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#!/usr/bin/perl -ws
use Parse::RecDescent;
sub evalop
{
my (@list) = @{[@{$_[0]}]};
my $val = shift(@list)->();
while (@list)
{
my ($op, $arg2) = splice @list, 0, 2;
$op->($val,$arg2->());
}
return $val;
}
my $parse = Parse::RecDescent->new(<<'EndGrammar');
main: expr /\s*\Z/ { $item{expr}->() }
| <error: expected expression in $item{__RULE__}>
expr: /for(each)?/ lvar range expr
{ my ($vname,$expr) = @item{"lvar","expr"};
my ($from, $to) = @{$item{range}};
sub { my $val;
no strict "refs";
for $$vname ($from->()..$to->())
{ $val = $expr->() }
return $val;
}
}
| lvar '=' addition
{ my ($vname, $expr) = @item{"lvar","addition"};
sub { no strict 'refs'; $$vname = $expr->() }
}
| addition
range: "(" expr ".." expr ")"
{ [ @item[2,4] ] }
addition: <leftop:multiplication add_op multiplication>
{ my $add = $item[1]; sub { ::evalop $add } }
add_op: '+' { sub { $_[0] += $_[1] } }
| '-' { sub { $_[0] -= $_[1] } }
multiplication: <leftop:factor mult_op factor>
{ my $mult = $item[1]; sub { ::evalop $mult } }
mult_op: '*' { sub { $_[0] *= $_[1] } }
| '/' { sub { $_[0] /= $_[1] } }
factor: number
| rvar
| '(' expr ')' { $item{expr} }
number: /[-+]?\d+(\.\d+)?/ { sub { $item[1] } }
lvar: /\$([a-z]\w*)/ { $1 }
rvar: lvar { sub { no strict 'refs'; ${$item{lvar}} } }
EndGrammar
print "> ";
while (<>) { # FOR DEMO CHANGE TO: while (<DATA>)
print eval {$parse->main($_)}||"", "\n\n> ";
}
__DATA__
$x = 2
$y = 3
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))
$x * $y
foreach $i (1..$y) $x = $x * 2 + $i
$x