No, currently it's an inline array of immutable 128-bit numeric values and we use bit-twiddling to pull the real and imaginary parts out. However, that's a temporary hack. (It's also why the mandel benchmark is relatively slow — all the bit-twiddling is not very efficient.)

The longer-term approach is still up in the air and that's what I was talking about above. My favorite approach at this point is to allow fields to be declared as const — which in Julia means write-once. Then if all fields are const the object is immutable and can automatically be stored in arrays inline.