A hail of mail with Hailgun

The Hailgun library

Very recently I was writing a shared service in Haskell and we realised that we would need to integrate with an email service provider. After a little bit of research I concluded that Mailgun was a great service for developers to send their emails so I looked for Mailgun integration libraries that looked like they were being groomed to be "the" Mailgun library for Haskell and I was disappointed to find that the existing libraries did not seem to be very comprehensive or did not make use of type safety.

So I wrote my own library and I called it "Hailgun". Then I uploaded it to Hackage.

I believe this library to be better because:

  • It supports simple email sending just like the existing libraries.
  • The type system is stricter, leaving less room for incorrect usage of the API. Giving you the assurance that you will be sending emails correctly.
  • There is a backlog of issues that has been roadmapped such that the library will have full Mailgun API support. It just needs time to develop it.
  • The support for the various parts of the API is being implimented and released incrementally so that people can get the benefits of the hailgun library now.
  • It comes with the hailgun-send executable out of the box. This executable can be used on the command line to send emails through the Mailgun API.
  • This library has been written in pure Haskell and does not use any FFI wrappers around another Mailgun library: the goal of this library is to work on any platform.

And this is just for version To people that read this in the future, you should check out the library on hackage to see what it impiliments now.

Sending a test email

I’m going to explain here how to send a test email using hailgun-send so that you can quickly see that the library works and can be used to send your emails in pure Haskell code.

If you don’t have a Mailgun account and a sandbox for it then go sign up for one. Once you have created your account you should have been given an API key and have a sandbox domain name. Please create a file called ‘hailgun.send.conf’ in the current directory and make the contents of the file:

mailgun-domain    = "sandbox-mailgun-domain.com"
mailgun-api-key   = "key-thatmailgungaveme9234uoah234"

Once you have that file in place you should then be able to use hailgun-send. For example, this is one invocation that I used:

hailgun-send \
  --from 'postmaster@sandbox17bd032a44ea44f5b540470a8ab4787f.mailgun.org' \
  --to 'robertmassaioli@massaioli.com' \
  --subject 'Hailgun v0.1.0 test email' \
  -x data/email.text \
  -m data/email.html

You will notice that I provided a from and to address, an email subject and the content of the email in two files. You should create the email.text and email.html files yourself or you could simply use the ones that are avaliable in the repository. Please note that the HTML version of the email is optional but the text version is always required.

You should see something like the following email appear in your inbox:

This email was delivered by Mailgun and sent using Hailgun.

This email was delivered by Mailgun and sent using Hailgun.

Concluding Words

In short you should now be able to send emails, via Mailgun, with Haskell and the library should be improved to have more API support as time goes by. I hope this is useful to some peoplee and, if a Mailgun developer should happen to swing by this post then please feel free to review my code.

Parsing WAVE files correctly in Haskell.

Wavy over WAVE

Playing with audio data is a ton of fun and something that I believe that Haskell could do very well. Processing audio data safely and efficiently seems to fit very well into Haskell’s model so, overy a year ago, I started working on and off on a WAVE file format parser. I have been working on it very infrequently (bus trips and other spare time) and I rewrote it once but today I am pleased to announce the release of the very first version of my ‘wavy’ package that lets you extract data from WAVE files in Haskell. The features of this release include:

  • Methods to Parse and Assemble Wave Files
  • Support for different orderings of RIFF chunks (via the riff library I wrote previously)
  • A split between the parsers for the container format and the data allowing efficient metadata parsing.
  • The ability to pase the data into Int64 or Float formats so that you can handle the data in whichever way that you please.
  • Example programs that make use of the library for your perusal and use.

Things which the library is currently missing include:

  • RIFX support
  • Direct support for maintaining RIFF chunks that are not mentioned in the WAVE specification.

