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This project demonstrates how to set up your own real Haskell project, and helps you get a feel for the Haskell ecosystem. Currently, teaching Haskell is not a goal of this project, but it may in the future. As for now, basic knowledge of Haskell is assumed, and this project is aimed at people who want to build a real program or library in Haskell.

Getting started with this project

The best way to get started with this project is to simply clone it, and poke around.

git clone https://github.com/joshcough/HaskellStarter.git

Here are the contents of the project:

$ tree -a -I .git
.
├── .gitignore
├── .travis.yml
├── LICENSE
├── README.md
├── Setup.hs
├── executables
│   └── Main.hs
├── haskell-starter.cabal
├── src
│   └── HaskellStarter
│       ├── CommitPrinter.hs
│       └── Util.hs
├── test
│   ├── DocTest.hs
│   ├── Main.hs
│   ├── Properties.hs
│   └── UnitTests.hs
└── travis
    ├── cabal-apt-install
    └── config

5 directories, 15 files

Some valuable commands to play with (all of which will be explained) are:

$ cabal --help
$ cabal update
$ cabal init
$ cabal install github
$ cabal build
$ cabal test
$ cabal repl
$ cabal install

Prerequisites

As stated, this project assumes some basic knowledge of Haskell. If you don't have that, I recommend Learn You a Haskell for Great Good!. Also, if you haven't already, go install the Haskell Platform.

ghci

In order to have some basic knowledge of Haskell, it's highly likely that you've played with ghci, but a refresher still helps.

ghci is a simple way to get started playing with Haskell, and is essential for testing out functions and types during development.

Run ghci at the command line by simply typing ghci:

$ ghci

Input expressions

prelude> 5 + 5
10
prelude> let x = 10
prelude> x + x
20

Get types with :t

prelude> :t x
x :: Num a => a
prelude> let x = 7 :: Int
prelude> :t x + x
x + x :: Int

Load files with :load

prelude> :load Goop
*Goop> --now run some expressions in Goop

Quit ghci

prelude> ^d

Hoogle

Hoogle is the go to place for finding:

  • Functions
  • Libraries
  • Documentation
  • Links to source code
  • etc...

Maybe we want to write a library that does some fun stuff with the Github API (http://developer.github.com/v3/). Let's go search Hoogle to see if there is anything already there to help us:

https://lmddgtfy.net/?q=!hoogle%20github

Yay, we've discovered a Github package — http://hackage.haskell.org/package/github, complete with everything listed above. Poke around and find out more. After you're done poking, you should install the package:

$ cabal install github

Cabal

While ghci is useful for playing with Haskell code, it doesn't enable you to build libraries and programs. Cabal (Common Architecture for Building Applications and Libraries) is installed with the Haskell Platform, and is the canonical tool for building Haskell code. This section explains using Cabal, but where it lacks, you can get more info at: http://www.haskell.org/cabal/.

Getting Started with Cabal

A few helpful commands for getting started with Cabal:

  • cabal --help shows you all the Cabal commands. Highly recommended.
  • cabal update updates Cabal so that it has all of the latest package information.
  • cabal init runs you through a series of questions to start a new project.
  • cabal build builds your project locally (into the dist directory).
  • cabal test builds your project and then runs all of your tests.
  • cabal repl runs GHCI with your project already loaded.
  • cabal install installs a package.
    • Given zero arguments, it will install your package.
    • Given any number of library arguments (like cabal install github which was used above), it will download those libraries from hackage, build, and install them.
    • Given --only-dependencies, it will install all your package's dependencies. This is the recommended way to install new libraries to a project, as it will choose a version to install that is compatible with your other dependencies.

This project is a working Cabal project, and this document explains the Cabal configuration, which is in haskellstarter.cabal. Let's get started by taking a quick peek at it, and we'll create a Haskell library.

Cabal file header

name:                haskell-starter
version:             0.1.0.0
synopsis:            A demo for getting a project started in Haskell.
homepage:            https://github.com/joshcough/HaskellStarter
license:             MIT
license-file:        LICENSE
author:              Josh Cough
maintainer:          [email protected]
category:            Demo
build-type:          Simple
cabal-version:       >=1.8

source-repository head
  type: git
    location: [email protected]:joshcough/HaskellStarter.git

Most of these fields are self explanatory, and are mostly for documentation, and only a couple of them are required. More information on what each of these fields means and which ones are required can be found at http://www.haskell.org/cabal/users-guide/developing-packages.html.

