View source for Haskell in 5 steps

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

__TOC__

== Install Haskell ==

The recommended way to get started with programming Haskell is the [http://hackage.haskell.org/platform/ 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 [https://www.haskell.org/community/ Haskell community].

[http://tryhaskell.org/ 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.

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

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:

<haskell>
Prelude> "Hello, World!"
"Hello, World!"
</haskell>

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

<haskell>
Prelude> putStrLn "Hello World"
Hello World
</haskell>

Using a Haskell compiler, such as [http://haskell.org/ghc GHC], you can
compile the code to a standalone executable. Create a source file
'''hello.hs''' containing:

<haskell>
main = putStrLn "Hello, World!"
</haskell>

And compile it with:

<pre>
    $ ghc -o hello hello.hs
</pre>

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

<pre>
    $ ./hello
    Hello, World!
</pre>

== 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:

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

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

We can now run '''fac''' on some argument:
<haskell>
Prelude> fac 42
1405006117752879898543142606244511569936384000000000
</haskell>

'''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:

<haskell>
fac n = if n == 0 then 1 else n * fac (n-1)
</haskell>

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

<haskell>
Hugs.Base> :load fac.hs
Main> fac 42
1405006117752879898543142606244511569936384000000000
</haskell>

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):

<haskell>
fac 0 = 1
fac n = n * fac (n-1)

main = print (fac 42)
</haskell>

which can then be compiled and run:

<pre>
    $ ghc -o fac fac.hs
    $ ./fac
    1405006117752879898543142606244511569936384000000000
</pre>

'''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:

<haskell>
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)
</haskell>

Compiling with -threaded and optimizations on:

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

''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:

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

Congratulations! You are now programming your multicore!

==== Troubleshooting ====

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

== Where to go from here ==

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

'''Tutorials'''
* [http://book.realworldhaskell.org/ Real World Haskell]
* [[Learn_Haskell_in_10_minutes|Haskell in 10 minutes]]
* [http://web.archive.org/web/20090306064337/http://darcs.haskell.org/yaht/yaht.pdf Yet Another Haskell Tutorial] (English)
* [http://www.haskell.org/tutorial/ A Gentle Introduction to Haskell] (English, [[Image:GentleFR.pdf|French PDF]])
* [http://learnyouahaskell.com/ 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 [[User_groups|community]].

[[Category:Tutorials]]

Languages: [[Haskell in 5 Schritten|de]] [[5 adımda Haskell|tur]] [[Cn/Haskell 入门五步走|zh/cn]] [[Tw/Haskell入門的5個步驟|zh/tw]] [[Haskell入門 5ステップ|ja]]