Getting the Code

The code is on hackage as the wavy library so you can install it by:

cabal update
cabal install wavy

Please feel free to give it a try. Probably the best way to quickly see it working is by finding a WAVE file on your machine:

locate '*.wav'

And then passing that wave file as the first argument into wave-info. For example:

$ wave-info /Applications/Steam.app/Contents/MacOS/Friends/friend_join.wav
File: /Applications/Steam.app/Contents/MacOS/Friends/friend_join.wav - 2s
     Audio Format:  Microsoft PCM
     Channels:      2
     Sample Rate:   44100
     Bits Per Sample:  16
     Byte Rate:  176400
     Block Alignment:  4

   INFO Metadata
     Creation Date: 2010-03-11
     Engineers:       - Kelly Thornton
     Creation Software: Sony Sound Forge 9.0

As you can see it can parse an audio file very quickly. The wave-info program is very efficient because the library is lazy and does not parse the data chunk unless you specifically require it.

Using the Library

I would recommend that you start by looking at the executables in the libraries source code for examples of how this library can be used in your applications. The wave-info source code can be found on BitBucket.

Once you have finished doing that then you can have a read throug the documentation on Hackage to get a full understanding of what methods the library provides.

If you have any questions then please do not hesitate to contact me or comment on the blog.

(This blog post was produced using pandoc)

Tutorial: The basics of cipher-aes

Cipher AES Tutorial

In this guide we are going to have a quick run through the cipher-aes library to show how it can be used in your own projects. We will just stick to simple ECB encryption and leave other encodings for a future guide.

Note: In order to follow along with this guide you will need a working knowledge of Haskell (Understanding the first few chapters of Real World Haskell should do the trick)

Import all the Things

But first lets get the imports out of the way:

module Main where

import qualified Crypto.Cipher.AES as CCA
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as BC
import qualified Data.ByteString.Base16 as B16
import Data.Char (chr)

You will notice that I like to qualify everything to give you a better idea of which libraries these functions are coming from.

How much seed data do we need?

Whenever you wish to begin the encryption you need to provide a seed. You can see that in the type of the initAES method:

initAES :: Byteable b => b -> AES

For the sake of this guide we are going to use a 32 byte hash in order to initialise the AES cipher. This library supports starting with 16, 24 and 32 byte hashes. In most programs that you write this initial seed is going to be called a "secret key" or "shared key"; it will often be stuck in a configuration file in a secure location and it should certainly be kept out of the source code. So developers often like specifying this secret key as a big hexadecimal String. However, even though you could read this string straight into a ByteString that would not be correct because each hexidecimal character only has 16 unique points of data (4-bits).

So the quesiton is: how many hexidecimal characters do we need in order to get 32 bytes of random seed data?

Lets just use Haskell to do it for us. We know that there are four bits of data in each hexidecimal character:

bitsPerHexChar :: Integer
bitsPerHexChar = 4

There are 8 bits per byte:

bitsPerByte :: Integer
bitsPerByte = 8

We want 32 bytes of seed data:

requiredSeedBytes :: Integer
requiredSeedBytes = 32

With that information we can work out how many hexidecimal characters are required:

requiredHexChars :: Integer
requiredHexChars = (requiredSeedBytes * bitsPerByte) `div` bitsPerHexChar

Now this only works because either requiredSeedBytes or bitsPerByte is divisible by 4. In our case they both are.

Loading the seed from Hexidecimal input

Now that we know that we need to load requiredHexChars many characters in order to initialise the AES encryption. However this is going to be given to us in this form:

type HexStream = B.ByteString

Where every byte in the stream has 4 bits of unique data. However, the initAES function expects the data to be a little more compressed than that. It expects 8 bits of unique data per per byte to make a true seed for the AES algorithm:

type InputSeed = B.ByteString

And before we even try to convert from one type to the other we should validate that our inputs are correctly formated Hexidecimal strings (validate your inputs):

validHex :: HexStream -> Either String HexStream
validHex input = if inputLength == requiredHexChars
   then Right input
   else Left $ "Expected " ++ show requiredHexChars ++ " hex characters but instead recieved " ++ show inputLength
      inputLength :: Integer
      inputLength = fromIntegral . B.length $ input