Creating a Library

Modules

In order to have a library, we need some code. 😄 I usually put my code in a directory called src, but any directory name you want is fine (provided it is configured). In this project we have two modules, HaskellStarter.Util, which just contains some simple utility functions, and HaskellStarter.CommitPrinter which uses the github library previously mentioned to print commits for a repo.

Let's take a look at HaskellStarter.Util:

module HaskellStarter.Util (extract, printAll) where

extract :: Show a => Either a c -> c
extract = either (error . show) id

printAll :: [String] -> IO ()
printAll xs = mapM_ print xs

Don't worry too much yet about what this code actually does; we'll document it shortly.

And HaskellStarter.CommitPrinter:

module HaskellStarter.CommitPrinter where

import Control.Applicative
import Github.Repos.Commits
import HaskellStarter.Util

getMessage :: Commit -> String
getMessage = gitCommitMessage . commitGitCommit

printCommitsFor :: String -> String -> IO ()
printCommitsFor user repo = do
  commits <- extract <$> commitsFor user repo
  printAll $ getMessage <$> commits

We will document this code shortly too, but do notice that it imports Github.Repos.Commits, which is a module in the github library.

Configuring the library in Cabal

A Cabal file can only have one library (but you're not required to have one). Here's the configuration for it:

library 
  hs-source-dirs: src
  exposed-modules:
    HaskellStarter.CommitPrinter
  other-modules:
    HaskellStarter.Util
  build-depends:
    base >= 4 && < 5, github >= 0.7.4
  • hs-source-dirs is a list of directories to find your source files in, relative to the root directory of your project. Here, we only use one, but you can also say: hs-source-dirs: src1 src2 goober/joober
  • exposed-modules specifies the modules in the library's public API.
  • other-modules specifies modules that aren't publicly exposed, but are still part of the library.
  • build-depends will be explained in the section Understanding Dependencies, but for now, it is enough to know that this library depends on two other libraries: base, and github.

A quick note about modules: Cabal must know about all of your modules in the library, so they must be specified in exposed-modules or other-modules. I find this good and bad — on one hand it allows you to have Haskell files in your source directory that you don't want to be compiled, on the other, it forces you to list all of your modules.

Using the Library with ghci

With some code in place and the library configured, it's time to play with it. Try this out:

$ cabal repl

This will bring you into ghci with all of your modules preloaded. Let's call a function:

*HaskellStarter.CommitPrinter> printCommitsFor "joshcough" "HaskellStarter"

Of course, this project has way too many bogus commits that just say "readme update" to actually show the output here, but it does work. Trust me.

Building and Installing your Library

Building your library is easy:

$ cabal build

If you want to build other projects that depend on your library, you can install it locally:

$ cabal install

Haddock

Let's add some documentation to the code, and then generate pretty html from it.

First HaskellStarter.Util:

{-|
 This module contains some simple utility functions.
 It exports two functions, extract and printAll.
 -}
module HaskellStarter.Util (extract, printAll) where

{-|
   Forcefully pull a value out of an Either.
   This function: 
     * Returns the result if the Either is a Right.
     * Dies with an error if the Either is a Left.
 -}
extract :: Show a => Either a c -> c
extract = either (error . show) id

{-|
   Print a list of Strings, one per line.
 -}
printAll :: [String] -> IO ()
printAll xs = mapM_ print xs

And HaskellStarter.CommitPrinter:

{-|
  This module allows you to print the commits messages
  in a github repo.
 -}
module HaskellStarter.CommitPrinter (printCommitsFor) where

import Control.Applicative
import Github.Repos.Commits
import HaskellStarter.Util

{-|
   Get the actual commit message from a Commit.
 -}
getMessage :: Commit -> String
getMessage = gitCommitMessage . commitGitCommit

{-|
  Print all of the commits messages for a given user and repo.
 -}
printCommitsFor :: String -> String -> IO ()
printCommitsFor user repo = do
  commits <- extract <$> commitsFor user repo
  printAll $ getMessage <$> commits

Hopefully this documentation helps explain what the code does. If not, feel free to fix it and send me a pull request.

