function eval { (x:tree; rho:Env) :Value   forwarded };
{ eval :Exp -> Env -> Value  Note: evaluates an Expression and returns a Value}
{POST: result tag is not deferval, weak head normal form}
   var func, switch :Value;
begin case x^.tag of
         ident:     eval:=applyenv(rho, x^.id);
         intcon:    eval:=mkint(x^.n);
         boolcon:   eval:=mkbool(x^.b);
         charcon:   eval:=mkchar(x^.ch);
         nilcon:    eval:=mkvalue(nilval);
         emptycon:  eval:=mkvalue(emptyval);
         lambdaexp: eval:=mkfunc(x, rho);
         application:
            begin func := eval(x^.fun, rho);
                  if func^.tag=funcval then
                     eval:=apply(func, defer(x^.aparam, rho))
                  else error('apply ~fn ')
            end;
         unexp:   eval:=U(x^.unopr, eval(x^.unarg, rho));
         binexp:  if x^.binopr=conssy then { cons should not eval its params }
                       eval:=O(x^.binopr, defer(x^.left,rho),
                                          defer(x^.right,rho))
                  else eval:=O(x^.binopr, eval(x^.left,rho),   {others strict}
                                          eval(x^.right,rho));
         ifexp:
            begin switch:=eval(x^.e1, rho);
                  if switch^.tag=boolval then
                     if switch^.b then eval:=eval(x^.e2, rho)
                                  else eval:=eval(x^.e3, rho)
                  else error('if ~bool  ')
            end;
         block:   eval:=eval( x^.exp, D(x^.decs, rho))
      end {case}
      ; evals := evals + 1 { statistics }
end {eval};

procedure force { ( v :Value )  forwarded } ;
   var fv :Value;
begin if v^.tag=deferval then
         begin fv := eval( v^.e, v^.r ); v^ := fv^ {overwrite} end
end;

{\fB Evaluate an Expression. \fP}

{Do not remove: Lazy.p, Strict.p, lazy.*.P, strict.*.P, lex.*.P, & syntax.*.P }
{ are released under Gnu `copyleft' General Public Licence (GPL)              }
{ - L. Allison, CSSE, Monash Uni., .au, 7/2003.                               }