ B ++ C
```

--------------------------------

### Unformatted Split Tokens

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

An example of code where the line-length is exceeded, requiring manual intervention.

```erlang
split_tokens([{TokenType, Meta, TokenValue} | Rest], TokenAcc, CommentAcc) ->
    split_tokens(Rest, [{TokenType, token_anno(erl_anno:to_term(Meta), #{}), TokenValue} | TokenAcc], CommentAcc).
```

--------------------------------

### Expression group alignment

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Discourages aligning expression groups in Erlang code.

```erlang
%% Bad
Module = Env#env.module,
Arity  = length(Args).

inspect(false) -> "false";
inspect(true)  -> "true";
inspect(ok)    -> "ok".
```

```erlang
%% Good
Module = Env#env.module,
Arity = length(Args).

inspect(false) -> "false";
inspect(true) -> "true";
inspect(ok) -> "ok".
```

--------------------------------

### Module Documentation Percentage Usage

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionComments/module_comments.md

Empirical results showing the percentage of module documentation comments using %, %%, and %%% for various projects.

```text
OTP
%: 0.7%
%%: 91.5%
%%: 7.8%

ejabberd
%: 0.0%
%%: 16.9%
%%: 83.1%

MongooseIM
%: 0.2%
%%: 40.1%
%%: 59.7%

Inaka
%: 8.3%
%%: 23.4%
%%: 68.3%

kazoo
%: 0.0%
%%: 0.0%
%%: 100.0%

WhatsApp
%: 17.4%
%%: 15.6%
%%: 67.0%
```

--------------------------------

### Function with Long Pattern

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

An example of a function head with a long pattern that might be split by the formatter.

```erlang
my_function(
    #user{name: Name, age: Age, ...},
    Arg2,
    Arg3
) ->
    ...
```

--------------------------------

### Dedent double pipes, just a little - Consistent alignment

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Demonstrates consistent 4-space alignment with the chosen formatting.

```erlang
[    function_with_long_name(A)
 || {A, B} <- Cs,
    filter(B)
]
```

--------------------------------

### Unary operator spacing

Source: https://github.com/whatsapp/erlfmt/blob/main/StyleGuide.md

Shows that spaces should not be used after symbolic unary operators in Erlang.

```erlang
%% Bad
Angle = - 45,
WithParens = - ( + 45 ).
```

```erlang
%% Good
Angle = -45,
WithParens = -(+45).
```

--------------------------------

### Container Layouts: Expanded

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

Example of a list formatted in 'expanded' layout, where each element is printed on a separate line.

```erlang
```erlang formatted expanded
[
    Foo,
    Bar
]
```
```

--------------------------------

### Unaligned Matrix

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionIgnore.md

An example of a matrix defined with alignment that would be lost if formatted by erlfmt.

```erlang
-define(DELTA_MATRIX, [
    [0,   0,   0,   0,   0,   0,   0,   0,   0, -17,   0,   0,   0,   0,   0,   0, -16,   0,   0,   0],
    [0,   0,   0, -15,   0,   0, -17,   0,   0,   0,   0,   0, -16,   0,   0,   0,   0,   0, -15,   0],
    [0,   0,   0, -17,   0,   0,   0,   0, -16,   0,   0,   0,   0, -15,   0,   0,   0,   0,   0,   0],
    [-17, 0,   0, -16, -16,   0,   0,   0, -15,   0,   0,   0,   0,   0,   0,   0,   0, -17,   0,   0],
    [-16, 0,   0, -15,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0, -17, -33, -16, -31, -15,   0],
    [0,   0,   0,   0,   0,   0,   0,   0,   0,   0, -18, -17, -16, -15, -14,   0,   0,   0,   0,   0],
    [0,  -1,  -1,  -1,  -1,  -1,  -1,  -1,   0,   1,   1,   1,   1,   1,   1,   1,   0,   0,   0,   0],
    [0,   0,  14,  15,  16,  17,  18,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,  15,  31],
    [16,  33,  17,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,  15,   0,   0,  16,   0,   0,  17],
    [0,   0,   0,   0,   0,   0,   0,   0,  15,   0,   0,   0,  16,   0,   0,   0,  17,   0,   0,   0],
    [0,   0,   0,  15,   0,   0,   0,   0,  16,   0,   0,   0,   0,  17,   0,   0,   0,   0,  15,   0],
    [0,   0,   0,   0,  16,   0,   0,   0,   0,   0,  17,   0,   0,  15,   0,   0,   0,   0,   0,   0],
    [16,  0,   0,   0,   0,   0,   0,  17,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0]
]).
```

--------------------------------

### Container Layouts: Collapsed

Source: https://github.com/whatsapp/erlfmt/blob/main/README.md

Example of a list formatted in 'collapsed' layout where all elements are printed in a single line.

```erlang
```erlang formatted collapsed
[Foo, Bar]
```
```

--------------------------------

### List with leading comma

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list where each element, except the last, is preceded by a comma.

```erlang
[ x
, y
, z
]
```

--------------------------------

### Compressed with 4 spaces and multiline expressions

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionListComprehensions.md

Example showing inconsistent handling of multiline expressions with 'compressed' and 4-space indentation.

```erlang
[function_with_long_name(
    A,
    AlongArgument
 )
    || A <- a_generator(),
        some_check(A)]
```

--------------------------------

### List with trailing comma

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionCommas.md

Example of a list where each element, except the last, is followed by a comma.

```erlang
[
  x,
  y,
  z
]
```

--------------------------------

### IO Format Tilde P style for lists

Source: https://github.com/whatsapp/erlfmt/blob/main/doc/FormattingDecisionLists.md

Shows the compact formatting of lists when using io:format("~p", [MyList]).

```erlang
[the_first_and,second_element_fits_on_one_line,
 but_the_third_cannot_also_fit]
```
Tokens:
Raw