We present a new approach to implementing arrays in a pure lazy functional programming language. The arrays can be updated destructively by using uniqueness typing, and the elements can be unboxed. We describe the implementation of these arrays in the functional programming language Clean 1.1. The performance of two sorting algorithms and a fast fourier transformation written in Clean using arrays is compared with similar programs written in C. The current implementation of Clean is on average about 25 percent slower than C for these programs.