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Haskell a la carte

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== Potages ==
 
== Potages ==
The best soup is made by combining well-known ingredients.
+
The best soup is made by combining the available ingredients.
   
 
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</haskell>
 
</haskell>
 
::The dot <hask>f . g</hask> is good old function composition <math>f \circ g</math>: first apply g, then apply f. Use it for squaring something twice.
 
::The dot <hask>f . g</hask> is good old function composition <math>f \circ g</math>: first apply g, then apply f. Use it for squaring something twice.
 
   
 
== Plats principaux ==
 
== Plats principaux ==

Revision as of 13:45, 14 December 2007


New to Haskell? This menu will give you a first impression. Don't read all the explanations, or you'll be starved before the meal.

Contents

1 Apéritifs

Foretaste of an excellent meal.

  qsort :: Ord a => [a] -> [a]
  qsort []     = []
  qsort (x:xs) = qsort (filter (<x) xs) ++ [x] ++ qsort (filter (>=x) xs))
Quicksort in three lines (!). Sorts not only integers but anything that can be compared.
  fibs = 1:1:zipWith (+) fibs (tail fibs)
The infinite list of fibonacci numbers. Just don't try to print all of it.
  linecount = interact $ show . length . lines
  wordcount = interact $ show . length . words
Count the number of lines or words from standard input.

2 Entrées

How to start eating?

  square x = x*x
The function f(x)=x\cdot x which maps a number to its square. While we commonly write parenthesis around function arguments in mathematics and most programming languages, a simple space is enough in Haskell. We're going to apply functions to arguments all around, so why clutter the notation with unnecessary ballast?
  square :: Integer -> Integer
  square x = x*x
Squaring again, this time with a type signature which says that squaring maps integers to integers. In mathematics, we'd write f:\mathbb{Z}\to\mathbb{Z},\ f(x)=x\cdot x. Every expression in Haskell has a type and the compiler will automatically infer (= figure out) one for you if you're too lazy to write down a type signature yourself. Of course, parenthesis are allowed for grouping, like in
square (4+2)
which is 36 compared to
square 4 + 2
which is 16+2=18.
  square :: Num a => a -> a
  square x = x*x
Squaring yet again, this time with a more general type signature. After all, we can square anything (
a
) that looks like a number (
Num a
). By the way, this general type is the one that the compiler will infer for
square
if you omit an explicit signature.
  average x y = (x+y)/2
The average of two numbers. Multiple arguments are separated by spaces.
  average :: Double -> Double -> Double
  average x y = (x+y)/2
Average again, this time with a type signature. Looks a bit strange, but that's the spicey currying. In fact,
average
is a function that takes only one argument (
Double
) but returns a function with one argument (
Double -> Double
).

3 Potages

The best soup is made by combining the available ingredients.

  (.) :: (b -> c) -> (a -> b) -> (a -> c)
  (.) f g x = f (g x)
 
  fourthPower = square . square
The dot
f . g
is good old function composition f \circ g: first apply g, then apply f. Use it for squaring something twice.

4 Plats principaux

5 Desserts

6 Vins