Erlang: commandline interface to factorial using escript

fact.erl

#! /bin/env escript

%% Simplest use of Erlang's escript and commandline arguments
%% for an interface to the tail-recursive factorial function.

-module(fact).

fact(N) ->
	fact(N, 1).

fact(N,Product) when N =< 1 ->
	Product;
fact(N,Product) ->
	fact(N-1, Product*N).

main([]) ->
	io:format("Usage:  fact  {integer}~n"),
	halt(1);
main([NumericString|_]) ->
	N = list_to_integer(NumericString),
	io:format("fact(~p) = ~p~n", [N, fact(N)]).

Hopefully this illustrates that making standalone executable scripts with Erlang really isn't much of a hurdle, even for day-1 beginners. To use it from a Unix-type commandline:

$ chmod +x fact.erl
$ ./fact.erl
Usage:  fact  {integer}
$ ./fact.erl 50
fact(50) = 30414093201713378043612608166064768844377641568960512000000000000

More usefully, two numbers would be taken from the commandline by matching on a 2-element list, and a very small additional looping function would use fact(N) to generate all factorials from the first number to the second.

thanks for sharing, very useful to see it applied like this!