use early methods for tag based operations

lib/methsel.g

@@ -135,9 +135,10 @@ end );
 ## <P/>
 ## Installing methods with <Ref Func="InstallMethod"/> for a tag based
 ## operation is possible.
-## (Installing such methods with the same requirements as the ones for the
-## tag based methods is not recommended, because this may lead to unwanted
-## effects.)
+## However, installing such methods with the same requirements as the ones
+## for the tag based methods will have no effect because the handling of
+## tag based methods gets installed with <Ref Func="InstallEarlyMethod"/>
+## and thus has higher priority.
 ## </Description>
 ## </ManSection>
 ## <#/GAPDoc>
@@ -166,25 +167,96 @@ BIND_GLOBAL( "NewTagBasedOperation",
  ADD_OBJ_MAP( _METHODS_TAG_BASED_DEFAULTS, oper, requirements );
 
  # install the method for 'oper' that uses the tag handling
- InstallMethod( oper,
- "tag based method",
- requirements,
- function( requ... )
- local method;
-
- method:= FIND_OBJ_MAP( methods, requ[1], fail );
- if method = fail then
- # Take the default method if there is one.
- method:= FIND_OBJ_MAP( methods, IS_OBJECT, fail );
- fi;
- if method = fail then
- # Calling 'TryNextMethod' would lead to a less useful error message,
- # and perhaps cause real trouble.
- Error( "no default installed for tag based operation <oper>" );
- fi;
+ if LENGTH( requirements ) = 1 then
+ InstallEarlyMethod( oper,
+ function( req1 )
+ local method;
+
+ if not requirements[1]( req1 ) then
+ TryNextMethod();
+ fi;
+
+ method:= FIND_OBJ_MAP( methods, req1, fail );
+ if method = fail then
+ # Take the default method if there is one.
+ method:= FIND_OBJ_MAP( methods, IS_OBJECT, fail );
+ fi;
+ if method = fail then
+ Error( "no default installed for tag based operation <oper>" );
+ fi;
+
+ return method( req1 );
+ end );
+ elif LENGTH( requirements ) = 2 then
+ InstallEarlyMethod( oper,
+ function( req1, req2 )
+ local method;
+
+ if not ( requirements[1]( req1 ) and
+ requirements[2]( req2 ) ) then
+ TryNextMethod();
+ fi;
+
+ method:= FIND_OBJ_MAP( methods, req1, fail );
+ if method = fail then
+ # Take the default method if there is one.
+ method:= FIND_OBJ_MAP( methods, IS_OBJECT, fail );
+ fi;
+ if method = fail then
+ Error( "no default installed for tag based operation <oper>" );
+ fi;
+
+ return method( req1, req2 );
+ end );
+ elif LENGTH( requirements ) = 3 then
+ InstallEarlyMethod( oper,
+ function( req1, req2, req3 )
+ local method;
 
- return CallFuncList( method, requ );
- end );
+ if not ( requirements[1]( req1 ) and
+ requirements[2]( req2 ) and
+ requirements[3]( req3 ) ) then
+ TryNextMethod();
+ fi;
+
+ method:= FIND_OBJ_MAP( methods, req1, fail );
+ if method = fail then
+ # Take the default method if there is one.
+ method:= FIND_OBJ_MAP( methods, IS_OBJECT, fail );
+ fi;
+ if method = fail then
+ Error( "no default installed for tag based operation <oper>" );
+ fi;
+
+ return method( req1, req2, req3 );
+ end );
+ elif LENGTH( requirements ) = 4 then
+ InstallEarlyMethod( oper,
+ function( req1, req2, req3, req4 )
+ local method;
+
+ if not ( requirements[1]( req1 ) and
+ requirements[2]( req2 ) and
+ requirements[3]( req3 ) and
+ requirements[4]( req4 ) ) then
+ TryNextMethod();
+ fi;
+
+ method:= FIND_OBJ_MAP( methods, req1, fail );
+ if method = fail then
+ # Take the default method if there is one.
+ method:= FIND_OBJ_MAP( methods, IS_OBJECT, fail );
+ fi;
+ if method = fail then
+ Error( "no default installed for tag based operation <oper>" );
+ fi;
+
+ return method( req1, req2, req3, req4 );
+ end );
+ else
+ Error( "tag based operations for ", LENGTH( requirements ),
+ " arguments are currently not supported" );
+ fi;
 
  return oper;
 end );