[WWcvs] pg: Adds a new context that is more limited by not allowing operations within the coefficients and exponents themselves.

[dpvc v. a. <we...@ma...>]

Log Message:
-----------
Adds a new context that is more limited by not allowing operations
within the coefficients and exponents themselves.  Access this context
using 

      Context("LimitedPolynomial-Strict");

Modified Files:
--------------
    pg/macros:
        contextLimitedPolynomial.pl

Revision Data
-------------
Index: contextLimitedPolynomial.pl
===================================================================
RCS file: /webwork/cvs/system/pg/macros/contextLimitedPolynomial.pl,v
retrieving revision 1.13
retrieving revision 1.14
diff -Lmacros/contextLimitedPolynomial.pl -Lmacros/contextLimitedPolynomial.pl -u -r1.13 -r1.14
--- macros/contextLimitedPolynomial.pl
+++ macros/contextLimitedPolynomial.pl
@@ -7,19 +7,29 @@
 
  ##########################################################
  #
- #  Implements a context in which students can only 
- #  enter (expanded) polynomials (i.e., sums of multiples
- #  of powers of x).
+ #  Implements a context in which students can only enter (expanded)
+ #  polynomials (i.e., sums of multiples of powers of x).
  #
  #  Select the context using:
  #
  #      Context("LimitedPolynomial");
  #
- #  If you set the "singlePowers" flag, then only one monomial of
- #  each degree can be included in the polynomial:
+ #  If you set the "singlePowers" flag, then only one monomial of each
+ #  degree can be included in the polynomial:
  #
  #      Context("LimitedPolynomial")->flags->set(singlePowers=>1);
  #
+ #  There is also a strict limited context that does not allow
+ #  operations even within the coefficients.  Select it using:
+ #
+ #      Context("LimitedPolynomial-Strict");
+ #
+ #  In addition to disallowing operations within the coefficients,
+ #  this context does not reduce constant operations (since they are
+ #  not allowed), and sets the singlePowers flag automatically.  In
+ #  addition, it disables all the functions, though they can be
+ #  re-enabled, if needed.
+ #
 
 =cut
 
@@ -38,8 +48,11 @@
   my $self = shift;
   my $super = ref($self); $super =~ s/LimitedPolynomial/Parser/;
   &{$super."::_check"}($self);
-  return if LimitedPolynomial::isConstant($self->{lop}) &&
-            LimitedPolynomial::isConstant($self->{rop});
+  if (LimitedPolynomial::isConstant($self->{lop}) &&
+      LimitedPolynomial::isConstant($self->{rop})) {
+    $self->checkStrict if $self->context->flag("strictCoefficients");
+    return;
+  }
   return if $self->checkPolynomial;
   $self->Error("Your answer doesn't look like a polynomial");
 }
@@ -95,6 +108,15 @@
   return 1;
 }
 
+#
+#  Report an error when both operands are constants
+#  and strictCoefficients is in effect.
+#
+sub checkStrict {
+  my $self = shift;
+  $self->Error("Can't use '%s' between constants",$self->{bop});
+}
+
 ##################################################
 
 package LimitedPolynomial;
@@ -157,11 +179,15 @@
 sub checkPolynomial {
   my $self = shift;
   my ($l,$r) = ($self->{lop},$self->{rop});
-  $self->Error("Addition is allowed only between monomials")
-    if $r->{isPoly};
+  $self->Error("Addition is allowed only between monomials") if $r->{isPoly};
   $self->checkPowers;
 }
 
+sub checkStrict {
+  my $self = shift;
+  $self->Error("You can only use addition for the terms of a polynomial",$self->{bop});
+}
+
 ##############################################
 
 package LimitedPolynomial::BOP::subtract;
@@ -170,11 +196,15 @@
 sub checkPolynomial {
   my $self = shift;
   my ($l,$r) = ($self->{lop},$self->{rop});
-  $self->Error("Subtraction is only allowed between monomials")
-    if $r->{isPoly};
+  $self->Error("Subtraction is allowed only between monomials") if $r->{isPoly};
   $self->checkPowers;
 }
 
+sub checkStrict {
+  my $self = shift;
+  $self->Error("You can only use subtraction between the terms of a polynomial",$self->{bop});
+}
+
 ##############################################
 
 package LimitedPolynomial::BOP::multiply;
@@ -192,6 +222,11 @@
   $self->Error("Multiplication can only be used between coefficients and variables");
 }
 
+sub checkStrict {
+  my $self = shift;
+  $self->Error("You can only use '%s' between a coefficent and a variable in a polynomial",$self->{bop});
+}
+
 ##############################################
 
 package LimitedPolynomial::BOP::divide;
@@ -210,6 +245,11 @@
   return 1;
 }
 
+sub checkStrict {
+  my $self = shift;
+  $self->Error("You can only use '%s' to form fractions",$self->{bop}) if $self->{lop}->class eq 'BOP';
+}
+
 ##############################################
 
 package LimitedPolynomial::BOP::power;
@@ -234,6 +274,11 @@
   return 1;
 }
 
+sub checkStrict {
+  my $self = shift;
+  $self->Error("You can only use powers of a variable in a polynomial");
+}
+
 ##############################################
 ##############################################
 #
@@ -366,6 +411,13 @@
   #
   $context->reduction->set("(-x)-y"=>0);
 
+  #
+  #  A context where coefficients can't include operations
+  #
+  $context = $main::context{"LimitedPolynomial-Strict"} = $context->copy;
+  $context->flags->set(strictCoefficients=>1, singelPowers=>1, reduceConstants=>0);
+  $context->functions->disable("All");  # can be re-enabled if needed
+
   main::Context("LimitedPolynomial");  ### FIXME:  probably should require author to set this explicitly
 }