That's pretty nice... Here is a straight port from Ruby to Perl6:
sub evaluate ($expr) {
my @stack;
for $expr.split {
@stack.push: do {
when /^ -?\d* $/ { $_ }
when <+ - * /> {
my $right = @stack.pop;
my $left = @stack.pop;
defined($left & $right) or fail "Stack underflow";
$left.$_: $right;
}
fail "Invalid token: $_";
}
}
@stack == 1 or fail "Invalid stack: @stack";
@stack[0];
}
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