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# Programming-Idioms.org

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1
Print a literal string on standard output
``putStrLn "Hello world!"``
2
Loop to execute some code a constant number of times
``replicateM_ 10 \$ putStrLn "Hello"``
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
``f = print "whatever"``
4
Create a function which returns the square of an integer
``````square x = x^2
``````
Alternative implementation:
``square x = x * x``
Alternative implementation:
``square x = x**2``
5
Declare a container type for two floating-point numbers x and y
``````data Point = Point
{ x :: Double
, y :: Double
}``````
Alternative implementation:
``````data Point = Point
{ x :: Double
, y :: Double
} deriving (Eq, Ord, Show, Read)``````
6
Do something with each item x of the list (or array) items, regardless indexes.
``````forM_ items doSomething
``````
7
Print each index i with its value x from an array-like collection items
``````mapM_ print (zip [0..] items)
``````
8
Create a new map object x, and provide some (key, value) pairs as initial content.
``````x = Data.Map.Strict.fromList [ ("red", "FF0000"), ("blue", "0000FF") ]

``````
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
``````data BT x = BEnd | BNode (BT x) x (BT x)
``````
10
Generate a random permutation of the elements of list x
``````shuffle x = if length x < 2 then return x else do
i <- System.Random.randomRIO (0, length(x)-1)
r <- shuffle (take i x ++ drop (i+1) x)
return (x!!i : r)
``````
11
The list x must be non-empty.
``(\l g -> l !! fst (randomR (0, length l) g))``
Alternative implementation:
``(l !!) <\$> randomRIO (0, length l - 1)``
12
Check if the list contains the value x.
list is an iterable finite container.
``x `elem` list``
Alternative implementation:
``````find _ [] = False
find n (x:xs)
| x == n = True
| otherwise = find n xs``````
13
Access each key k with its value x from an associative array mymap, and print them.
``````let f k v = [show k, " = ", show v]
mapped = Map.mapWithKeys f mymap
in putStrLn \$ intercalate "," \$ mapped``````
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
``````pick(a,b)=
return.(+a).(*(b-a))=<<System.Random.randomIO::IO(Double)
``````
Alternative implementation:
``````pick a b = do
r <- System.Random.randomIO :: IO Double
return a + (r * (b - a))
``````
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
``````let pick (a, b) = randomRIO (a, b) :: IO Integer
in pick (1, 6) >>= print

``````
16
Call a function f on every node of binary tree bt, in depth-first infix order
``````inorder Ø = []
inorder (BT left pivot right) =
inorder left ++ pivot : inorder right
f <\$> inorder bt
``````
Alternative implementation:
``````instance Functor BT where
fmap f (BT l x r) = BT (fmap f l) (f x) (fmap f r)

fmap f bt``````
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
``````data Tree a = Node {
value :: a,
children :: [Tree a]
}``````
18
Call a function f on every node of a tree, in depth-first prefix order
``````preordered (Node pivot left right) =
pivot : preordered left ++ preordered right
preordered Ø = []
f <\$> (preordered tree)
``````
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
``````reverse x
``````
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
``````search x m = head [ (i, j) | (i, r) <- zip [0..] m,
(j, c) <- zip [0..] r, c == x]

