Perl6 - Matt's Programming Blag

A familiar example of a factorial function using a switch (given/when) statement:

sub factorial (Int $i) { given $i { when $i < 0 { fail "Not a Natural Number" } when 0 { 1 } when 1 { 1 } default { $i * factorial $i-1 } } } use Test; isa_ok factorial(-1), Failure, "Factorial for -1 fails"; ok factorial(0) == 1, "Factorial for 0"; ok factorial(1) == 1, "Factorial for 1"; ok factorial(5) == 120, "Factorial for a larger integer";

Using the meta [*] list operator we can calculate the product of a range 1 upto the desired number:

sub factorial { if $^i < 0 { fail "Not a Natural Number"; } else { [*] 1..$i; } } use Test; isa_ok factorial(-1), Failure, "Factorial for -1 fails"; ok factorial(0) == 1, "Factorial for 0"; ok factorial(1) == 1, "Factorial for 1"; ok factorial(5) == 120, "Factorial for a larger integer";

Finally we can introduce a postfix ! operator for producing the factorial of a number, 5! == 120 being true.

multi sub postfix:<!> (Int $i where $i < 0) is tighter(&infix:<*>) { fail "Not a Natural Number in Factorial" } multi sub postfix:<!> (Int $i where 0|1) is tighter(&infix:<*>) { 1 } multi sub postfix:<!> (Int $i where $i >= 1) is tighter(&infix:<*>) { $i * ($i-1)! } use Test; isa_ok -1!, Failure, "Factorial for -1 fails"; ok 0! == 1, "Factorial for 0"; ok 1! == 1, "Factorial for 1"; ok 5! == 120, "Factorial for a larger integer";