Then we will want a function that converts from one to the other and squishes two 4 bit chunks together to form an 8 bit number. We can do that like so by using the base16-bytestring library:

toSeed :: HexStream -> Either String InputSeed
toSeed input = if B.null errors 
   then Right seedData 
   else Left "The input data was not made up of Hexidecimal characters."
      (seedData, errors) = B16.decode input

We can quickly join the two to have a useful function for loading seed data from hex streams:

toValidSeed :: HexStream -> Either String InputSeed
toValidSeed input = toSeed =<< validHex input

In order to actually make use of these methods we need to have a hexidecimal string to start off with as our input. Here is one that I generated for this guide (you will need to generate another truly random one for your production applications):

chosenHexStream :: HexStream
chosenHexStream = BC.pack "e5d6834e0e52a78a47fc1c8887ca0e0ecd0863df89e6a3eebf7085bd131bb854"

We can then use this seed to create an InputSeed but it might not parse so we want to encode that in the types:

potentialInputSeed :: Either String InputSeed
potentialInputSeed = toValidSeed chosenHexStream

Using the AES library

Now that we have our input seed we can initialise an AES encryption context:

aesEnc :: Either String CCA.AES
aesEnc = fmap CCA.initAES potentialInputSeed

And with it we can start to do some encryption! Woo! However, we need some data to try and encrypt…hmmm. Lets make some random test strings:

testData0 :: B.ByteString
testData0 = BC.pack $ "It might seem crazy what I’m about to say" 
   ++ "Sunshine she’s here, you can take a break"
   ++ "I’m a hot air balloon that could go to space"
   ++ "With the air, like I don’t care baby by the way"

testData1 :: B.ByteString
testData1 = BC.pack "B-b-b-baby, you just ain't seen n-n-nothin' yet Here's something that you never gonna forget"

testData2 :: B.ByteString
testData2 = BC.pack $ "There's a calm surrender to the rush of day" 
   ++ "When the heat of a rolling wind can be turned away"

So, with this test data we can now try and run some encryption. Lets look at the most basic encryption method:

encryptECB :: AES -> ByteString -> ByteString

Okay, that type seems pretty self explanatory, give me an AES context and the thing that you want to encrypt and I’ll run some ECB encryption over it and give the result to you in a Strict ByteString. So you would think that we could just do something like this:

broken :: Either String B.ByteString
broken = fmap (flip CCA.encryptECB testData0) aesEnc

After all, it even compiles! But, surprisingly, that does not work. Instead you get the following error message:

Encryption error: input length must be a multiple of block size (16). Its length is: 173

As you can see the cipher-aes library apparently requires that all data be aligned to a block size of 16 bytes. I know that the correct units are bytes based on the fact that testData0 is 173 units long. To solve this problem we need to make sure that we are always giving the encryptECB function a bytestring that always meets that boundary. The most sensible way I can think to do that is to use a zero padded bytestring. So lets try and build a funciton that will do that for us:

type PaddedByteString = B.ByteString

zeroPadData :: B.ByteString -> PaddedByteString
zeroPadData input = input `B.append` padding
      padding = BC.replicate requiredPadding (chr 0)
      requiredPadding = case inputLength `mod` 16 of
         0 -> 0
         x -> 16 - x
      inputLength = B.length input

Now with this new zeroPadData function we can build an encryption function that will always work:

safeEncryptECB :: CCA.AES -> B.ByteString -> B.ByteString
safeEncryptECB enc input = CCA.encryptECB enc (zeroPadData input)

With this new safe encryption function we can encrypt all of our test data. However, before we do that lets first try and show what happens when you go in the other direction.