``````
21
Swap the values of the variables a and b
``````swap (a,b) = (b,a)
``````
22
Extract the integer value i from its string representation s (in radix 10)
``let i = read s :: Integer``
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
``````s <- showFFloat (Just 2) x ""
``````
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
``````s = "ネコ"
``````
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
``````peopleToGreet <- newChan
writeChan peopleToGreet "Alan"
``````
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
``````x = [ [ j**(1/i) | j <- [1..n] ] | i <- [1..m] ]
``````
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
``````x = [ [ [ k**(i/j) | k<-[1..p] ] | j<-[1..n] ] | i<-[1..m] ]
``````
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
``sortBy (comparing p) items``
Alternative implementation:
``List.sortOn p items``
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
``take i items ++ drop (1 + i) items``
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
``mapM_ (forkIO . f) [1..1000]``
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
``````f i = if i > 1 then f (i-1) * i else 1
``````
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
``exp = (^)``
33
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
``````putMVar x . f =<< takeMVar x
``````
34
Declare and initialize a set x containing unique objects of type T.
``````x = empty :: Set T
``````
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
``````compose :: (A -> B) -> (B -> C) -> A -> C
compose = flip (.)``````
Alternative implementation:
``````compose :: (A -> B) -> (B -> C) -> A -> C
compose f g x = g (f x)``````
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
``````compose f g = g . f
``````
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
``````curry range
``````
Alternative implementation:
``````addThem :: Num a => a -> a -> a

add5 :: Num a => a -> a
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
``````t = drop i (take j s)
``````
39
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise.
``````ok = word `isInfixOf` s
``````
40
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices.
``````datatype Node = Int
datatype Adjacencies = [ Node ]
datatype Graph = [ Adjacencies ]
``````
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
``````t = reverse s :: String
``````
42
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
``sequence_ [ print v | v <- a, [ u | u <- b, u == v] == [] ]``
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
``````(print . head . filter (<0) . concat) m
``````
44
Insert the element x at position i in the list s. Further elements must be shifted to the right.
``````take i s ++ x : drop i s
``````
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
``````Control.Concurrent.threadDelay 5000000
``````
46
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled.
``````t :: String
t = take 5 s``````
Alternative implementation:
``````t :: T.Text
t = T.take 5 s``````
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
``````t = drop (length s - 5) s
``````
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
``````s = unlines [
"several"
,"lines"
,"of"
,"text"]``````
Alternative implementation:
``````s :: String
s = "We will put a backslash to take a break\
\and then a backslash to resume"``````
49
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
``chunks = words s``
50
Write a loop that has no end clause.
``````forever (getLine >>= putStrLn)
``````
Alternative implementation:
``let x = x in x``
Alternative implementation:
``forever \$ pure ()``
51
Determine whether the map m contains an entry for the key k
``````(/=Nothing) (lookup k m)
``````
Alternative implementation:
``Map.member k m``
52
Determine whether the map m contains an entry with the value v, for some key.
``````elem v (elems m)
``````
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
``y = intercalate ", " x``
Alternative implementation:
``````{-# LANGUAGE OverloadedStrings #-}
y :: T.Text
y = T.intercalate ", " x``````
54
Calculate the sum s of the integer list or array x.
``s = sum x``
55
Create the string representation s (in radix 10) of the integer value i.
``let s = show i``
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
``````mapConcurrently f [1..1000]
print "Finished"``````
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
``````y = filter p x
``````
58
Create the string lines from the content of the file with filename f.
``do lines <- readFile f; putStr lines``
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
``hPutStrLn stderr (show (x) ++ " is negative")``
60
Assign to x the string value of the first command line parameter, after the program name.
``````x <- return.head =<< System.Environment.getArgs
``````
61
Assign to the variable d the current date/time value, in the most standard type.
``````d <- System.Posix.Time.epochTime
``````
62
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
``Just i <- (y `isPrefixOf`) `findIndex` (tails x)``
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
``````allchanged [] _ _ = []
allchanged input from to = if isPrefixOf from input
then to ++ allchanged (drop (length from) input) from to
else head input : allchanged (tail input) from to

