Haskell in 5 steps

Haskell is a general purpose, purely functional programming language. This page will help you get started as quickly as possible.

Install Haskell

The recommended way to get started with programming Haskell is the Haskell Platform. The Platform comes with GHC, the de-facto standard Haskell compiler, and other tools that will help you program Haskell. The installation should be supported on most operating systems. If you encounter difficulty, feel free to reach out to the Haskell community.

Try Haskell provides a less complete but quicker way to give Haskell a shot.

Start Haskell

If you have installed the Haskell Platform, open a terminal and type ghci (the name of the executable of the GHC interpreter) at the command prompt. Alternatively, if you are on Windows, you may choose WinGHCi in the Start menu.

    $ ghci
    GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
    Loading package base ... linking ... done.
    Prelude>

And you are presented with a prompt. The Haskell system now attentively awaits your input.

Write your first Haskell program

If you've learned to program another language, your first program probably was "Hello, world!", so let's do that:

Prelude> "Hello, World!"
"Hello, World!"

The Haskell system evaluated the string, and printed the result. Or we can try a variation to print directly to standard output:

Prelude> putStrLn "Hello World"
Hello World

Using a Haskell compiler, such as GHC, you can compile the code to a standalone executable. Create a source file hello.hs containing:

main = putStrLn "Hello, World!"

And compile it with:

    $ ghc -o hello hello.hs

You can then run the executable (./hello on Unix systems, hello.exe on Windows):

    $ ./hello
    Hello, World!

Haskell the calculator

Let's do something fun. In Haskell, your first true program is the factorial function. So back to the interpreter now and let's define it:

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

This defines a new function called fac which computes the factorial of an integer.

We can now run fac on some argument:

Prelude> fac 42
1405006117752879898543142606244511569936384000000000

Congratulations! Programming made easy. Note that if you're using Hugs, you'll need to load the definition of fac from a file, fac.hs, containing:

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

And run it with Hugs as follows (this also works in GHCi):

Hugs.Base> :load fac.hs
Main> fac 42
1405006117752879898543142606244511569936384000000000

We can of course compile this program, to produce a standalone executable. In the file fac.hs we can write (and let's use elegant pattern matching syntax just for fun):

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

main = print (fac 42)

which can then be compiled and run:

    $ ghc -o fac fac.hs
    $ ./fac
    1405006117752879898543142606244511569936384000000000

Great!

Write your first parallel Haskell program

Haskell has good support for parallel and multicore programming. We can write a parallel program by adding `par` to expressions, like so:

import Control.Parallel

main = a `par` b `par` c `pseq` print (a + b + c)
    where
        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)

Compiling with -threaded and optimizations on:

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

Note that in versions of GHC prior to GHC 7, you can leave off the -rtsopts flag

And now we can run our multicore program. Here across two cores:

$ time ./A +RTS -N2
1405006117752879898543142606244511569936384005711076
./A +RTS -N2  2.14s user 0.02s system 149% cpu 1.449 total

Congratulations! You are now programming your multicore!

Troubleshooting

If you encounter a Could not find module ‘Control.Parallel’ error, please install the parallel library with cabal install --lib parallel

Where to go from here

There are many good Haskell tutorials and books. Here are some we recommend:

Tutorials

• Real World Haskell
• Haskell in 10 minutes
• Yet Another Haskell Tutorial (English)
• A Gentle Introduction to Haskell (English, File:GentleFR.pdf)
• Learn You A Haskell For Great Good!

For a complete list of textbooks, references and tutorials:

• Books and tutorials

Join the community!

Talk to others in the Haskell community.

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