With the docs in our source code, generating pretty html from them is simple:

$ cabal haddock

Which outputs this info:

Haddock coverage:
 100% (  3 /  3) in 'HaskellStarter.Util'
 100% (  2 /  2) in 'HaskellStarter.CommitPrinter'
Documentation created: dist/doc/html/haskell-starter/index.html

Now, open up dist/doc/html/haskell-starter/index.html and see the glory. Notice that only publicly exposed modules are added to the documentation.

Understanding Dependencies

TODO

Executables

A library is a collection of code that you can depend on, but cannot actually execute. Fortunately, you can build executables with Cabal very easily. To do this, we first need a module with a main function. Here is executables/Main.hs from this project:

module Main where

import HaskellStarter.CommitPrinter
import System.Environment

main :: IO ()
main = do
  args <- getArgs
  printCommitsFor (args !! 0) (args !! 1)

This is a command line program that takes two arguments — a username and a project name, and prints the commits for that project.

Configuring an executable in Cabal

Configuring an executable in Cabal is very simple:

executable githubCommitPrinter
  hs-source-dirs: executables
  main-is: Main.hs
  build-depends: base < 5, haskell-starter
  • executable githubCommitPrinter starts the executable block, and names it. You may have many different executables in one Cabal file.
  • hs-source-dirs is a list of directories to find source files.
  • main-is specifies the Haskell file that contains the main function. main must have type IO ().
  • build-depends is the same as it is in the library definition. Notice here that githubCommitPrinter depends on the haskell-starter library. Cabal doesn't implicitely add your library to executables.

Installing and Running Executables

cabal install installs all executables in your project, as well as the library (if there is one). By default, Cabal installs executables to ~/.cabal/bin, or %appdata\cabal\bin on Windows. By adding that to your PATH, you can run your executables immediately.

$ githubCommitPrinter joshcough HaskellStarter

Tests

In Haskell and Cabal there are a lot of different test libraries and frameworks, and it's difficult to choose which to use. Here, I'll explain briefly:

  • HUnit – a library for writing unit tests
  • QuickCheck – a library for writing properties
  • test-framework – a framework for organizing and running unit tests and properties
  • doctest – inject tests directly into your documentation

HUnit

HUnit is "is a unit testing framework for Haskell, similar to the JUnit tool for Java."

It allows you to write very simple assertions of this form:

actual @?= expected

And if actual doesn't equal expected, then you will get a test error.

I've written a very simple example in test/UnitTests.hs* that I think is very self explanatory:

module UnitTests where

import Test.Framework.Providers.API
import Test.Framework.Providers.HUnit
import Test.HUnit

tests = testGroup "HUnit tests" [
  testCase "a passing test!"       $ 5 @?= 5
 ,testCase "another passing test!" $ 6 @?= 6 ]

*This doesn't currently import anything in the library. The reason for this is a little subtle — HaskellStarter.Utils is the only module with easily testable functions, but it is not exposed, so we don't have access to it! There are a couple ways around this, but the easiest is to simply expose all modules that you wish to test.

We will see how to run these tests shortly.

QuickCheck

QuickCheck is a library for writing properties for your functions, and a framework for testing those properties with random inputs. Here is a simple example property that for all lists of integers, tests that the reverse of the reverse of that list is the equal to that list.

prop_list_reverse_reverse :: [Int] -> Bool
prop_list_reverse_reverse list = list == reverse (reverse list)

This property is put into a module in tests/Properties.hs:

{-# LANGUAGE TemplateHaskell #-}
module Properties where

import Test.Framework.Providers.QuickCheck2
import Test.Framework.TH
import Test.QuickCheck

prop_list_reverse_reverse :: [Int] -> Bool
prop_list_reverse_reverse list = list == reverse (reverse list)

prop_list_length :: [Int] -> Int -> Bool
prop_list_length list i = length (i : list) == 1 + length list

tests = $testGroupGenerator

This module has a couple of very simple properties in it (once again, unrelated to the code in the library). We will see how to run these shortly, as well.

test-framework

Before we can run our tests, we need to package them up into a module with a main function. We do that using test-framework. Here are the contents of test/Main.hs:

module Main where

import Properties
import UnitTests
import Test.Framework.Runners.Console (defaultMain)

main = defaultMain $ [UnitTests.tests, Properties.tests]

This module provides a main function using the defaultMain function from test-framework. It takes a list of test groups as an argument, and runs all the tests in those groups. It also takes care of printing fancy messages for successes and failures.

