Why are the elements of the matrix and vector types in the F# Powerpack mutable?

Question

F# is often promoted as a functional language where data is immutable by default, however the elements of the matrix and vector types in the F# Powerpack are mutable. Why is this?

Furthermore, for which reason are sparse matrices implemented as immutable as opposed to normal matrices?

Answer 1

The standard array type ('T[]) in F# is also mutable. You're mostly correct -- F# is a functional language where data immutability is encouraged, but not required. Basically, F# allows you to do write both mutable/imperative code and immutable/functional code; it's up to you to decide the best way to implement the code for your specific application.

Another reason for having mutable arrays and matrices is performance -- it is possible to implement very fast algorithms with immutable types, but users writing scientific computations usually only care about one thing: achieving maximum performance. That being that case, it follows that the arrays and matrices should be mutable.