Uniqueness Types

Felix has a special type combinator uniq which is used to specify the value of its argument type is exclusively owned. It is usually applied to pointers.

First lets write a procedure to convert a varray of chars to upper case:

proc toupper(x: varray[char]) {
  for i in 0 ..< x.len.int
    perform set(x,i,toupper(get(x,i)));

Varray’s are passed by reference, so this modifies the varray in place. So how would we use this in a function?

fun upcase(x: varray[char]) {
  var y = varray x; // copy
  toupper y;
  return y;

We can’t just call toupper on the parameter x and return it because in Felix functions are not allowed to have side effects.

But consider this case:

var upped = upcase$ varray("Hello World");
println$ upped.str;

This is inefficient because in upcase we copy the array, modify the copy and return it, but the argument x is then no longer reachable, so it will be garbage collected.

Why not just modify the argument directly in this case:

fun upcase(x: uniq varray[char]) {
  toupper x,peek;
  return x;

 var upped = upcase$ box (varray("Hello World"));
 println$ upped.str;