Hathy

I had always been interested in writing a simple LISP interpreter. Well, had some free time today, and thought, why not create a parser that can return the results of mathematical expressions that are entered as LISP expressions. Nothing fancy, but here is a class that can take in a file containing math expressions and return the results of each line as an array.

package Processor;

use strict;
use warnings;

#Creates the object, nothing else
sub new {
	my $class=shift;
	my $self={};
	bless $self, $class;
	$self;
}

#Set the file to be processed, or return the currently
#selected file
sub file{
	my $self=shift;
	my $filename=shift;
	$self->{filename}=$filename if defined $filename;
	$self->{filename};
}

#Process each line of the file as an expression, and load
#an array with the results
sub process_file{
	my $self=shift;
	return unless defined $self->{filename};
	my @results=();
	open FILE, $self->{filename} or die ("Could not open file") ;
	while(<FILE>){
		push @results, ($self->process_line($_));
	}
	@results;
}

#Processes LISP style expressions that are valid
sub process_line{
	my $self=shift;
	my $line=shift;
	my @opstack=();
	my @operandStack=();
	for(split(" ",$line)){
		push @opstack,$_ if $_ eq "(" or 
				    $_ eq "+" or 
				    $_ eq "-" or 
				    $_ eq "*" or
				    $_ eq "/";
		push @operandStack,$_ if /^\d+$/;
		if($_ eq ")"){
			my $op1=pop @operandStack;
			my $op2=pop @operandStack;
			my $op=pop @opstack;
			pop @opstack;
			if ($op eq "+"){
				push @operandStack,($op1+$op2);
			}elsif ($op eq "-"){
				push @operandStack,($op2-$op1);
			}elsif ($op eq "*"){
				push @operandStack,($op1*$op2);
			}elsif ($op eq "/"){
				push @operandStack,($op2/$op1);
			}
		}
	}
	return "Invalid expression" unless @operandStack==1;
	$operandStack[0];
}

1;

I sure do hope to add variables to this, so that results can be used from one line of the file to another.