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Revision as of 00:17, 2 March 2006
Useful Idioms that will blow your mind (unless you already know them :)
This collection is supposed to be comprised of short, useful, cool, magical examples, which should incite the reader's curiosity and (hopefully) lead him to a deeper understanding of advanced Haskell concepts. At a later time I might add explanations to the more obscure solutions. I've also started providing several alternatives to give more insight into the interrelations of solutions.
More examples are always welcome, especially "obscure" monadic ones.
List/String Operations
 split at whitespace
 "hello world" > ["hello","world"]
words
takeWhile (not . null) . unfoldr (Just . (second $ drop 1) . break (==' '))
fix (\f l > if null l then [] else let (s,e) = break (==' ') l in s:f (drop 1 e))
 splitting in two (alternating)
 "1234567" > ("1357", "246")
foldr (\a (x,y) > (a:y,x)) ([],[])
(map snd *** map snd) . partition (even . fst) . zip [0..]
transpose . unfoldr (\a > if null a then Nothing else Just $ splitAt 2 a)
 this one uses the solution to the next problem in a nice way :)
 splitting into lists of length N
 "1234567" > ["12", "34", "56", "7"]
unfoldr (\a > if null a then Nothing else Just $ splitAt 2 a)
takeWhile (not . null) . unfoldr (Just . splitAt 2)
 sorting by a custom function
 length > ["abc", "ab", "a"] > ["a", "ab", "abc"]
sortBy length
map snd . sortBy fst . map (length &&& id)
 the so called "Schwartzian Transform" for computationally more expensive functions.
 lazy substring search
 "ell" > "hello" > True
substr a b = any (a `elem`) $ map inits (tails b)
Mathematical Series, etc
 factorial
 6 > 720
product [1..6]
foldl1 (*) [1..6]
(!!6) $ unfoldr (\(n,f) > Just (f, (n+1,f*n))) (1,1)
fix (\f n > if n <= 0 then 1 else n * f (n1))
 powers of two series
iterate (*2) 1
unfoldr (\z > Just (z,2*z)) 1
 fibonacci series
unfoldr (\(f1,f2) > Just (f1,(f2,f1+f2))) (0,1)
fibs = 0:1:zipWith (+) fibs (tail fibs)
fib = 0:scanl (+) 1 fib
 prime numbers
 example of a memoising caf (??)
primes = sieve [2..] where
sieve (p:x) = p : sieve [ n  n < x, n `mod` p > 0 ]
unfoldr sieve [2..] where
sieve (p:x) = Just(p, [ n  n < x, n `mod` p > 0 ])
Monad Magic
 all combinations of a list of lists.
 these solutions are all pretty much equivalent in that they run in the List Monad. the "sequence" solution has the advantage of scaling to N sublists.
 "12" > "45" > ["14", "15", "24", "25"]
sequence ["12", "45"]
[[x,y]  x < "12", y < "45"]
do { x < "12"; y < "45"; return [x,y] }
"12" >>= \a > "45" >>= \b > return [a,b]
 all combinations of letters
(inits . repeat) ['a'..'z'] >>= sequence
 apply a list of functions to an argument
 even > odd > 4 > [True,False]
map ($4) [even,odd]
sequence [even,odd] 4
 apply a function to two other function the same argument
 (lifting to the Function Monad (>))
 even 4 && odd 4 > False
liftM2 (&&) even odd 4
liftM2 (>>) putStrLn return "hello"
 forward function concatenation
(*3) >>> (+1) $ 2
foldl1 (flip (.)) [(+1),(*2)] 500
 perform functions in/on a monad, lifting
fmap (+2) (Just 2)
liftM2 (+) (Just 4) (Just 2)
 [still to categorize]
(id >>= (+) >>= (+) >>= (+)) 3  (3+3)+(3+3) = 12
(join . liftM2) (*) (+3) 5  64
mapAccumL (\acc n > (acc+n,acc+n)) 0 [1..10]  interesting for fac, fib, ...
do f < [not, not]; d < [True, False]; return (f d)  [False,True,False,True]
do { Just x < [Nothing, Just 5, Nothing, Just 6, Just 7, Nothing]; return x }
Other
 simulating lisp's cond
case () of ()  1 > 2 > True
 3 < 4 > False
 otherwise > True
 match a constructor
 this is better than applying all the arguments, because this way the data type can be changed without touching the code (ideally).
case a of Just{} > True
_ > False
{
TODO, IDEAS:
more fun with monad, monadPlus (liftM, ap, guard, when)
fun with arrows (second, first, &&&, ***)
liftM, ap
lazy search (searching as traversal of lazy structures)
innovative data types (i.e. having fun with Maybe sequencing)
LINKS:
bananas, envelopes, ... (generic traversal)
why functional fp matters (lazy search, ...)
}