x2 = allchanged x y z
``````
64
Assign to x the value 3^247
``````x :: Integer
x = 3^247``````
65
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
``s = Numeric.showFFloat (Just 1) (100*x) "%"``
66
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
``z = x ^ n``
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
``````binom n k = product [1+n-k..n] `div` product [1..k]
``````
68
Create an object x to store n bits (n being potentially large).
``````x :: Integer
x = sum [bit i | i <- [1..n], wannaset i]
``````
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
``````System.Random.mkStdGen s
``````
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
``````getStdGen
``````
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
``````putStrLn . unwords =<< System.Environment.getArgs
``````
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
``````x = gcd a b
``````
Alternative implementation:
``````gcd a b
|   a==b =a
|   a>b = gcd(a-b) b
|   otherwise = gcd a (b-a)``````
Alternative implementation:
``````gcd x y =  gcd' (abs x) (abs y)
where
gcd' a 0  =  a
gcd' a b  =  gcd' b (a `rem` b)``````
75
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
``````x = lcm a b
``````
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
``s x = showIntAtBase 2 intToDigit x ""``
Alternative implementation:
``foldl (\acc -> (++ acc) . show) "" . unfoldr (\n -> if x == 0 then Nothing else Just (x `mod` 2, x `div` 2))``
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
``let i = ( 0 :+ 1 ) ; x = 3 * i - 2 in x * i``
Alternative implementation:
``x = (0 :+ 1) * ((0 - 2) :+ 3)``
78
Execute a block once, then execute it again as long as boolean condition c is true.
``````doowhile c b = do a <- b; if c a
then doowhile c b
else return a

doowhile (=="") getLine
gg``````
79
Declare the floating point number y and initialize it with the value of the integer x .
``````y = fromInteger x :: Double
``````
80
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
``````y = truncate x
``````
81
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
``````y = floor (x + 1/2)
``````
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
``````sum [ 1 | r <- tails s, isPrefixOf t r ]
``````
Alternative implementation:
``length . filter (isPrefixOf t) . tails \$ s``
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
``````r :: String -> Bool
r = (=~ "htt+p")
``````
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
``````c = Data.Bits.popCount i
``````
85
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
``addingWillOverflow x y = y > maxBound - x``
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
``exitSuccess``
88
Create a new bytes buffer buf of size 1,000,000.
``````createBuf :: Int -> IO (V.IOVector Word8)
createBuf n = V.new n

main :: IO ()
main = do
buf <- createBuf 1000000
return ()``````
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
``````sqrt' :: Int -> Either String Int
sqrt' x | x < 0 = Left "Invalid argument"
sqrt' x         = Right (sqrt x)``````
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
``````module Foo (Foo, getX) where

import Data.IORef

data Foo = Foo { xRef :: IORef Integer }

93
Implement the procedure control which receives one parameter f, and runs f.
``control f = f``
94
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
``````print (dynTypeRep (toDyn x))
``````
95
Assign to variable x the length (number of bytes) of the local file at path.
``````filesize = withFile path ReadMode hFileSize
``````
96
Set the boolean b to true if string s starts with prefix prefix, false otherwise.
``b = prefix `isPrefixOf` s``
97
Set boolean b to true if string s ends with string suffix, false otherwise.
``b = isSuffixOf suffix s``
98
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
``d = posixSecondsToUTCTime . secondsToNominalDiffTime . MkFixed \$ toInteger ts``
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
``x = showGregorian d``
100
Sort elements of array-like collection items, using a comparator c.
``result = sortBy c items``
105
Assign to the string s the name of the currently executing program (but not its full path).
``s <- getProgName``
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
``dir <- getCurrentDirectory``
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
``dir <- takeDirectory `fmap` getExecutablePath``
Alternative implementation:
``dir = unsafePerformIO (takeDirectory `fmap` getExecutablePath)``
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
``b = null (dropWhile isSpace s)``
Alternative implementation:
``````blank :: Bool
blank = all isSpace s``````
111
From current process, run program x with command-line parameters "a", "b".
``spawnProcess x ["a","b"]``
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
``mapM_ print (Map.toList mymap)``
113
Print each key k with its value x from an associative array mymap, in ascending order of x.
Multiple entries may exist for the same value x.
``mapM_ print \$ sortBy (comparing snd) \$ Map.toList mymap``
Alternative implementation:
``mapM_ print . sortOn snd \$ Map.toList mymap``
114
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types.
``b = x == y``
115
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
``b = d1 < d2``
116
Remove all occurrences of string w from string s1, and store the result in s2.
``````remove :: String -> String -> String
remove w "" = ""
remove w s@(c:cs)
| w `isPrefixOf` s = remove w (drop (length w) s)
| otherwise = c : remove w cs

