Distributed Coding Prefunc: Some Basic Erlang & Erlang Shell Basics

Erlang LogoThe first few parts of this series include:

This continues the series, with the intent of getting to “Distributed Coding” with this all ramping up around Erlang and distributed programing in general. In this entry we’ll jump into some of the basics of Erlang and the shell that we can use to test and write some code.

Erlang Basics : The Shell & Some Integers

Before getting to deep into things, here’s some basic shell usage commands and integers to get a feel for the Erlang shell. To start the shell just type erl, which will present the following in the terminal.

$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1>

To exit the shell just type “q().” and hit enter like this.

$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> q().
ok
2> $

It’ll execute a second line that will dump you back out to the terminal. One thing to note, is that when you execute “q().” it drops to a second line numbered two above. If we start the shell back up again and type in some actual code we’ll see that the numbers increment. The idea is similar to each line of code in a code file.

Start up the shell again and type the following.

$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> q().
ok
2> Adrons-MacBook-Air-2:erlangMagic adronhall$ clear
Adrons-MacBook-Air-2:erlangMagic adronhall$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> -69.
-69
2> 2#1212
2> 2#1212.
* 1: syntax error before: 212
2> 2#1010.
10
3> $A.
65
4> $B.
66
5> $a.
97
6> 2*2+100.
104
7>

What I’ve done here is enter some values, primarily all integers. These integers, on the first line is a negative integer. By entering a “.” at the end, the lines executes and returns the value. In this case, the value is “-69”.

The second line, I entered “2#1212” without entering a period to finish the line of code. Thus, nothing executes.

The third line I entered the same thing as the second, “2#1212” which is not valid, giving us a syntax error. On the next line I tweak my value just a bit, entering a valid hexadecimal number “2#1010” with the appropriate period at the end. The line executes and it displays the decimal integer “10”.

So far at this point, this shows us several things:

  • Every line must end with a period to execute.
  • We’ve seen a negative integer, a positive hexadecimal integer and an invalid integer and the error that throws “* 1: syntax error before: 212”.
  • We’ve seen that when the line in the shell successfully executes we get the next incremental line, such as “-69.” was on the first line, the first successful second line was the “2#1010.” value and as we move through each line above the code line increases.
  • A valid integer includes -69, but also includes a hexadecimal representation of an integer such as “#2#1010”.

Moving beyond these values the “$A”, “$B” and “$a” are all ASCII representations and return their numeral values.

On line six I actually perform a basic calculator type multiplication and addition of integer values, which returns the integer value 104.

Now that we have the shell start and stop figured out, that gives us a platform in which to start stepping through some of the basics of Erlang.

Erlang Floats

Above we’ve seen how regular integers are represented such as 2, 100 or -69 along with hexadecimal and ASCII numeric representations. For floats, we’re looking at similar uses. Spool up the shell again and try out these examples.

1> 21.21.
21.21
2> -1234.1234.
-1234.1234
3> 3042.1.
3042.1
4> 650.21
4> 650.21.
* 2: syntax error before: 650.21
4> 650.21
4> 650.21.
* 2: syntax error before: 650.21
4> 650.21.
650.21

Lines 1-3 show some standard floats. On the first part of line four I left off the period to show that it won’t accept the line, even with a period in it, because of the numbers after that period. In that second line number four I’ve added the period, but it throws a syntax error.

Then note I did the exact same thing again, to show that entering a float and then adding the period, will still leave the float broken because of the way the Erlang compiler is waiting to execute the code. It’s almost like the compiler sees “650.21650.21” which of course doesn’t mean anything. However on the fifth line of the number four lines, I finally get a good execution and it displays the float I’ve entered “650.21”. Be sure when you’re fixing values like this, the compiler could be stuck in mid execution, because without the period at the very end, being the last character on the line, the compiler is still waiting for the period to actually start the execution.

Mathematical Operators, You Know Those Things That Help Math Explain the Universe!

Erlang has the standard operators that you’re used to, especially if you’ve done anything with a language based around C syntax such as JavaScript, Java, C#, C++, Objective-C… well, pretty much every regularly used static typed language and more. In functional languages these operators are pretty common too. If any don’t look familiar, be sure to get intimate with them. You’ll find them all over Erlang code.

The + and – characters act as addition and subtraction, but also as unary operators. In order of precedence, the unary operations of the + and – have the highest precedence and the addition and subtraction usage of the + and – have the lowest precedence.

The * and / signify multiplication and division. They’re precedence is just below that of the unary operators of + and -.

