View Source erl_pp (stdlib v6.0)
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
Summary
Types
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 option quote_singleton_atom_types is used to add quotes to all singleton
atom types.
Functions
Equivalent to attribute(Attribute, none)
Same as form/1,2, but only for attribute Attribute.
Equivalent to expr(Expression, none)
Equivalent to expr(Expression, 0, Options)
Prints one expression.
Equivalent to exprs(Expressions, none)
Equivalent to exprs(Expressions, 0, Options)
Same as form/1,2, but only for the sequence of expressions in
Expressions.
Equivalent to form(Form, none)
Pretty prints a Form, which is an abstract form of a type that is returned by
erl_parse:parse_form/1.
Equivalent to function(Function, none)
Same as form/1,2, but only for function Function.
Equivalent to guard(Guard, none)
Same as form/1,2, but only for the guard test Guard.
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
-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.
-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).
-type options() :: hook_function() | [option()].
Functions
-spec attribute(Attribute) -> io_lib:chars() when Attribute :: erl_parse:abstract_form().
Equivalent to attribute(Attribute, none)
-spec attribute(Attribute, Options) -> io_lib:chars() when Attribute :: erl_parse:abstract_form(), Options :: options().
Same as form/1,2, but only for attribute Attribute.
-spec expr(Expression) -> io_lib:chars() when Expression :: erl_parse:abstract_expr().
Equivalent to expr(Expression, none)
-spec expr(Expression, Options) -> io_lib:chars() when Expression :: erl_parse:abstract_expr(), Options :: options().
Equivalent to expr(Expression, 0, Options)
-spec expr(Expression, Indent, Options) -> io_lib:chars() when Expression :: erl_parse:abstract_expr(), Indent :: integer(), Options :: options().
Equivalent to expr(Expression, Indent, 0, Options)
-spec expr(Expression, Indent, Precedence, Options) -> io_lib:chars() when 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).
-spec exprs(Expressions) -> io_lib:chars() when Expressions :: [erl_parse:abstract_expr()].
Equivalent to exprs(Expressions, none)
-spec exprs(Expressions, Options) -> io_lib:chars() when Expressions :: [erl_parse:abstract_expr()], Options :: options().
Equivalent to exprs(Expressions, 0, Options)
-spec exprs(Expressions, Indent, Options) -> io_lib:chars() when Expressions :: [erl_parse:abstract_expr()], Indent :: integer(), Options :: options().
Same as form/1,2, but only for the sequence of expressions in
Expressions.
-spec form(Form) -> io_lib:chars() when Form :: erl_parse:abstract_form() | erl_parse:form_info().
Equivalent to form(Form, none)
-spec form(Form, Options) -> io_lib:chars() when 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.
-spec function(Function) -> io_lib:chars() when Function :: erl_parse:abstract_form().
Equivalent to function(Function, none)
-spec function(Function, Options) -> io_lib:chars() when Function :: erl_parse:abstract_form(), Options :: options().
Same as form/1,2, but only for function Function.
-spec guard(Guard) -> io_lib:chars() when Guard :: [erl_parse:abstract_expr()].
Equivalent to guard(Guard, none)
-spec guard(Guard, Options) -> io_lib:chars() when Guard :: [erl_parse:abstract_expr()], Options :: options().
Same as form/1,2, but only for the guard test Guard.
-spec legalize_vars(Function) -> erl_parse:abstract_form() when Function :: 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.