s2 = remove w s1``````
117
Set n to the number of elements of the list x.
``let n = length x``
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
``y = Set.fromList x``
119
Remove duplicates from the list x.
Explain if the original order is preserved.
``nub x``
120
Read an integer value from the standard input into the variable n
``n <- (read :: String -> Int) <\$> getContents``
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
``data Suit = SPADES | HEARTS | DIAMONDS | CLUBS deriving (Enum)``
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
``let x' = assert isConsistent x``
124
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
``````binSearch :: Ord a => a -> [a] -> Maybe Int
binSearch _ [] = Nothing
binSearch t l = let n = div (length l) 2
(a, m:b) = splitAt n l in
if t < m then binSearch t a
else if t > m then aux (binSearch t b)
else Just n where
aux :: Maybe Int -> Maybe Int
aux (Just x) = Just (x+n+1)
aux _ = Nothing``````
126
Write a function foo that returns a string and a boolean value.
``````foo :: (String, Bool)   -- optional signature
foo = ("String", True)``````
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
``````if c1 then f1
else if c2 then f2
else f3``````
Alternative implementation:
``````let x | c1 = f1
| c2 = f2
| c3 = f3``````
133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
``````containsIgnoreCase :: String -> String -> Bool
containsIgnoreCase s word = isInfixOf (map toLower word) (map toLower s)``````
134
Declare and initialize a new list items, containing 3 elements a, b, c.
``items = [a, b, c]``
135
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
``delete x items``
136
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
``filter (/= x) items``
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
``b = all isDigit s``
138
Create a new temporary file on the filesystem.
``````withTempFile \$ \f -> do
-- do something with f``````
139
Create a new temporary folder on filesystem, for writing.
``````withTempDirectory \$ \d -> do
-- do something with d``````
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
``delete k m``
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
``mapM_ print \$ items1 ++ items2``
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
``````s :: String
s = printf "%x" 999``````
143
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
``````interweave :: [a] -> [a] -> [a]   -- optional signature
interweave [] ys = ys
interweave xs [] = xs
interweave (x:xs) (y:ys) = x : y : interweave xs ys

main = mapM_ print \$ interweave items1 items2
``````
Alternative implementation:
``````mapM_ print . concat
\$ zipWith (\a b -> [a,b]) items1 items2``````
144
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
``b = doesFileExist fp``
146
Extract floating point value f from its string representation s
``read s :: Double``
147
Create string t from string s, keeping only ASCII characters
``````f = filter isAscii
t = f s``````
Alternative implementation:
``t = filter isAscii s``
148
Read a list of integer numbers from the standard input, until EOF.
``````read <\$> getLine :: IO [Integer]

-- reading space separated list of ints
map read . words <\$> getLine :: IO [Int]``````
150
Remove the last character from the string p, if this character is a forward slash /
``slashscrape p = if last p == '/' then init p else p``
151
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
``p' = dropWhileEnd (== pathSeparator) p``
152
Create string s containing only the character c.
``s = [c]``
153
Create the string t as the concatenation of the string s and the integer i.
``t = s ++ show i``
155
Delete from filesystem the file having path filepath.
``removeFile filePath``
157
Initialize a constant planet with string value "Earth".
``planet = "Earth"``
158
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
``````randomSample :: Int -> [a] -> IO [a]
randomSample 0 x = pure []
randomSample k x = do
i <- randomRIO (0, length x - 1)
let (a, e:b) = splitAt i x
l <- randomSample (k-1) (a ++ b)
pure (e : l)``````
159
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries)
``````data Trie v
= Branch Char (Map Char (Trie v))
| Leaf Char v``````
160
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
``````detectArch :: IO ()
detectArch = do
case arch == "x86_64" of
True -> do f64
False -> case arch == "x86" of
True -> do f32``````
161
Multiply all the elements of the list elements by a constant c
``map (*c) elements``
162
execute bat if b is a program option and fox if f is a program option.
``````do
args <- getArgs
when ("b" `elem` args) bat
when ("f" `elem` args) fox``````
163
Print all the list elements, two by two, assuming list length is even.
``````everySecond :: [a] -> [a]
everySecond [] = []
everySecond (_:[]) = []
everySecond (_:x:xs) = x : everySecond xs
everySecond' :: [a] -> [a]
everySecond' = everySecond . (undefined :)

