Erlang has been around since I was 7, so by this point I had 23 years to say hello to Erlang, but just did not get to it until today.. At 10 I was busy with chess, at 15 with girls, at 20 girlfriends, at 25 I don’t remember (that was a quite frequent condition at that age), and only now at 3 o’clock in the beautiful Wednesday morning, here in one of many Chicago hotels, I can really say it outloud: “Erlang ! { me, Hello }.”, or as we all use to saying it “System.out.println( “Hello to Erlang from me” );”, or using the language of our grandparents: “Me: Hello Erlang!”
I already spent about 30 minutes playing with it – 5 minutes with erl interpreter, 5 minutes coding, and 20 minutes figuring out how the heck I can execute it from command line with passing parameters to functions. So here it goes 30 minute summary in “count yourself” number of sentences…
From “Erlang – Quick Start“, I stole a factorial (fac) function:
-module(test). -export([fac/1]). fac(0) -> 1; fac(N) -> N * fac(N-1). |
“Compile the program by typing c(test) then run it” said Erlang site, so I did:
3> c(test). {ok,test} 4> test:fac(20). 2432902008176640000 5> test:fac(40). 815915283247897734345611269596115894272000000000 6> |
What I wanted to do now is to have a more useful factorial that can actually take a number parameter and run from a command line. See, Erlang is cool with numbers, because it uses arbitrary-sized integers when it does integer arithmetic, and I wanted to see that 10000.. digit number on my screen, but I wanted to do it from command line…
Hence, reading further the Erlang’s documentation, in Erlang How To FAQ I found “How to run an Erlang program directly from the unix shell:”
matthias >erl -compile hello matthias >erl -noshell -s hello hello_world -s init stop hello, world |
Great, I thought, let’s run my factorial function then:
$ erl -noshell -s test fac 4 -s init stop {"init terminating in do_boot",{undef,[{test,'test:fac',[['4']]},{init,start_it,1},{init,start_em,1}]}} Crash dump was written to: erl_crash.dump init terminating in do_boot () |
I tried many different combinations, prefixes, suffixes, but nothing seemed to work. Google (at this time) did not really help, and I spend another 5-10 minutes by going to actually study the language from multiple online resources I could find. After aggregating the knowledge, here goes a solution:
-module( matematika ). % module name = file name -export( [factorial/1] ). % exporting a factorial function, that takes exactly 1 argument % public, since exported above factorial( [CommandLineParameter] ) -> % converting the input parameter to an integer, so we can use it in (private) fac below Number = list_to_integer( atom_to_list( CommandLineParameter ) ), FactorialResult = fac( Number ), io:format( "~w! = ~w~n" ,[Number, FactorialResult] ), % pretty much like printf in C init:stop(). % need to explicitly stop it % private, since not exported fac( 0 ) -> 1; % if 0, then return 1 fac( N ) -> N * fac( N - 1 ). % else return fac of N minus 1 |
(I put %comments above for myself, so I can make sense of it tomorrow)
Let’s run it now:
$ erl -noshell -s matematika factorial 10 |
10! = 3628800
Awesome! My first stolen and adapted Erlang creation! Let’s see that factorial of .. let’s say 2500 :)
$ erl -noshell -s matematika factorial 2500 |
2500!=162888842416926354689668105747439663365399942834366593333
761170598517395953006666015681181171091114301822189949967063775
407379642957266480360849144773982699565766503949953039081536069
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520807127737619451831790023662437656379915366899692425817099473
955735537991551620610205879561628364536090561091825520933523438
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Now, THAT is cool!