Hello World: Haskell

Today’s challenge: write my first program using Haskell. Let’s start!


Searching “Hello World Haskell” on Google gives me the following tutorial: Haskell in 5 steps.

Install Haskell

First step is to install the Haskell Platform. Main components are the GHC (Glasgow Haskell Compiler) and Cabal (Common Architecture for Building Applications and Libraries). I decided to use brew for simplicity.

brew update
brew install ghc
brew install cabal-install

Using the REPL

You can run Haskell REPL using ghci command:

$ ghci
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help

Here you can run any command:

Prelude> "Hello, World!"
"Hello, World!"
Prelude> putStrLn "Hello World"
Hello World
Prelude> 3 ^ 5
Prelude> :quit
Leaving GHCi.


Create hello.hs.

putStrLn "Hello World"

Then run ghc compiler.

$ ghc hello.hs
[1 of 1] Compiling Main             ( hello.hs, hello.o )
Linking hello ...

Output executable is named hello. You can run it as any other executable.

$ ./hello
Hello, World!

Real code

Now a bit more code: factorial calculator. First step is to define factorial function. You can do in a single line:

let fac n = if n == 0 then 1 else n * fac (n-1)

Or split definition on multiple lines:

fac 0 = 1
fac n = n * fac (n-1)

And put everything into factorial.hs. Now you can load the function inside the console:

Prelude> :load factorial.hs
[1 of 1] Compiling Main             ( factorial.hs, interpreted )
Ok, modules loaded: Main.
*Main> fac 42

Or write a main function then compile and run your executable:

fac 0 = 1
fac n = n * fac (n-1)
main :: IO ()
main = print (fac 42)

N.B. First time I compiled the source code I needed to add main :: IO () to avoid compiling error: The IO action 'main' is not defined in module 'Main'.

Now your executable runs well:


Going parallel

Haskell is cool because the pure functional nature and the parallel/multicore vocation so this beginner’s tutorial add some tips about that.

First of all you need to get the Parallel lib:

$ cabal update
Downloading the latest package list from hackage.haskell.org
$ cabal install parallel
Resolving dependencies...
Downloading parallel-
Configuring parallel-
Building parallel-
Installed parallel-

Then you can write your parallel software using `par` expression.

import Control.Parallel
main = a `par` b `par` c `pseq` print (a + b + c)
a = ack 3 10
b = fac 42
c = fib 34
fac 0 = 1
fac n = n * fac (n-1)
ack 0 n = n+1
ack m 0 = ack (m-1) 1
ack m n = ack (m-1) (ack m (n-1))
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

Compile it with -threaded flag.

$ ghc -O2 --make parallel.hs -threaded -rtsopts
[1 of 1] Compiling Main             ( parallel.hs, parallel.o )
Linking parallel ...

And run it

./parallel +RTS -N2

Now, many details aren’t clear to me. From the meaning of `pseq` to the level of parallelism used. Moreover I have no idea of what are the options used with compiler and the ones for the executable. There is definitely more then “Hello, World!”.

Next step would be Haskell in 10 minutes, another beautiful introduction to the language with links for more complex topics, or Real World Haskell (by Bryan O’Sullivan, Don Stewart, and John Goerzen) or any other useful tutorial listed on 5 steps guide.

See you to the next language 🙂