There’s some inconsistency in passed arguments for normal and exponential distributions. Consider the pdfs:
result = pdf_normal(x, loc, scale)
result = pdf_exp(x, lambda)

While the normal distribution procedure uses loc and scale (reminiscent of scipy) as opposed to mu and sigma, the exponential distribution procedure uses lambda rather than scale (also often referred to as beta, which is 1/lambda) and is missing loc.

As discussed on fortran-lang discourse, I suggest introducing more consistency in the stats distribution procedures API. I have made some modifications to stdlib_stats_distribution_exponential (and associated examples and tests) to:

1. use scale instead of lambda, and
2. allow passing an additional location parameter loc.

With the suggested modifications in the PR, rvs, pdf and cdf procedures can now be called as such:
result = rvs_exp(x, loc, scale),
result = pdf_exp(x, loc, scale), and
result = cdf_exp(x, loc, scale).