Decrypting your AES data

Now that we have function that can encrypt our data we really want a function that can go in the other direction and decrypt it. Lets take a look at the decryptECB function from the cipher-aes library:

decryptECB :: AES -> ByteString -> ByteString

Once again this one is pretty simple, given an AES content and an encoded string of data it will decode it back into the original format. So lets try and write a function that will do the reverse of the operation that we did in the safeEncryptECB function. It is important to note that the data also needs to come back in 16 byte blocks otherwise it cannot be decoded. We can ensure that and use types to handle the errors instead of the error command:

safeDecryptECB :: CCA.AES -> B.ByteString -> Either String B.ByteString
safeDecryptECB enc encodedData = if alignment /= 0
   then Left $ "Error: encoded data should have been 16 byte aligned but was off by: " ++ show alignment
   else Right . fst . BC.spanEnd (== (chr 0)) $ CCA.decryptECB enc encodedData
      alignment = B.length encodedData `mod` 16

This function handles encrypting and decrypting the data safely. Which is especially important because often the encrypted data has the potential to be modified by other systems before coming back to us.

However, it is really important to note that the safeEncryptECB and safeDecryptECB functions are not inversions of eachother. Specifically, if you have an input string that legitimately has trailing nul characters and you encrypt it and then decrypt it then those characters will be stripped in the final output. That is something to be wary of. However, the set of strings that do not end in nul characters will be invertable by these functions.

Now that we have all of this we can really bring it all together.

Bringing it all together

Now that we have put in all of that effort we can really bring it all together with a function that will encrypt the data, show it to us encrypted, and then decrypt it again. Lets give that a try:

-- A hobbits tale...
thereAndBackAgain :: CCA.AES -> B.ByteString -> IO ()
thereAndBackAgain enc input = do
   putStrLn $ "Data is: " ++ show input
   let encData = safeEncryptECB enc input
   putStrLn $ "Encrypted data: " ++ show encData
   case safeDecryptECB enc encData of
      Left error -> putStrLn error
      Right originalData -> putStrLn $ "Original data was: " ++ show originalData

newline = putStrLn ""

We can run that little snippet of code on our test data and watch as it encrypts and decrypts our data. But we also want to make sure that invalid data is handled correctly too, so why don’t we also run something that actually modifies the data before trying to decrypt it again and see what happens:

showErrorsHappening :: CCA.AES -> IO ()
showErrorsHappening enc = do
   let testData0Enc = safeEncryptECB enc testData0
   print $ safeDecryptECB enc (testData0Enc `B.append` (BC.pack "modified"))

And now that we have done that all that is left is to actually run the code:

main = do
   putStrLn "Welcome to the cipher-aes guide by Robert Massaioli."
   putStrLn $ "We calculated that we need " ++ show requiredHexChars ++ " hexidecimal characters in the seed in order to initialise the AES cipher."
   case aesEnc of
      Left error -> putStrLn $ "Error parsing chosenHexStream: " ++ error
      Right aes -> do
         putStrLn "Parsed successfully."
         thereAndBackAgain aes testData0
         thereAndBackAgain aes testData1
         thereAndBackAgain aes testData2
         putStrLn "Showing an error happening by data that is too long:"
         showErrorsHappening aes
         putStrLn "All tests ran successfully"

This guide is actually a literate Haskell source file so you can run it and watch the results printed on the screen. Please check out the repository on BitBucket for this guide to give it a try. If you have any questions then please post them below and thankyou for reading!

(Note: This guide was designed to be converted into HTML for use in WordPress via pandoc.)

Read and write a RIFF (or RIFX)

What is RIFF anyway?

The RIFF file format is an old file format that is used as a container format for WAVE files among other things. Recently I decided that I wanted to write some pure Haskell code that could parse this file format so that I can start working my way towards building audio libraries in pure Haskell.

So you wrote a Haskell RIFF package did you?

Yes. You can view the results of my efforts on hackage: The riff package. You can even view the code on BitBucket if you like.