Two that are a slight bit unique, is the usage of “div” and “rem” for division and remainder, for integer numbers. This begs to bring up the point that whenever an integer is involved in multiplication, division, addition, subtraction or other operations, it is coerced into a float when being operated on against another float. Such as 2 + 3.47 would mean that 2 is turned into a float before it is added to 3.47.

Here’s some examples of operations occurring between numbers via the shell.

$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1> +15.
15
2> +2.
2
3> -11.
-11
4> 11 div 5.
2
5> 12 div 5
5> .  
2
6> 15 div 5.
3
7> 14 div 5.
2
8> (12 + 345) div 12.
29
9> (3+2)/5.
1.0
10> 2*3*4.12.
24.72
11> 1+1+1+1+13+-2.
15
12> 2/3 + 3/4 - (1/2 + 2/3) - 1.
-0.75
13> 14 rem 5.
4
14> 

These examples show a number of effects of how the operators work. Notable is how everything looks very much like simple math being done. This is one of the more readable aspects of Erlang. Note also that the rounding generally goes down.

Distributed Coding Prefunc: Ubuntu Erlang Dev & EUnit

Erlang LogoAfter installing Erlang on OS-X and then getting QuickCheck installing via Erlang, I wanted to expand the OS options I’m using to Ubuntu (i.e. Linux). So in this entry I’m going to cover the Erlang install, a quick eunit bit, and then a QuickCheck install and sample. The first step in geting Erlang installed is deciding how you want to install it. So far, it isn’t like Rails where it is pretty important which method you pick to how well it will work for you on Ubuntu. For Erlang all methods get you started with a good version that is working. The method I used was simply to install with apt-get.

sudo apt-get install erlang erlang-doc

After installing, always a good idea to run things and make sure they’re all content and happy with your machine. Startup the erlang shell.

erl

Then run some of the commands. Some I’ve found that will present you with useful and interesting information ist he erlang:system_info function with appropriate parameter passed. The otp_release parameter will get the version of erlang, the cpu_topology shows you the processor outlay for you machine (or in this case my virtual machine, with a single processor core allocated to it), and allocated_areas shows a bit about system memory allocations.

Eshell V5.9.1  (abort with ^G)
1> erlang:system_info(otp_release).
"R15B01"
2> erlang:system_info(cpu_topology).
[{processor,{logical,0}}]
3> erlang:system_info(allocated_areas).
[{sys_misc,80748},
 {static,1007616},
 {atom_space,98328,73387},
 {atom_table,95961},
 {module_table,9084},
 {export_table,50316},
 {export_list,240960},
 {register_table,180},
 {fun_table,3266},
 {module_refs,2048},
 {loaded_code,3437028},
 {dist_table,403},
 {node_table,227},
 {bits_bufs_size,0},
 {bif_timer,80200},
 {link_lh,0},
 {process_table,262144},
 {ets_misc,52504}]
[{processor,{logical,0}}]

Now that erlang is effectively installed we can write a little sample code. To do this I created a directory called “TestingErlang” and in it placed an Erlang code file called “eunit_tests.erl”. Note: I’m using Sublime 2 on Ubuntu, so exchange that for whatever text editor you’re using for your Erlang coding.

adron@ubuntu:~/Codez$ mkdir TestingErlang
adron@ubuntu:~/Codez$ cd TestingErlang
adron@ubuntu:~/Codez/TestingErlang$ sublime eunit_tests.erl

Add the header file include.

-define(NOTEST, true).
-include_lib("eunit/include/eunit.hrl").

Adding the header file include will cause all the function with _test() or _test_() to automatically be exported. An exported function of test() is created that can be used for running all of the unit tests. This also will include the preprocessor macros of EUnit for writing tests. So now throw a super simple test into the file.

You may want to, amid the automatic export of the methods ending in _test() or _test_() not name them this, and you’ll then need to add a line at the top of your code file like this.

-export([reverse_test/0]).

After this, add the function test as shown below.

reverse_test() -> lists:reverse([1,2,3]).

The complete code file should look like this.

-module(eunit_test).
-define(NOTEST, true).
-include_lib("eunit/include/eunit.hrl").
-export([reverse_test/0]).

reverse_test() -> lists:reverse([1,2,3]).

Build it and call the function.

Eshell V5.9.1  (abort with ^G)
1> c(eunit_test).
{ok,eunit_test}
2> eunit_test:reverse_test().
[3,2,1]
3> 

BOOM! Passing. So now we know we have a good Erlang install and eunit is setup and usable. In the following blog entries I have in the works, we’ll dive deeper into what and how Erlang works from an extremely basic level all the way to diving into some of the more complex features.