mapM_ print (zip (everySecond list) (everySecond' list))``````
Alternative implementation:
``````pair :: [a] -> [(a, a)]
pair [] = []
pair (x:[]) = error "List had odd length"
pair (x:y:xs) = (x, y) : pair xs

mapM_ print (pair list)``````
165
Assign to the variable x the last element of the list items.
``````foo :: [a] -> Maybe a
foo [] = Nothing
foo xs = Just \$ last xs

x = foo items``````
Alternative implementation:
``x = last items``
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
``ab = a ++ b``
167
Create the string t consisting of the string s with its prefix p removed (if s starts with p).
``t = Maybe.fromMaybe s \$ List.stripPrefix p s``
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
``````t :: String
t = if w `isSuffixOf` s then take (length s - length w) else s``````
169
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
``n = length s``
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
``````getMapSize :: Map.Map k v -> Int
getMapSize m = Map.size m``````
171
Append the element x to the list s.
``xs = s ++ [x]``
172
Insert value v for key k in map m.
``newM = Map.insert k v m``
173
Number will be formatted with a comma separator between every group of thousands.
``````s :: Int -> String
s = intersperseN ',' 3 . show

intersperseN :: a -> Int -> [a] -> [a]
intersperseN x n = uncurry (<>) . foldr alg ([], [])
where
alg a (buf', acc)
| length buf >= n = ([], (x:buf) <> acc)
| otherwise = (buf, acc)
where buf = a:buf'``````
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
``b = x >= x1 && x <= x2 && y >= y1 && y <= y2``
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
``````rectCenter :: Num a => (a, a) -> (a, a) -> (a,a)
rectCenter (x1,y1) (x2,y2) = ((x1+x2)/2,(y1+y2)/2)``````
180
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing.
``````do
x <- listDirectory d
-- do something with x``````
182
Output the source of the program.
``main = putStr s >> print s where s = "main = putStr s >> print s where s = "``
186
Exit a program cleanly indicating no error to OS
``exitSuccess``
189
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P.
``y = (map t . filter p) x``
191
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
``when (foldl1 (||) \$ map (a > x)) f``
198
Abort program execution with error condition x (where x is an integer value)
``exitWith (ExitFailure 4)``
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
``hypo x y = sqrt \$ x**2 + y**2``
202
Calculate the sum of squares s of data, an array of floating point values.
``sumOfSquares = sum . map (^2)``
203
Calculate the mean m and the standard deviation s of the list of floating point values data.
``````mean dat = sum dat / (fromIntegral \$ length dat)

stddev dat = sqrt . mean \$ map ((**2) . (m -)) dat
where
m = mean dat``````
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
``````do
foo <- catch (getEnv "FOO") (const \$ pure "none" :: IOException -> IO String)
-- do something with foo``````
206
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
``````case str of
"foo" -> foo
"bar" -> bar
"baz" -> baz
"barfl" -> barfl``````
215
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
``````--              BaseString      PadChar      MinOutputLength     PaddedString/Output
padLeft ::       String ->     Char ->          Int ->                String
padLeft s c m = let
isBaseLarger =  length s > m
padder s c m False = [ c | _ <- [1..(m-length s)]] ++ s
padder s _ _ True = s
in
218
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
``a `intersect` b``
219
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
``t= unwords \$ words s``
220
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
``t = (a, b, c)``
221
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
``t = filter (`elem` ['0'..'9']) s``
222
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
``````findIndex x items = fst <\$> (find ((==x) . snd) . zip [0..]) items