That package contains:

  • The riff library with the following features:
    • The ability to parse both RIFF and RIFX files. (Only perfectly formatted RIFF files are currently supported, we currently have no best effort support)
    • Convenience methods to make parsing / assembling RIFF files easier.
    • Written in pure Haskell so that you can run your code everywhere and be assured by all of the nice type safety that Haskell gives you.
  • A riff-structure executable that will print out the structure of all of the riff files that you provide it with.
  • A riff-convert executable that will let you convert RIFX files into RIFF files and vice versa.
  • A riff-identity executable that is pretty useless for practical purposes (it just makes a clone of the RIFF file you give it) but great for testing the library and it serves as a good code example.
  • Complete documentation coverage so that you know how to use each and every method in the library and what the limitations are.

You can give it a try today to read some RIFF files and it is all pretty self explanitory. I hope somebody gets some good use out of this. I am going to try and keep this library small and focused; please feel free to contribute and let me know what you think. And if you use it for something then especially let me know. It would make me very happy.

Fixing a Play Framework Java Compiler Version Warning

So I was doing some play development when compiling my code gave me the following warnings:

[warn] there were 1 feature warning(s); re-run with -feature for details
[warn] one warning found
[warn] warning: /home/robert/installed/play-2.2.1/framework/../repository/cache/com.atlassian.connect/ac-play-java_2.10/jars/ac-play-java_2.10-0.6.4.jar(com/atlassian/connect/play/java/controllers/AcController.class): major version 51 is newer than 50, the highest major version supported by this compiler.
[warn] It is recommended that the compiler be upgraded.
[warn] warning: /home/robert/installed/play-2.2.1/framework/../repository/cache/com.atlassian.connect/ac-play-java_2.10/jars/ac-play-java_2.10-0.6.4.jar(com/atlassian/connect/play/java/AC.class): major version 51 is newer than 50, the highest major version supported by this compiler.
[warn] It is recommended that the compiler be upgraded.
[warn] warning: /home/robert/installed/play-2.2.1/framework/../repository/cache/com.atlassian.connect/ac-play-java_2.10/jars/ac-play-java_2.10-0.6.4.jar(com/atlassian/connect/play/java/AcHost.class): major version 51 is newer than 50, the highest major version supported by this compiler.
[warn] It is recommended that the compiler be upgraded.
[warn] warning: /home/robert/installed/play-2.2.1/framework/../repository/cache/com.atlassian.connect/ac-play-java_2.10/jars/ac-play-java_2.10-0.6.4.jar(com/atlassian/connect/play/java/CheckValidOAuthRequest.class): major version 51 is newer than 50, the highest major version supported by this compiler.
[warn] It is recommended that the compiler be upgraded.
[warn] warning: /home/robert/installed/play-2.2.1/framework/../repository/cache/com.atlassian.connect/ac-play-java_2.10/jars/ac-play-java_2.10-0.6.4.jar(com/atlassian/connect/play/java/token/CheckValidToken.class): major version 51 is newer than 50, the highest major version supported by this compiler.
[warn] It is recommended that the compiler be upgraded.
[warn] 5 warnings

In this particular case you should notice that it says “major version 51 is newer than 50″. This means that we are using a Java 6 Compiler when we should be using a Java 7 compiler and you can confirm this by:

$ javac -version                       
javac 1.6.0_27

As you can see it is, in-fact, the Java 6 compiler (1.6 means Java version 6). So what do we do to fix this problem? Simple, you just make it so that a Java 7 version of javac is higher in your PATH environment variable than the one that is currently being used. You’ll know when you have it working and setup correctly when you see something like this:

$ javac -version
javac 1.7.0_40

For more information on how to change your path variable see this StackOverflow post.

Connecting Haskell HUnit tests to Cabal TestSuite

The Cabal logo.This is a quick guide that should explain how to connect Haskell HUnit tests to the Cabal TestSuite. I’m going to explain the simplest possible method in which you could do so and then leave it up to you to take the method further. For this post I am going to be using the following versions of Haskell software:

We are going to go through this guide in the following order:

  • Understand the data structures in Cabal TestSuite and their expected use cases.
  • Understand the data structures in Test.HUnit
  • Walk through a simple way to combine the two.

