Functional Programming
Chris Eidhof Eelco Lempsink
Amsterdam.rb, October 27, 2008
( tp
, ui
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Functional programming in Ruby
(1..4).collect {|i| i*i }
#=> [1, 4, 9, 16]
(1..10).inject {|sum, n| sum + n}
#=> 55

Functional programming with Haskell
(1..4).collect {|i| i*i }
map (λi → i ∗ i) [1 . . 4] -- [1,4,9,16]
(1..10).inject {|sum, n| sum + n}
foldr (λn sum → sum + n) 0 [1 . . 10] -- 55

What is Haskell?
Haskell is a
• strongly typed
• lazy
• purely functional
programming language

Everything is a function
• No “assignment”
• Referential transparancy
• Partial application, no parentheses needed
a=5
a=6 -- Invalid!
squares xs = map (λx → x ∗ x) xs

Why should I learn Haskell?
• Functional Programming gives you a fresh way to look at
problems
• Even if you don’t use a functional language
• Python, Ruby, C# all use ideas from FP
• Google’s Map/Reduce and Hadoop are large scale FP

Lists
data [a] = [ ]
| a : [a]

Some standard functions
sum [ ] =0
sum (x : xs) = x + sum xs
product [ ] =1
product (x : xs) = x ∗ product xs
and [ ] = True
and (x : xs) = x ∧ and xs

Everything is a fold!
foldr (⊕) e [ ] =e
foldr (⊕) e (x : xs) = x ⊕ (foldr (⊕) e xs)
sum = foldr ( + ) 0
product = foldr ( ∗ ) 1
and = foldr ( ∧ ) True

Even map!
map f = foldr (λx tail → (f x) : tail) [ ]

Infinite lists...
primes = sieve [2 . . ]
sieve (p : xs) = p : sieve [x | x ← xs
, x ‘mod‘ p ≡ 0
]

QuickSort
qsort [ ] = []
qsort (x : xs) = qsort (filter (<x) xs)
+ [x ]
+
+ qsort (filter ( x) xs)
+

What we didn’t tell you
• Function composition
• Types
• Purity and I/O
• Monads
• Foreign Function Interface
• ...

Haskell in the Real World
• Large applications (XMonad, Darcs, GHC, Yi)
• Lots of libraries (Today on hackage: 814 libraries and
programs)
• Used by large companies (Credit Suisse, Galois, Microsoft,
ABN Amro . . . )
• Lots of ways to learn Haskell ((free) textbooks, university
courses, haskell wiki, blogs)

Functional programming
• ... sharpens your brain
• ... is lots of fun
• ... can help you become a better programmer

More information
• http://haskell.org
• http://hackage.haskell.org
• http://book.realworldhaskell.org/
• http://tupil.com