PRACTICAL HASKELL

Haskell is famous for having a steep learning curve. As a web developer we’re used to clear tutorials that we can understand and complete within an hour or two. Haskell introduces many new concepts not found in other languages, but we can learn it faster by spending as much time coding as we do reading.

This is the first of a tutorial series intended to introduce Haskell by coding things that work.

1. Getting Started with Stack
2. Importing Code
3. Using Monads
4. Build a JSON API

In this article we will show you how to get Haskell installed, how to set up a new project, and run your code.

Tools and Names

GHC is the compiler for Haskell. It takes Haskell source code and turns it into an executable.

Cabal is the package description format. You’ll have a file called my-project.cabal with information about your project. There’s an executable called cabal too, but we are going to use stack instead.

Stack is a package manager. It reads my-project.cabal and stack.yaml to link in third party code. It will install GHC for you too.

Installing Stack

The only thing you need to download is Stack. It will install everything else for you.

• Download Stack

Then, follow the instructions on the download page for your operating system. Here’s what I did on a mac:

1. Unzip the file by double clicking it

2. Move it to /usr/local/bin

   $ mv stack-0.1.3.1-x86_64-osx /usr/local/bin/stack

3. Give it executable permissions

   $ chmod +x /usr/local/bin/stack

Alternatively on OSX you can install via homebrew:

brew updatebrew install haskell-stack

Check to make sure it is working

$ stack --versionVersion 0.1.3.1, Git revision 908b04205e6f436d4a5f420b1c6c646ed2b804d7

Setting up a new project

We’re going to need a few files to get our first project going: some haskell source code, a stack.yaml and a .cabal file. We can create these files by hand, but stack has a template feature we can use instead. Let’s call our project “my-project” and use the simple template.

$ stack new my-project simple...

This creates a directory named my-project. Let’s see what’s inside:

LICENSESetup.hsstack.yamlmy-project.cabalsrc/  Main.hs

The stack.yaml config file tells stack which version of GHC and your dependencies to use.

flags: {}packages:- '.'extra-deps: []resolver: lts-3.1

We use the my-project.cabal config file to store settings like the project name, and license. In the next article, Importing Code, we’ll edit this file to add dependencies.

name:                my-projectversion:             0.1.0.0synopsis:            Simple project template from stackdescription:         Please see README.mdhomepage:            http://github.com/githubuser/my-project#readmelicense:             BSD3license-file:        LICENSEauthor:              Sean Hessmaintainer:          [e-mail]:           2010 Author Herecategory:            Webbuild-type:          Simplecabal-version:       >=1.10executable my-project  hs-source-dirs:      src  main-is:             Main.hs  default-language:    Haskell2010  build-depends:       base >= 4.7 && < 5

Last but not least, we have some source code. src/Main.hs is the main module for our program. This is where the Haskell happens.

module Main wheremain :: IO ()main = do  putStrLn "hello world"

Installing GHC

Stack will install the correct version of GHC for our project. This is cool because everyone working on your project will be on the same version. Let’s give it a shot: run this in your project folder

$ stack setup

Which results in:

Downloaded lts-3.1 build plan.    Caching build planFetched package index.Populated index cache.Downloaded ghc-7.10.2.Installed GHC.stack will use a locally installed GHCFor more information on paths, see 'stack path' and 'stack exec env'To use this GHC and packages outside of a project, consider using:stack ghc, stack ghci, stack runghc, or stack exec

Run the Code!

Now we’re ready to run some code! Let’s use stack to fire up GHCI: the Haskell REPL.

$ stack ghciConfiguring GHCi with the following packages: my-projectGHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for helpPrelude> 

There are other great tutorials that will teach you how to use ghci, but here are a few examples:

Prelude> 2 + 1517Prelude> 5 == 5True

We can Use the :load command to load our code. Note that you can tab-complete module names.

Prelude> :load Main[1 of 1] Compiling Main             ( src/Main.hs, interpreted )Ok, modules loaded: Main.

Then we can run our program by typing main

Main> mainhello world

Making Changes

We can use ghci to test our changes as we go. Let’s make a greet function! Add this to src/Main.hs

greet name = "Hello " ++ name ++ "!"

Now go back to ghci and type :reload or :r

Main> :r[1 of 1] Compiling Main   (src/Main.hs, interpreted)Ok, modules loaded: Main.

We can test greet without it being used in main

Main> greet "bobby""Hello bobby!"

Let’s add it to our main program! Edit src/Main.hs

main = do  putStrLn (greet "bobby")  putStrLn (greet "World")

Reload again with :r and then run main

Main> :rMain> mainHello bobby!Hello World!

Building an Executable

When you are ready to ship, you can build an executable with stack build

$ stack build

It will tell you where the executable is, but it’s easier to run it with stack exec. It will run anything in your main function.

$ stack exec my-projectHello bobby!Hello World!

Other Resources

• Learn You a Haskell for Great Good - Good (free) introductory Haskell book.
• Prelude Documentation - All the functions that come built in
• Complete source code for this tutorial
• Configure your text editor to underline errors for you. Check out Editor Setup

Assignment

1. Read chapter 3 of Learn You a Haskell and add a type declaration to greet

2. Use the getLine function to read a name from the command-line, and print out a greeting to that name. Will require using the Prelude Documentation, and probably some googling.

Answers

What’s Next

In the next article, Importing Code, we show you how to use other built-in modules and 3rd party code.