After that is done then you should be able mesh the two together easily. If you already feel that you understand the first two sections then feel free to skip straight to the final section to see how the two are joined together.

Understanding the structure of Cabal TestSuite

I am going to attempt to connect them together for the sake of running all of my test cases easily using Cabal. It also means that other people reading my project for the first time will be able to follow conventions and run my test cases with ease. The first thing that you have to understand is the structure of the Cabal TestSuite. Here is the source code for Distribution.TestSuite:

data Test
    = Test TestInstance
    | Group
        { groupName     :: String
        , concurrently  :: Bool
            -- ^ If true, then children of this group may be run in parallel.
            -- Note that this setting is not inherited by children. In
            -- particular, consider a group F with "concurrently = False" that
            -- has some children, including a group T with "concurrently =
            -- True". The children of group T may be run concurrently with each
            -- other, as long as none are run at the same time as any of the
            -- direct children of group F.
        , groupTests    :: [Test]
    | ExtraOptions [OptionDescr] Test

You will notice that this data structure has three constructors. The first constructor Test expects a single test instance, the Group constructor gathers a number of TestSuite test cases under the same name and also, interestingly, gives us the option to run the test cases concurrently. The final constructor ExtraOptions lets us pass extra options to test cases. For the sake of making the integration very simple so that you can get up and running in minutes we will be ignoring everything except the Test constructor. Now you may have noticed that the Test constructor expects a TestInstance but what does that even look like. Here is the code that makes up a TestInstance:

data TestInstance = TestInstance
    { run       :: IO Progress      -- ^ Perform the test.
    , name      :: String           -- ^ A name for the test, unique within a
                                    -- test suite.
    , tags      :: [String]         -- ^ Users can select groups of tests by
                                    -- their tags.
    , options   :: [OptionDescr]    -- ^ Descriptions of the options recognized
                                    -- by this test.
    , setOption :: String -> String -> Either String TestInstance
        -- ^ Try to set the named option to the given value. Returns an error
        -- message if the option is not supported or the value could not be
        -- correctly parsed; otherwise, a 'TestInstance' with the option set to
        -- the given value is returned.

So a TestInstance expects to be able to run something that will return a progress, it expects to have a name, a list of zero or more tags, a list of zero or more options and a method that allows you to set new options and get back a modified test. The name is pretty self explanatory, it lets you name the test. The list of tags to selectively run different test cases is extremely useful and large companies that choose to write a ton of test cases and have them run in automated builds (like in Bamboo) will use this feature a massive amount to be very selective in the test cases that they run. The options are partially useful too but we will be ignoring them for the sake of speed of development. Out of all of these fields the run is perhaps the most interesting. Lets take a look at the Progress data structure that is supposed to be the result of the IO action:

data Progress = Finished Result
              | Progress String (IO Progress)

data Result = Pass
            | Fail String
            | Error String
  deriving (Eq, Read, Show)

Now the Progress data structure is actually quite interesting. It has one constructor that tells us that a test case has Finished and that the Result of the test was a Pass, Fail or Error. That is the easy part to understand, the next constructor seems to say that the progress of this test case is that it is still in Progress; it also provides a message and gives an IO action to continue the progress. This is a way of reporting progress of the test cases before the test case finishes; this would be really useful if you have some very long running test cases and you wanted to get them to give you messages sooner rather than later. For the sake of our simple test cases we will only be using the Finished constructor and the Result data structure to show success.

Now you understand how the Cabal Distribution expects Test cases to be structured. You have Test cases that have TestInstances. The TestInstances are capable of being run and reporting their Progress. Eventually the Progress will be Finished and you will be able to get a result which will be reported back to you on the screen. Now we have to learn the structure of the HUnit module so that we can figure out how to put the two together.

