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Re^2: Using overload to parse arithmetic terms

by blokhead (Monsignor)
on Feb 07, 2006 at 14:41 UTC ( #528524=note: print w/replies, xml ) Need Help??

in reply to Re: Using overload to parse arithmetic terms
in thread Using overload to parse arithmetic terms

To expand (and generalize) on this a bit, let each type of operation have a precedence. Then as you walk the expression tree, you need to parenthesize operations that have a lower precedence than the expression they appear within.

A good way to do this is via OO, representing your operation types as subclasses:

package Op; sub new { my $pkg = shift; bless [@_], $pkg; } sub children { @{ $_[0] } } sub display { my ($self, $precedence) = @_; $precedence = 0 if not defined $precedence; my $s = join $self->op, map { $_->display($self->precedence) } $self->children; $s = "($s)" if $precedence > $self->precedence; return $s; } package Op::Addition; @ISA = qw[Op]; sub precedence { 10 } sub op { " + " } package Op::Multiplication; @ISA = qw[Op]; sub precedence { 20 } sub op { " * " } # ... package Op::Term; @ISA = qw[Op]; sub precedence { 100 } sub display { $_[0][0] }
Then you just call the display method on an Op tree. For example,
Op::Multiplication->new( Op::Term->new(5), Op::Addition->new( Op::Term->new(6), Op::Term->new(7) ) )->display; # 5 * (6 + 7) Op::Addition->new( Op::Term->new(5), Op::Multiplication->new( Op::Term->new(6), Op::Term->new(7) ) )->display; # 5 + 6 * 7
And you can see how easy it would be to add new types of operations, due to the beauty of subclassing.


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Re^3: Using overload to parse arithmetic terms
by japhy (Canon) on Feb 07, 2006 at 15:38 UTC
    That is a beautiful approach and technique. ++ well deserved.

    Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
    How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
Re^3: Using overload to parse arithmetic terms
by esskar (Deacon) on Feb 07, 2006 at 16:07 UTC
    perfect ... thanks.

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