It’s certainly pure in the Haskell sense: the inputs are two immutable values, and the output is another immutable value. It’s even pure in some Rusty senses: it could take String or &str as input and returns a String — in a model where String is a value, it’s pure.

A lot of modern programming logically involves telling a computer to what to compute and not particularly caring how it gets computed. Yet we mostly lack a language in which one can specify computations and then then run the result portably and safely.