posted by lucjansz

]]>xs ++ ys = foldr (:) ys xs ≡ { 1 } (++) ys xs = foldr (:) ys xs ≡ { 2 } (++) ys = foldr (:) ys ≡ { 3 } (++ ys) = foldr (:) ys ≡ { 4 } (++) = foldr (:)

This contains several errors:

- The first equality is wrong because the arguments are flipped: xs ++ ys ≡ (++) xs ys
- The second equality is true, although since the arguments of append are flipped, the function doesn't have the correct behaviour
- The third equality is wrong because it's flipping the arguments again. Although actually, (++ ys) isn't a valid way to define a function, so you can't write this down in Haskell. But if this were valid Haskell, the the behaviour of this append operator would be correct again.
- The fourth equality is wrong again because it once again flips the arguments.

A correct way to define append would be:

(++) = flip (foldr (:))

Having said all that, I really like this series.

Keep on going Erik!

posted by Tom Lokhorst

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a -> b // function a -> b -> c // curried function (a -> b) -> c // but a -> (b -> c) // vs

Apparently, if history be the judge, currying ought to be called Schönfinkeling and curried functions Schönfinkeled functions after the true inventor of this transformation, Moses Schönfinkel. Sad fate: his papers were burned for heating by his neighbors and he ended up in a sanatorium! Maybe currying is safer after all.

posted by exoteric

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EDIT: Ah, I see what you mean, I thought you meant the first "=" sign. sorry about that, anyway. I'll keep this here because it's neat.

Put this in a Haskell file:

import Test.QuickCheck prop_Concat :: [Int] -> [Int] -> Bool prop_Concat xs ys = xs ++ ys == foldr (:) ys xs

This code defines a quickCheck property, which is a way of automatically generating tests in Haskell. You specify what you expect to be true for the inputs, and it generates tons of data for you and verifies that the property is indeed true.

Then open it in GHCi (or hugs, I think) and do:

*Main> quickCheck prop_Concat OK, passed 100 tests

posted by sylvan

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To be clear to everyone: What I'm saying is that the **equalities** are wrong. The first definition is perfectly fine.

Also sylvan: Very nince demo of QuickCheck, and a good thing you put a type signature on
`prop_Concat`

.

When I started using QuickCheck I didn't do that and GHCi defaulted that kind of a function to:
`prop_Concat :: [()] -> [()] -> Bool`

So the tests all ran OK and I ended up submitting a wrong solution to an exercise to my teacher...

posted by Tom Lokhorst

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A comment about implementing takeWhile and dropWhile using foldr. These are functions to take or drop elements at the beginning of a list. So, I think it would be easier to implement them using foldl (fold left). Can it be done with foldr? I’m not sure. The drawback to implement them with fold is that they would be useless when dealing with infinite lists. That is because fold(r|l) consume the whole list before producing a result.

Here is my take on both using foldl

takeWhile' :: (a -> Bool) -> [a] -> [a] takeWhile' p = snd . foldl (\(e,v) x -> if e then (e,v) else if p x then (False,v++[x]) else (True,v)) (False,[]) dropWhile' :: (a -> Bool) -> [a] -> [a] dropWhile' p = snd . foldl (\(e,v) x -> if e then (e,v++[x]) else if p x then (True,v) else (False,v++[])) (False,[])

posted by paks8150

]]>takeWhile p = foldr (\x xs -> if p x then x : xs else []) []

Also, foldr most definitely does not consume the whole list before producing a result. Take this definition:

foldr :: (a -> b -> b) -> b -> [a] -> b foldr f z [] = z foldr f z (x:xs) = f x (foldr f z xs)

As you can see in the cons case; foldr calls

`f`

with `x`

and the result of a recursive call.However, since Haskell is lazy,

`f`

gets executed `f`

decides to never inspects its second argument, the recursive call will never be evaluated. So that's why you can do:
`takeWhile (<4) [0..]`

However, you are right about foldl.