Understanding the structure of the HUnit Module

The Test.HUnit module provides some similar structures to write HUnit test cases. Specifically it has a Test data structure that looks like this:

-- | The basic structure used to create an annotated tree of test cases.
data Test
    -- | A single, independent test case composed.
    = TestCase Assertion
    -- | A set of @Test@s sharing the same level in the hierarchy.
    | TestList [Test]
    -- | A name or description for a subtree of the @Test@s.
    | TestLabel String Test

As you can see we have a tree based data structure with a TestCase being the smallest node that can be defined. Each test case asserts something. You can also have a list of tests that make up one larger test and you can use labels to give names to other tests or groups of tests. I’m going to just assume that you know how to write HUnit test cases (as that is not the point of this document) but how do you run a test case?

You could use the performTest function but it is quite low level and would take to long to wire up. So instead we can use the runTestTT function to do the job for us with a much simpler return type:

-- | Provides the \"standard\" text-based test controller. Reporting is made to
--   standard error, and progress reports are included. For possible
--   programmatic use, the final counts are returned.
--   The \"TT\" in the name suggests \"Text-based reporting to the Terminal\".

runTestTT :: Test -> IO Counts
runTestTT t = do (counts', 0)                  return counts'

As you can see we just give this function our test cases and it gives us the results in the form of a Counts object that looks like this:

-- | A data structure that hold the results of tests that have been performed
-- up until this point.
data Counts = Counts { cases, tried, errors, failures :: Int }
  deriving (Eq, Show, Read)

From this Counts object we can derive the number of test cases that we tried to run as well as any failures or errors that occurred. We will use that in the next section to join the two together.

Combining HUnit and Cabal TestSuite together

Now that we know how it all works we can, quite easily, join the two together. Now because both Cabal TestSuite and HUnit define a Test data structure we need to have qualified imports and that makes the code a little harder to read but I am sure that you will see straight through it. Therefore I am just going to show you the final results and then just highlight the important sections:

module Test where

import qualified Distribution.TestSuite as TS
import qualified Test.HUnit as HU

test1 = HU.TestCase (HU.assertEqual "one equals three" 1 3)

hunitTests = HU.TestList [HU.TestLabel "Test 1" test1]

runHUnitTests :: HU.Test -> IO TS.Progress
runHUnitTests tests = do
   (HU.Counts cases tried errors failures) <- HU.runTestTT tests
   return $ if errors > 0
      then TS.Finished $ TS.Error "There were errors in the HUnit tests"
      else if failures > 0
         then TS.Finished $ TS.Fail "There were failures in the HUnit tests"
         else TS.Finished TS.Pass

tests :: IO [TS.Test]
tests = return [ TS.Test hunit ]
    hunit = TS.TestInstance
        { TS.run = runHUnitTests hunitTests
        , TS.name = "HUnit Test Cases"
        , TS.tags = ["hunit"]
        , TS.options = []
        , TS.setOption = \_ _ -> Right hunit

Breaking down the code segment above:

  • The first two highlights are lines 3 and 4 which show us importing the TestSuite as TS and the HUnit test classes as HU. You’ll have to remember that for the remainder of the tests.
  • The next highlight is the definition of the hunitTests which are HUnit Test classes. You can provide this wherever you want and, to make your code nicer to read, you should probably split this off into a separate test module and then import it back in.
  • The next highlight shows the function that does the majority of the work. It runs the test cases using runTestTT and then inspects the result counts to figure out what Result type to return back to the Cabal TestSuite.
  • In the next highlight you can see that we are using the runHUnitTests function to provide a runner for the TestInstance. At this point in time we have fully connected our HUnit test cases and our Cabal TestSuite and you should be able to “cabal install –enable-tests” and watch your HUnit test cases run.

That is all for this guide and I hope that it help you get your test cases up and running quickly.

Further reading: This is obviously a very simple integration of the two and you have plenty of room to improve that integration dramatically. However, I have spotted a library that seems to be in development (and not on Hackage) called cabal-test-hunit that you might want to look at for more information.

