# 08. haskell functions

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- 1.Functions Sebastian RettigEvery function in haskell consumes exactly Every function in haskell consumes exactlyone parameter and returns aa value. one parameter and returns value.

2. Functional Programming No Variables Functions only, eventually stored in Modules Behavior do not change, once defined Function called with same parametercalculates always the same result Function definitions (Match Cases) Recursion (Memory) 3. Haskell Features Pure Functional Programming Language Lazy Evaluation Pattern Matching and Guards List Comprehension Type Polymorphism 4. Nice to remember (1)Typeclasses: define properties of the types like an interface Eq can be compared Ord can be ordered (>, =, a -> a -> a1. we have a Typeclass restriction for Ord2. we have 2 Parameters of Types, who have amembership for the Ord Typeclass3. we have a return value of the same type But, why is the syntax not separating the parametersbetter from the result set? 7. Curried Functions (2) every function in haskell consumes exactly oneparameter and returns a value so we could write the function header instead of:max :: (Ord a) => a -> a -> a also in the following way:max :: (Ord a) => a -> (a -> a) max is a function which consumes a parameter andreturns a function these function consumes a parameter and returns avalue (the max of both values) Partial Application 8. Curried Functions (3) some examples: :t max returnsmax :: (Ord a) => a -> a -> a max 4 5 returns 5 (max 4) 5 returns 5 :t max 4 returns max 4 :: (Num a, Ord a) => a -> a let foo = max 4foo 5 returns 5 :t max 4 5 returnsmax 4 5 :: (Num a, Ord a) => a 9. Curried Functions (4) we can also partial apply infix functions(/10) 2 returns0.2(10/) 2 returns5(++ bar) foo returns foobar(bar ++) foo returns barfoo:t (`elem` [a..z])(`elem` [a..z]) :: Char -> Bool:t (a `elem`)(`elem` [a..z]) :: [Char] -> Bool:t elemelem :: Eq a => a -> [a] -> Bool 10. Pointless Style lets say, we have the following function:maxWithFour :: (Num a, Ord a) => a -> amaxWithFour x = max 4 x what is the result of max 4 again?:t max 4 returns max 4 :: (Num a, Ord a) => a -> a max 4 returns already a function which consumes aparameter, so we can simplify our function:maxWithFour :: (Num a, Ord a) => a -> amaxWithFour = max 4 but how can we achieve that with the following function?fn x = ceiling (negate (tan (cos (max 50 x)))) 11. Function Application (1) used with ($) has the following header::t ($)($) :: (a -> b) -> a -> b compared with SPACE (normal function application) ($) has lowest priority ($) is right associative f $ g $ a b = (f (g (a b))) SPACE has highest priority SPACE is left associative f a b c= (((f a) b) c) 12. Function Application (2) what can we do with ($) ? helps us to avoid parentheses: instead of:sum (map sqrt [1..20]) we can write:sum $ map sqrt [1..20] allows us also to wrap a value in a function:map ($ 3) [(4+), (10*), (^2), sqrt]returns [7.0,30.0,9.0,1.7320508075688772] 13. Function Composition (1) Definition: used with (.) has the following header::t (.)(.) :: (b -> c) -> (a -> b) -> a -> c composing two functions produces a new function good for on-the-fly pass of a function to the nextfunction 14. Function Composition (2) what can we do with (.) ? helps us also to avoid parentheses unwraps the parameter out of parentheses instead of:map (xs -> negate (sum (tail xs))) [[1..5],[3..6],[1..7]] we can write:map (negate . sum . tail) [[1..5],[3..6],[1..7]] 15. Composition vs. Application (1) IMPORTANT to know the difference between ($) and (.) best to compare both headers again: ($) :: (a -> b) -> a -> b (.) :: (b -> c) -> (a -> b) -> a -> c combination of both allows us parenthesis-less writing shorter code and mostly easier to read possibility to write pointless-style functions with more then one function call 16. Composition vs. Application (2) example with parameter: foo xs = sum (filter (> 10) (map (*2) xs)) foo xs = sum $ filter (> 10) $ map (*2 ) xs example without parameter (pointless-style): foo xs = sum (filter (> 10) (map (*2) xs)) foo = sum . filter (> 10) . map (*2) 17. Useful Types Maybe:data Maybe a = Nothing | Just a contains maybe a type a or Nothing good for handling e.g. Hashmap lookup Nothing if key not exist, else Value of type a Either:data Either a b = Left a | Right bderiving (Eq, Ord, Read, Show) can contain type a or type b Left for e.g. Error Messages, Right for value like an extended Maybe with information for Nothing 18. Functor Typeclass (1)class Functor f wherefmap :: (a -> b) -> f a -> f b for things that can be mapped over !!! Functor needs Types with 1 Typeparameter !!! fmap gets a function and a type and maps the function overthe type variable Instance for List: Remember: List has 1 Typeparameter [a], but is just Syntactic Sugar for ([] a)instance Functor [] wherefmap = map Example: fmap (*2) [2,3,4] returns [4,6,8] 19. Functor Typeclass (2)class Functor f wherefmap :: (a -> b) -> f a -> f b Instance for Maybeinstance Functor Maybe wherefmap g (Just x) = Just (g x)fmap g Nothing = Nothing Example: fmap (+3) Nothing returns Nothing fmap (+3) (Just 4) returns (Just 7) 20. Functor Typeclass (3)class Functor f wherefmap :: (a -> b) -> f a -> f b Instance for Eitherinstance Functor (Either a) wherefmap f (Right x) = Right (f x)fmap f (Left x) = Left x Either is a type with 2 Typeparameters! we have to permanently include the first parameter in theinstance (curried function, partial application) fmap do not change the 1st Typeparameter, only the 2nd Left Type-Constructor of Either is often used for ErrorMessages 21. Functor Typeclass (4)class Functor f wherefmap :: (a -> b) -> f a -> f b Example: fmap (+3) (Left 4) returns (Left 4) fmap (+3) (Right 4) returns (Right 7) what happens, if we try to do that?fmap (+) (Just 4) lets look at the type::t fmap (+) (Just 4)fmap (+) (Just 4) :: Num a => Maybe (a -> a) partial application, BUT we can not use the Functor instanceon the result! we need an extension Applicative Functors 22. Sources[1] Haskell-Tutorial: Learn you a Haskell (http://learnyouahaskell.com/,2012/03/15)[2] The Hugs User-Manual (http://cvs.haskell.org/Hugs/pages/hugsman/index.html, 2012/03/15)[3] The Haskellwiki (http://www.haskell.org/haskellwiki, 2012/03/15)

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