NAME

       erl_pp - The Erlang pretty printer.

DESCRIPTION

       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.

DATA TYPES

       hook_function() =
           none |
           fun((Expr :: erl_parse:abstract_expr(),
                CurrentIndentation :: integer(),
                CurrentPrecedence :: integer() >= 0,
                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() =
           {hook, hook_function()} |
           {encoding, latin1 | unicode | utf8} |
           {quote_singleton_atom_types, boolean()} |
           {linewidth, integer() >= 1} |
           {indent, integer() >= 1}

              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() = hook_function() | [option()]

EXPORTS

       attribute(Attribute) -> io_lib:chars()

       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) -> io_lib:chars()

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

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

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

              Types:

                 Expression = erl_parse:abstract_expr()
                 Indent = integer()
                 Precedence = integer() >= 0
                 Options = options()

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

       exprs(Expressions) -> io_lib:chars()

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

       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) -> io_lib:chars()

       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) -> io_lib:chars()

       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) -> io_lib:chars()

       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) -> erl_parse:abstract_form()

              Types:

                 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.

KNOWN LIMITATIONS

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

SEE ALSO

       erl_eval(3erl), erl_parse(3erl), io(3erl)