There's a Catch

• Perl uses applicative order evaluation for functions

• If it works for Perl, why not for -calculus?

• Answer: It doesn't always work for Perl

• Example: You can't write a function that behaves like if:

        sub my_if {
          my ($cond, $then, $else) = @_;
          if ($cond) { return $then }
          else       { return $else }
        }

        sub fact {
          my $n = shift;
          return my_if($n == 0, 1, $n*fact($n-1));
        }

• Infinite loop on all inputs

• If we use Perl functions as -expressions, we always get applicative-order evaluation

• For IF and Y, this doesn't work

• We can't change Perl's evalutation order

Next

Copyright © 1999 M-J. Dominus