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)