erl_pp (stdlib v7.3)

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The Erlang pretty printer.

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

All functions can have an optional argument, which specifies a hook that is called if an attempt is made to print an unknown form.

Note that if the functions in this module are used to convert abstract code back to Erlang source code, the enclosing function should first be processed by legalize_vars/1 in order to ensure that the output is semantically equivalent to the abstract code.

Known Limitations

It is not possible to have hook functions for unknown forms at other places than expressions.

See Also

erl_eval, erl_parse, io

Summary

Types

hook_function()

Optional argument HookFunction, shown in the functions described in this module, defines a function that is called when an unknown form occurs where there is to be a valid expression. If HookFunction is equal to none, there is no hook function.

option()

The option quote_singleton_atom_types is used to add quotes to all singleton atom types.

options()

Functions

attribute(Attribute)
attribute(Attribute, Options)

Same as form/1,2, but only for attribute Attribute.

expr(Expression, Indent, Options)
expr(Expression, Indent, Precedence, Options)

Prints one expression.

exprs(Expressions, Options)
exprs(Expressions, Indent, Options)

Same as form/1,2, but only for the sequence of expressions in Expressions.

form(Form)
form(Form, Options)

Pretty prints a Form, which is an abstract form of a type that is returned by erl_parse:parse_form/1.

function(Function)
function(Function, Options)

Same as form/1,2, but only for function Function.

guard(Guard)
guard(Guard, Options)

Same as form/1,2, but only for the guard test Guard.

legalize_vars(Function)

The Erlang compiler will, when expanding records to tuples, introduce new variables in the abstract representation. As the expansion is done on the abstract representation, the compiler can safely name the new variables with names that are not syntactically valid in Erlang source code (the name starts with a lowercase letter), thus ensuring the uniqueness of the new names.

Types

hook_function()

(not exported) 
-type hook_function() ::
          none |
          fun((Expr :: erl_parse:abstract_expr(),
               CurrentIndentation :: integer(),
               CurrentPrecedence :: non_neg_integer(),
               Options :: options()) ->
                  io_lib:chars()).

Optional argument HookFunction, shown in the functions described in this module, defines a function that is called when an unknown form occurs where there is to be a valid expression. If HookFunction is equal to none, there is no hook function.

The called hook function is to return a (possibly deep) list of characters. Function expr/4 is useful in a hook.

If CurrentIndentation is negative, there are no line breaks and only a space is used as a separator.

option()

(not exported) 
-type option() ::
          {hook, hook_function()} |
          {encoding, latin1 | unicode | utf8} |
          {quote_singleton_atom_types, boolean()} |
          {linewidth, pos_integer()} |
          {indent, pos_integer()}.

The option quote_singleton_atom_types is used to add quotes to all singleton atom types.

The option linewidth controls the maximum line width for formatted lines (defaults to 72 characters).

The option indent controls the indention for formatted lines (defaults to 4 spaces).

options()

(not exported) 
-type options() :: hook_function() | [option()].

Functions

attribute(Attribute) ≡ attribute(Attribute, none)

attribute(Attribute, Options) -> io_lib:chars()

Types

Attribute = erl_parse:abstract_form()
Options = options()

Same as form/1,2, but only for attribute Attribute.

expr(Expression, Indent, Options) ≡ expr(Expression, Indent, 0, Options)

expr(Expression, Indent, Precedence, Options) -> io_lib:chars()

Types

Expression = erl_parse:abstract_expr()
Indent = integer()
Precedence = non_neg_integer()
Options = options()

Prints one expression.

It is useful for implementing hooks (see section Known Limitations).

exprs(Expressions, Options) ≡ exprs(Expressions, 0, Options)

exprs(Expressions, Indent, Options) -> io_lib:chars()

Types

Expressions = [erl_parse:abstract_expr()]
Indent = integer()
Options = options()

Same as form/1,2, but only for the sequence of expressions in Expressions.

form(Form) ≡ form(Form, none)

form(Form, Options) -> io_lib:chars()

Types

Form = erl_parse:abstract_form() | erl_parse:form_info()
Options = options()

Pretty prints a Form, which is an abstract form of a type that is returned by erl_parse:parse_form/1.

function(Function) ≡ function(Function, none)

function(Function, Options) -> io_lib:chars()

Types

Function = erl_parse:abstract_form()
Options = options()

Same as form/1,2, but only for function Function.

guard(Guard) ≡ guard(Guard, none)

guard(Guard, Options) -> io_lib:chars()

Types

Guard = [erl_parse:abstract_expr()]
Options = options()

Same as form/1,2, but only for the guard test Guard.

legalize_vars(Function) -> Result

(since OTP 25.0)

Types

Function = Result = erl_parse:abstract_form()

The Erlang compiler will, when expanding records to tuples, introduce new variables in the abstract representation. As the expansion is done on the abstract representation, the compiler can safely name the new variables with names that are not syntactically valid in Erlang source code (the name starts with a lowercase letter), thus ensuring the uniqueness of the new names.

The above strategy leads to problems if a user wants to convert the abstract representation, using the functions of this module back to Erlang source code. Typically, pattern variables are output as atoms thus changing the sematics of the program. To solve this problem legalize_vars/1, when run on the abstract representation of a function, will return an equivalent function where all variables will have syntactically valid names.