
local portray(+TermClass, +TransPred, +Options),  export portray(+TermClass, +TransPred, +Options)

   Defines a portray transformation for the functor or type specified by TermClass.

Arguments
   +TermClass          Term in the form Atom, Atom/Integer or type(Type).
   +TransPred          Term in the form Atom/Integer.
   +Options            Possibly empty list of option flags.

Type
   Syntax Settings

Description
   This declaration is used to define a portray transformation on a class of
   terms.  Portray transformations are performed when a term is written by
   one of the predicates write/1,2, writeln/1,2, print/1,2, display/1,2 or
   printf/2,3 without the 'T' output option.

   The TermClass specifies to which terms the transformation will be
   applied:

Name/Arity
	transform all terms with the specified functor
type(Type)
	transform all terms of the specified type, where Type is one of
	compound, string, integer, rational, float, breal, goal, atom, meta.

   The +TransPred argument specifies the predicate that will perform the
   transformation.  TransPred must be of arity 2 or 3 and be in the form:

    trans_function(OldTerm, NewTerm [, Module]) :- ... .

   At transformation time, the system will call TransPred in the module
   where local portray/3 was invoked.  The term to transform is passed
   as the first argument, the second is a free variable which the
   transformation should bind to the transformed term, and the optional
   third argument is the module where the term is going to be printed.

   Options is a list which may be empty or contain one of the following
   type specification atoms:

term
	Transform all terms (this is the default)
clause
	Transform only if the term is printed as a program clause,
	i.e. by printf/2,3 with the 'C' output option.
goal
	Transform only if the term is printed as a goal,
	i.e. by printf/2,3 with the 'G' output option.

   and possibly some of the following options:

protect_arg
	Disable transformation of subterms (optional)
top_only
	Consider only the whole term, not subterms (optional)

   The visibility of portray declarations is controlled by the module
   system.  Transformations only take place when the portray
   declarations is visible in the module where the term is being
   printed.  The visibility is local or exported, depending on the
   declaration.


   The functor no_macro_expansion/1 can be wrapped around specific
   instances of a term to prevent it from being transformed. 


   Portray declarations are being treated as 'write'-macros in
   current_macro/4 and erase_macro/2.


Resatisfiable
      No.

Fail Conditions
      None.



Exceptions
     4 --- One or more arguments not instantiated.
     5 --- TermClass not of form Atom, Atom/Integer or type(Type).
     5 --- TransPred not of form Atom/Integer.
     5 --- Options not a list or contains invalid flags.
     6 --- Arity of TransPred is not 2 or 3.
     6 --- Illegal flag in Options.
   161 --- Transformation already defined in the current module for TermClass

Examples
   
% Example showing use of write macros

    [eclipse 1]: [user].

     tr_s(0, 0).
     tr_s(s(S), N) :- tr_s(S, N1), N is N1+1.

     :- local portray(s/1, tr_s/2, []).

    yes.
    [eclipse 2]: write(s(s(s(0)))).
    3
    yes.


Error:
   local portray(X, trx/2, []).              (Error 4).
   local portray(a/1, tra/2, [c]).           (Error 6).


See Also
   portray_goal / 2, macro / 3, printf / 2, printf / 3, current_macro / 4, erase_macro / 2
