You can use regex to check for prime numbers:

alias isprime {
  return $+($,$iif($regex($str(1,$1),/^1?$|^(11+?)\1+$/),false,true))
}

For the iscomposite alias, you can use $iif() to shrink it a bit:

alias iscomposite {
  return $+($,$iif($isprime($1) == $true,false,true))
}

Note that I didn't make the regex pattern. I've applied it to mIRC.