Distributed Coding Prefunc: Installing QuickCheck for Great Testing

Erlang LogoA few weeks ago I kicked off this series of “Distributed Coding Prefunc: Up and Running with Erlang” and had wanted to keep up the momentum, but as life goes I had to tackle a few other things first. But now, it’s time to get back on track with some distributed computing. I intend to write tests with my samples, as I often do, I decided to take a stab at .

Before going forward, note that there is QuickCheck for Haskell and there is a QuickCheck for Erlang. Since the point of this “Distributed Coding Prefunc” is to get started coding with Erlang from zero, I’ll be talking about the Erlang version here. This version is created by John Hughes and Koen Claessen, starting the Quviq Company in 2006.

To download QuickCheck choose the version you intend to use, I’ve chosen the commercial license version from the download page.

At the command prompt, install QuickCheck by running Erlang and then run the install with these commands.

Launch Erlang:

$ erl
Erlang R15B01 (erts-5.9.1)  [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.1  (abort with ^G)
1>

Then execute the install:

1> eqc_install:install().

If the execution of the install displays this error, you’ll need to use sudo.

Installing ["pulse-1.27.7","eqc-1.27.7","eqc_mcerlang-1.27.7"].
Failed to copy pulse-1.27.7--copy returned {error,eacces}??
** exception exit: {{error,eacces},"pulse-1.27.7"}
     in function  eqc_install:'-copy_quickcheck/3-lc$^0/1-0-'/3 (../src/eqc_install.erl, line 63)
     in call from eqc_install:install2/4 (../src/eqc_install.erl, line 44)

Kill Erlang with a ctrl+c and restart Erlang with the sudo command.

$ sudo erl

Now when you install you should see the following result or something similar. You’ll be asked to continue, select lowercase ‘y’ to continue. It may be different for some, but when I hit uppercase ‘Y’ (I suppose I got overzealous to install QuickCheck) it finished as if I’d hit no or something else.

1> eqc_install:install().
Installation program for "Quviq QuickCheck" version 1.27.7.
Installing in directory /usr/local/lib/erlang/lib.
This will delete conflicting versions of QuickCheck, namely
    []
Proceed? y
Installing ["pulse-1.27.7","eqc-1.27.7","eqc_mcerlang-1.27.7"].
Quviq QuickCheck is installed successfully.
Looking in "/Users/adronhall"...   .emacs not found
Could not find your .emacs file!
Try install("path-to-emacs-file") or install(new_emacs).
Bookmark the documentation at /usr/local/lib/erlang/lib/eqc-1.27.7/doc/index.html.
ok

You’ll note above, I don’t currently have emacs installed. The reason it looks for emacs is because QuickCheck has templates/ops mode for emacs. So if you use emacs you’re in luck. I on the other hand, don’t, so I’ll just be using this from wherever I’m using it.

In addition to the lack of emacs, another important thing to note from the message is the link to documentation. Once you get this link open it up and check out the docs. They’re broken out into easily readily topic spaces and are a good place to do initial reference checking while you’re writing up your specs.

If you have a license, it is important to note, that if you’ve used sudo with your installation you’ll need to kill your running Erlang session and start it anew without sudo. Otherwise you’ll run into issue down the road trying to use the libs (unless of course you want to go hack on your permissions manually). Once you’re ready to register the software it’s simply one command, where xxxxx is your license key.

eqc:registration("xxxxxxxxxxxx").

Alright, next time we’re on to next steps…

Distributed Coding Prefunc: Up and Running With Erlang

Erlang LogoBefore diving into architecture, coding, descriptions and other things related to distributed computing over the coming months. It helps to become familiar with a language like Erlang. I’m going to dive immediately into getting Erlang up and running before any theory, description or otherwise, so following the most direct installation…

Installing Erlang

This is easy on OS-X. Pending of course you have the XCode and Developer Tools installed.

curl -O http://erlang.org/download/otp_src_R15B01.tar.gz
tar zxvf otp_src_R15B01.tar.gz
cd otp_src_R15B01

Then compile the latest XCode and tools you can use the LLVM Compiler.

CFLAGS=-O0 ./configure --disable-hipe --enable-smp-support --enable-threads \
--enable-kernel-poll --enable-darwin-64bit
make
sudo make install

For more information on building Erlang you can also check out the Erlang Organization Site. It’s that simple, so now that it is up and running you should be able to check that all is right with the install by pulling a version.

erl -version

In my follow up blog entry I’m going to take you through the Rebar Riak Core Templates. This will get you up and running with an Erlang Application. This application can then be used either as a stand alone Erlang App for whatever you want to build with it or as a great starting point to build against Riak.