Configuring a test suite in Cabal

Before we can run our tests, we need to configure our test suite in Cabal:

-- configuration for Unit tests and properties
test-suite unit-tests-and-properties
  type:           exitcode-stdio-1.0
  main-is:        Main.hs
  hs-source-dirs: test
  build-depends:
    base,
    HUnit,
    QuickCheck                 >= 2.4,
    test-framework             >= 0.6,
    test-framework-hunit,
    test-framework-quickcheck2 >= 0.2,
    test-framework-th          >= 0.2

Don't worry too much about the details here. Just know that the tests are in the test directory, and Main.hs is in there. Hopefully soon I'll be able to provide more info here, and/or make the configuration slightly less verbose.

You'll need to configure with cabal configure --enable-tests before tests can be run. If that fails for lack of dependencies, cabal install --enable-tests --only-dependencies will install them.

Running Tests

Now that we have our test suite configured, running it is very easy:

$ cabal test

You can also pass the --enable-tests flag to cabal install, which will run all of your tests, and only install the library if all of the tests pass:

$ cabal install --enable-tests

doctest

"doctest is a small program, that checks examples in Haddock comments. It is similar to the popular Python module with the same name."

Let's add some tests into our documentation for the extract function in HaskellStarter.Util:

{-|
   Forcefully pull a value out of an Either.
   This function: 
     * Returns the result if the Either is a Right.
     * Dies with an error if the Either is a Left.
  >>> extract $ Right 10
  10
  >>> extract $ Right "hello, world"
  "hello, world"
 -}
extract :: Show a => Either a c -> c
extract = either (error . show) id

In case it's hard to notice, I've added:

  >>> extract $ Right 10
  10
  >>> extract $ Right "hello, world"
  "hello, world"

The first example says to run the extract function with Right 10, and expect to get back the value 10. The second is nearly identical.

In order to run doctests, we need to have a main function, and configure it in Cabal.

Here are the contents of test/Doctest.hs:

module Main where

import System.FilePath.Glob (glob)
import Test.DocTest (doctest)

main = glob "src/**/*.hs" >>= doctest

and here is the Cabal test suite configuration:

test-suite doctest
  type:           exitcode-stdio-1.0
  main-is:        DocTest.hs
  hs-source-dirs: test
  build-depends:  base, doctest == 0.9.*, Glob == 0.7.*

All test-suite configurations get ran when you execute cabal test, so this there is nothing else additional needed.

Travis

Travis is a service that works with Github for building your project automatically after a git push. It notifies you via email if the build fails. This project is already set up to use Travis, and technically all you need to do that is have a .travis.yml file in the root of your project with these contents:

language: haskell
script:
  - cabal install --enable-tests

To enable your GitHub repo to build on Travis: setup a Travis account, go to https://travis-ci.org/profile, find the repo you want to build, and simply flip the switch from Off to On. That's it.

The version in this project is slightly more complicated in order to allow the Travis build to pull dependencies even what Hackage is down. You shouldn't really need to worry about understanding those changes though, but if you're curious, check out .travis.yml and the travis directory.

Publishing Your Library

TODO (But for now: http://hackage.haskell.org/upload)

Further Reading

Here's a bunch of links (in no particular order) that I found useful in creating this project, and for Haskell development in general.

Hoogle:

Hackage:

Libraries:

Haddock:

doctest:

Testing:

Books:

Cabal:

Travis:

Other:

Other getting started links:

TODO

This section contains things that still need to be done in this project. If you feel like something is missing and it isn't in this list, please let me know!

Definitely:

  • Explain what Hackage is.
  • Maybe give more attention to writing modules. This is for people with just basic ghci experience.
  • All of the Dependencies section, especially base.
  • Explain imports better, especially from the executable.
  • Upload my a project to Hackage, and explain how to do that.
  • Cabal sandboxes

Maybe:

  • cabal unpack - you can pull something, unpack it, and fix it
  • HLint
  • Test coverage
  • Benchmarks

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A project that demonstrates getting up and running with Haskell.

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