foldl :: (a -> b -> a) -> a -> [b] -> a foldl f z [] = z foldl f z (x:xs) = foldl (f z x) xs

`foldl`

first recurses, before executing the `f`

function that produces the result value.
So calling the `takeWhile'`

, defined below, with an infinite list will result in an infinite computation.

takeWhile' p = foldl (\ys x -> if p x then ys ++ [x] else ys) []

posted by Tom Lokhorst

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So much to learn.

posted by paks8150

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reverse = foldl (flip (:)) []

The 'foldl' expresses the eagerness required by reverse, and the 'flip (' expresses the all-pairs transposition.

posted by piersh

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Microsoft would be well served to upgrade to Apple's QuickTime, but I'm confident that would never happen.

posted by HaskellGuy

]]>C

posted by Charles

]]>Shouldn't it be

reverse' = foldr (\x xs -> xs++[x]) []

posted by pangron

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2) Express the comprehension [f x | x <- xs, p x] using the functions map and filter.

mapfilter :: (a -> b) -> (a -> Bool) -> [a] -> [b] mapfilter f p = map f . filter p

3) Redefine map f and filter p using foldr.

map' :: (a -> b) -> [a] -> [b] map' f = foldr (\x v -> f x:v) [] filter' :: (a -> Bool) -> [a] -> [a] filter' p = foldr (\x v -> if p x then x:v else v) []

posted by paks8150

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import Text.Printf import Control.Exception import System.CPUTime time :: IO t -> IO t time a = do start <- getCPUTime v <- a end <- getCPUTime let diff = (fromIntegral (end - start)) / (10^12) printf "Computation time: %0.3f sec\n" (diff :: Double) return v len1 = sum . map ( \ _ -> 1 ) len2 = foldr ( \ _ n -> n+1) 0 limit = 500000 main = do putStrLn "Len1:" time $ len1 [1..limit] `seq` return () putStrLn "Len2:" time $ len2 [1..limit] `seq` return ()

I get

*Main> :run main Len1: Computation time: 0.719 sec Len2: Computation time: 1.141 sec

posted by BJG

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map :: (a -> b) -> [a] -> [b]

map f [] = []

map f (x:xs) = f x : Main.map f xs

map' :: (a -> b) -> [a] -> [b]

map' f [] = []

map' f xs = Main.foldr (\x xs -> f x : xs) [] xs

filter :: (a -> Bool) -> [a] -> [a]

filter f [] = []

filter f (x:xs) | f x = x : Main.filter f xs

| otherwise = Main.filter f xs

filter' :: (a -> Bool) -> [a] -> [a]

filter' f [] = []

filter' f xs = Main.foldr (\x xs -> if f x then x : xs else xs) [] xs

-- Need Integral class, since method div is provided there

even :: Integral a => a -> Bool

even a = if ((a `div` 2)*2) == a then True else False

foldr :: (a -> b -> b) -> b -> [a] -> b

foldr f v [] = v

foldr f v (x:xs) = f x (Main.foldr f v xs)

Sohail Qayum Malik

posted by Aeon

]]>length'' = (foldr (\x xs -> x + xs) 0) . (foldr (\_ xs -> [1] ++ xs) [])

all' :: (a -> Bool) -> [a] -> Bool

--all' p as = foldr (\x xs -> x && xs) True $ foldr (\x xs -> if p x then True : xs else False : xs) [True] as

all' p xs = foldr (\x xs -> p x && xs) True xs

any' :: (a -> Bool) -> [a] -> Bool

any' p xs = foldr (\x xs -> p x || xs) False xs

takewhile' :: (a -> Bool) -> [a] -> [a]

takewhile' p xs = foldr (\x xs -> if p x then x : xs else xs) [] xs

dropwhile' :: (a -> Bool) -> [a] -> [a]

dropwhile' p xs = foldr (\x xs -> if not (p x) then x : xs else xs) [] xs

Sohail Qayum Malik

posted by Aeon

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