The Internals of a Quartz Clock Mechanism

A cheap $3 Quartz Clock


Most of the world has seen and used a clock; but how many of you have actually opened up your clocks to see what is inside and how it works. In this guide inside clock mechanism I will attempt to discover as much as I can about how clocks work and how they are controlled internally; it’s going to be a bit of a journey of discovery and I hope you enjoy it as much as I will. For this video I bought a cheap clock from KMart for AUD$3 (you can see it on the right).

I must prefix this guide with the following statement: I know nothing about the construction of clocks, but I am going to attempt to reason about this one anyway. In this video I mention how the solenoid works and how it creates the clocks “tick” but I do not know if that information is accurate. Please feel free to correct me in the comments if I got it wrong.

Gear Ratios

After doing all of that work making the video I decided that it would also be a good idea to record the gear ratios that are present inside the clocks and discovered that they both have exactly the same gear ratios on every part. I am just going to write down the details for each gear and explain how it is connected to the next gear:

Gear Name Teeth on Top Half Teeth on Bottom Half Connection to Next Gear
Pinion / Motor Gear 12 Magnet Top to Top
Pinion To Seconds 48 8 Bottom to Bottom
Seconds 8 60 Top to Top
Seconds To Minutes 64 8 Bottom to Top
Minutes 60 16 Bottom to Top
Minutes To Hours 48 8 Bottom to Top
Minutes To Hours 32 NA NA – No Next Gear

Now that we know these gear ratios we can work out how long it will take each gear to make a complete revolution. We can do this because we know that the small pinion gear takes 2 seconds to make a complete revolution (because it makes half revolution per tick). Therefore we can work back the time it takes for each gear to perform a full revolution by starting with the pinion gear and working it forwards.

Gear Name Calculation Seconds per Full Rotation
Pinion / Motor Gear Given 2
Pinion to Seconds 2 * 48 / 12 8
Seconds 8 * 60 / 8 60 (One Minute)
Seconds to Minutes 60 * 64 / 8 480
Minutes 480 * 60 / 8 3600 (One Hour)
Minutes to Hours 3600 * 48 / 16 10800
Hours 10800 * 32 / 8 43200 (Twelve Hours)

Now the first thing that we should notice is that the seconds hand takes one minute to rotate once, the minutes hand takes an hour to rotate once and the hours hand takes twelve hours to rotate once: this is excellent. If that did not work correctly then this would not be a very accurate clock at all; it would be completely wrong.

The other thing that you should notice is that Minutes to Hours gear will rotate once every three hours (10800 seconds). But remember that this is also the gear that we indirectly spin by using the control on the back of the Quartz Clock Mechanism. This control gear has 16 teeth internally and it is connected to the top of the “Minutes to Hours” gear which has 48 teeth. This means that every full revolution of the control dial on the back of Quartz Clock will rotate the “Minutes to Hours” gear by one quarter. This means a quarter of three hours which is 45 minutes. That tells you exactly the granularity that you have with the control dial on the back of the device. You give it one full spin for 45 minutes worth of time. This means that it takes sixteen (12 * 60 / 45) complete revolutions of the control dial to rotate the full twelve hours around the clock. However, since you can go in both directions with the control dial, it will only take a maximum of 8 spins to set the clock to any time that you like.

The Minutes to Hours Gear with the teeth highlighted in groups of five.

I used GIMP to draw dots on photos of the gears in order to count the number of teeth per gear.

For those of you that are wondering how I counted the number of teeth on each gear then the answer is simple. I took photos of each and every gear and marked the photos such that increments of five teeth were easy to see and thus the number of teeth on the entire gear was trivial to calculate. For example, on the gear above I can see 9 groups of 5 gears and an extra three spare, making 48 teeth in total. A much simpler task than individually counting each and every tooth; much more accurate too.

Concluding Words

Hopefully you have learned something fun from this video and you now understand fully how a Quartz Clock works on the inside. Please feel free to comment below. I will happily take any comments that you may have and would be more than interested in discussing anything about Quartz Clocks.