Haskell a la carte: Difference between revisions
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== Potages == | == Potages == | ||
The best soup is made by combining | 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.
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.
Entrées
How to start eating?
square x = x*x
- The function 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 . 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 tosquare 4 + 2
which is 16+2=18.
- Squaring again, this time with a type signature which says that squaring maps integers to integers. In mathematics, we'd write . 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 :: 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 forsquare
if you omit an explicit signature.
- Squaring yet again, this time with a more general type signature. After all, we can square anything (
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
).
- Average again, this time with a type signature. Looks a bit strange, but that's the spicey currying. In fact,
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 : first apply g, then apply f. Use it for squaring something twice.
- The dot