In Perl 6, that means that foo is a function that takes a value of type T and returns a value of type T, for any type T. Let???s look inside that definition more mathematically: for every set T, x ??? T ??? foo(x) ??? T. You might as well not write the body of the function, for the only function that this could possibly be is the identity! (Note that the only function in Haskell with this signatur