Parser

[Anonymous Monk]

My apologies, I didn't have time to clean it up, I was in a hurry:

Missing all of the functions
Missing some operators
Missing post-processor that puts the lines back together

its essentially a recursive decent parser for basic, which does a translation to write out an if-then-else construct as a series of if-then and gotos using labels. It prefixes all the lines of the result with the original line number, with the aim of helping a post-processor then go through and correct the original GOTO lines and labels to point to the correct places.

it isn't well tested, but it will handle nested if/then/elses correctly. It could be expanded to handle compound statements with one line of code (mind you, in perl, thats not saying much).

The penalty is that it has to know everything about the language. Its basically half way to a basic compiler for Parrot once you add the functions and ops.

#!/usr/bin/perl

use Data::Dumper;

sub generic_node {
        my ($re, $tree) = @_;
        $current =~ s/^\s*//;
        if ($current !~ /$re/i) {
                return 0;
        }
        $current =~ s/$re//i;
        push @{$tree}, $1;
        return 1;
};

sub generic_node_create {
        my ($re, $tree, $name) = @_;
        $current =~ s/^\s*//;
        if ($current !~ /$re/i) {
                return 0;
        }
        $current =~ s/$re//i;
        push @{$tree}, [$name, $1];
        return 1;
};

sub generic_check {
        my ($re, $tree) = @_;
        $current =~ s/^\s*//;
        return $current =~ s/$re//i;
};

sub operator {
        generic_node_create('^([+-=*\/])',shift,"OPERATOR");
};

sub number {
        generic_node_create('^(\d+)',shift,"NUMBER");
};

sub string {
        generic_node_create('^\"([^\"]+)\"',shift,"STRING");
};

sub numeric_variable {
        generic_node_create('^([A-Z]\d)',shift,"NUMERIC_VAR");
};

sub string_variable {
        generic_node_create('^([A-Z]\$)',shift,"STRING_VAR");
};

sub equals {
        generic_node_create('^([=])',shift,"EQUALS");
};

sub expression {
        my $tree = shift;
        my $t = [EXPRESSION];

        string_variable($t) ||
        numeric_variable($t) ||
        string($t) ||
        number($t) ||
        (generic_check('^[\(]',$t) && expression($t) && generic_check(
+'^[\)]',$t)) ||
        return 0;
        
        if (operator($t)) {
                expression($t) || return 0;
        }

        push @{$tree},$t;       
        return 1;
};

sub assignment {
        my $tree = shift;
        my $t = [ASSIGNMENT];

        numeric_variable($t) || string_variable($t) || return 0;


        equals($t) || return 0;

        expression($t) || return 0;

        push @{$tree},$t;
        return 1;
};

sub comment {
        if (generic_check('^REM')) {
                return 1;
        }
};

sub if_condition {
        my $tree = shift;
        my $t = [IF_CONDITION];

        generic_check('^IF',$t) || return 0;

        expression($t) || die "Invalid IF statement, expected expressi
+on";

        generic_check('^THEN',$t) || die "Invalid IF statement, expect
+ed THEN";

        statement($t) || die "Invalid IF construct, expected statement
+ after THEN";

        if (generic_check('^ELSE',$t)) {
                statement($t) || die "Invalid IF/ELSE construct, expec
+ted statement after ELSE";
        }

        push @{$tree},$t;
        
        return 1;       
};

sub goto_statement {
        my $tree = shift;
        my $t = [GOTO];

        generic_check('^GOTO',$t) || return 0;

        number($t) || die 'GOTO must be followed by a number';

        push @{$tree},$t;
};

sub statement {
        my $tree = shift;
        my $t = [STATEMENT];

        assignment($t) ||
        comment($t) ||
        if_condition($t) ||
        goto_statement($t) ||
        return 0;

        push @{$tree},$t;
};


#
# Pretty-print routines
#

sub print_expression {
        my ($subelement) = @_;

        if ($subelement->[0] eq "EXPRESSION") {
                if ($subelement->[2][0] ne "OPERATOR") {
                        return "(".print_expression($subelement->[1]).
+")";
                } else {
                        return "(".print_expression($subelement->[1]).
+" ".$subelement->[2][1]." ".prin
t_expression($subelement->[3]).")";
                }
        }
        if ($subelement->[0] eq "NUMBER") {
                return $subelement->[1];
        }
        if ($subelement->[0] eq "STRING") {
                return '"'.$subelement->[1].'"';
        }       
        if ($subelement->[0] eq "STRING_VAR") {
                return $subelement->[1];
        }       
        if ($subelement->[0] eq "NUMBER_VAR") {
                return $subelement->[1];
        }       
        
};

sub print_statement {
        my ($statement) = @_;
        my $s = "";
        $level++;
        my $subelement = $statement->[1];
        if ($subelement->[0] eq "IF_CONDITION") {

                add_line("IF ".print_expression($subelement->[1])." TH
+EN GOTO THEN_$level");
                if ($subelement->[3]) {
                        print_statement($subelement->[3]);
                }
                add_line("GOTO END_$level");
                add_line("THEN_$level:");
                print_statement($subelement->[2]);                    
+  
                add_line("END_$level:");
        }
        if ($subelement->[0] eq "GOTO") {
                add_line("GOTO ".$subelement->[1][1]);
        }
        if ($subelement->[0] eq "ASSIGNMENT") {
                add_line($subelement->[1][1]." = ".print_expression($s
+ubelement->[3]));
        }
        if ($subelement->[0] eq "COMMENT") {
                add_line("REM - comment here");
        }
        $level--;
};

sub add_line {
        push @lines, [$current_line, shift];
};

sub pretty_print {
        my $tree = shift;
        
        $ln = shift(@{$tree});
        
        print_statement($tree->[0]);
};



while (<>) {
        chomp($_);
        $current = $_;

        $tree = [];

        generic_node('^(\d+)', $tree) || die "Line number invalid";
        $current_line = $tree->[0];
        
        statement($tree) || die "Invalid statement";
        print Dumper($tree);
        $level = 0;
        pretty_print($tree);
};

for (@lines) {
        print $_->[0]." ".$_->[1]."\n";
}