Can we create pure functions in Fortran which generate random numbers?

Question

My goal is to write a pure function using random numbers which can be used in a DO CONCURRENT structure. The compiler does not seem to permit this.

mwe.f95:8         


        

Answer 1

You'll need pure random number generator. It is quite possible to make, say, for Linear Congruential Generator, where seed (being 64bit unsigned integer) is the same as state and is the same as return value. In that case state/seed is kept externally outside the sampling routine, passed explicitly and on getting it back from RNG is stored

Answer 2

This is completely against the concept of pureness. True pure functions, as found in true functional languages, should always return the same result for the same input. Fortran pure functions can read module variables and therefore are more complex.

It is not even a good idea to have any function, not just a pure function, to return pseudo-random numbers. When you have more function calls in an expression the Fortran compiler is permitted to evaluate the function just once. That is even more likely, or better justified, when that function is pure.

I would suggest to just use regular DO loops and call random_number or other custom PRNG subroutine. Even if you want automatic parallelization or similar , the compilers are normally capable to treat regular DO loops equally well as DO CONCURRENT.