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Haskell a la carte

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[[Categories:Tutorials]]
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[[Category:Tutorials]]
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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.
   
 
== Apéritifs ==
 
== Apéritifs ==
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qsort :: Ord a => [a] -> [a]
 
qsort :: Ord a => [a] -> [a]
 
qsort [] = []
 
qsort [] = []
qsort (x:xs) = qsort (filter (<x) xs) ++ [x] ++ qsort (filter (>x) xs))
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qsort (x:xs) = qsort (filter (<x) xs) ++ [x] ++ qsort (filter (>=x) xs))
 
</haskell>
 
</haskell>
 
::Quicksort in three lines (!). Sorts not only integers but anything that can be compared.
 
::Quicksort in three lines (!). Sorts not only integers but anything that can be compared.
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== Entrées ==
 
== Entrées ==
How to eat?
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How to start eating?
   
 
*
 
*
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square x = x*x
 
square x = x*x
 
</haskell>
 
</haskell>
::The function <math>f(x)=x\cdot x</math> 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 supply arguments all around, so why clutter the notation with unnecessary ballast?
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::The function <math>f(x)=x\cdot x</math> 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?
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*
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<haskell>
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square :: Integer -> Integer
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square x = x*x
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</haskell>
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:: Squaring again, this time with a ''type signature'' which says that squaring maps integers to integers. In mathematics, we'd write <math>f:\mathbb{Z}\to\mathbb{Z},\ f(x)=x\cdot x</math>. 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 <hask>square (4+2)</hask> which is 36 compared to <hask>square 4 + 2</hask> which is 16+2=18.
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*
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<haskell>
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square :: Num a => a -> a
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square x = x*x
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</haskell>
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:: Squaring yet again, this time with a more general type signature. After all, we can square anything (<hask>a</hask>) that looks like a number (<hask>Num a</hask>). By the way, this general type is the one that the compiler will infer for <hask>square</hask> if you omit an explicit signature.
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*
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<haskell>
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average x y = (x+y)/2
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</haskell>
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:: The average of two numbers. Multiple arguments are separated by spaces.
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*
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<haskell>
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average :: Double -> Double -> Double
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average x y = (x+y)/2
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</haskell>
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::Average again, this time with a type signature. Looks a bit strange, but that's the spicey ''currying''. In fact, <hask>average</hask> is a function that takes only one argument (<hask>Double</hask>) but returns a function with one argument (<hask>Double -> Double</hask>).
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== Potages ==
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The best soup is made by combining well-known ingredients.
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*
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<haskell>
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(.) :: (b -> c) -> (a -> b) -> (a -> c)
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(.) f g x = f (g x)
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fourthPower = square . square
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</haskell>
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::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.
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== Plats principaux ==
 
== Plats principaux ==

Revision as of 13:44, 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 well-known 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