Erlang: parallel-map and parallel-foreach

In a post to erlang-questions last summer, Erlang on the niagara, Erlang super-hero Joe Armstrong posts a simple "parallel-map" implementation that he uses in place of some of his simple "iterative-foreach" calls to quickly achieve dramatic speed improvements by taking advantage of multiple processors. The post got me thinking, particularly in light of recent discussions on side-effects and purity, about the real differences and guarantees of such simple things as map and foreach.

Principally, map is (1) supposed to be called with a function with no side effects, (2) guarantees order of return to match the input list, and (3) makes no guarantee on order of function calls, while foreach is (1) used explicitly (and solely) for its side effects as (2) it has no return value (other than the atom ok for success) and (3) guarantees the order of function calls to match the order of the input list. Now, Joe's code was for a parallel-map implementation -- returning a list of results from the application of the given function, no guaranteed order of execution -- when it was actually being used to replace a foreach statement. It happened to work out nicely that his code did not rely on any order of execution promises.

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Erlang fizzbuzzery: Project Euler Problem 1

Another excercise in solving a simple problem using multiple programming idioms in the same language, Erlang. This time the simple problem is Problem 1 from Project Euler: "If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000." In particular, the solutions presented are (1) a simple single-function loop which uses if to check for termination and matching numbers, (2) a simple loop using Erlang's guard syntax, and (3) a few solutions using some of the functions of the lists module, one using Erlang's list comprehension syntax.

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Erlang fizzbuzzery

Herein, a few different ways of implementing "a" solution to the FizzBuzz problem, showing examples of some different programming styles in Erlang solving the same problem.

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Erlang example: "parallel primes"

The code for this example is fairly simplistic and does many things wrong. Nevertheless it does provide some examples of several Erlang constructs. Hopefully this little example is self-explanatory enough to be useful on its own for now, but I hope to now add some commentary as to its construction and flaws. First, it is obviously by far not a good way to generate a list of primes. The point is not to generate primes well, but to demonstrate, using a fairly easy-to-understand problem domain, how one might write a simple program that (1) spawns a set of worker processes to operate upon work items from a queue, (2) accumulate results from their work in an accumulation process, and (3) retrieve those results.

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