i = fromMaybe -1 (findIndex x items)``````
224
Insert the element x at the beginning of the list items.
``items2 = x : items``
Alternative implementation:
``f(x:xs)= x:xs``
226
Remove the last element from the list items.
``````pop :: [a] -> [a]
pop [] = []
pop xs = init xs

let items2 = pop items``````
Alternative implementation:
``let items2 = init items``
237
Assign to c the result of (a xor b)
``c = xor a b``
240
Lists a and b have the same length. Apply the same permutation to a and b to have them sorted based on the values of a.
``let (outA,outB) = unzip \$ sort \$ zip a b``
243
Print the contents of the list or array a on the standard output.
``print a``
245
Print the value of object x having custom type T, for log or debug.
``print x``
246
Set c to the number of distinct elements in the list items.
``c = count . nub \$ items``
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
``(a, b, c) = (42, "hello", 5.0)``
252
Assign to the variable x the string value "a" if calling the function condition returns true, or the value "b" otherwise.
``x = if condition then 'a' else 'b'``
Alternative implementation:
``````let x | condition = "a"
| otherwise = "b"``````
253
Print the stack frames of the current execution thread of the program.
``````msgStacktraced :: HasCallStack => String -> IO ()
msgStacktraced msg = putStrLn (msg ++ "\n" ++ prettyCallStack callStack)``````
254
Replace all exact occurrences of "foo" with "bar" in the string list x
``replaced = map (\e -> if e == "foo" then "bar" else e) x``
255
Print the values of the set x to the standard output.
The order of the elements is irrelevant and is not required to remain the same next time.
``print x``
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
``forM_ (reverse [0..5]) print``
Alternative implementation:
``foldl  (\res x-> x:res) [] [0..5]``
260
Declare a new list items of string elements, containing zero elements
``````var :: [String]
var = []``````
263
Write two functions log2d and log2u, which calculate the binary logarithm of their argument n rounded down and up, respectively. n is assumed to be positive. Print the result of these functions for numbers from 1 to 12.
``````log2d :: Double -> Integer
log2d = floor . logBase 2

log2u :: Double -> Integer
log2u = ceiling . logBase 2

main :: IO ()
main = print \$ [log2d, log2u] <*> [1..12]``````
264
Pass a two-dimensional integer array a to a procedure foo and print the size of the array in each dimension. Do not pass the bounds manually. Call the procedure with a two-dimensional array.
``````foo :: [[a]] -> IO ()
foo a = print (length a, length \$ head a)

main :: IO ()
main = foo [[1, 2, 3], [4, 5, 6]]``````
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
``s = concat \$ replicate n v``
268
Define a type vector containing three floating point numbers x, y, and z. Write a user-defined operator x that calculates the cross product of two vectors a and b.
``````data Vector a = Vector a a a

infixl 7 ×
(×) :: Num a => Vector a -> Vector a -> Vector a
Vector x1 y1 z1 × Vector x2 y2 z2 = Vector (y1 * z2 - z1 * y2) (z1 * x2 - x1 * z2) (x1 * y2 - y1 * x2)``````
Alternative implementation:
``````data Vector a = Vector a a a

infixl 7 `x`
x :: Num a => Vector a -> Vector a -> Vector a
Vector x1 y1 z1 `x` Vector x2 y2 z2 = Vector (y1 * z2 - z1 * y2) (z1 * x2 - x1 * z2) (x1 * y2 - y1 * x2)``````
274
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
``t = filter (not . isSpace) s``
281
You have a Point with integer coordinates x and y. Create a map m with key type Point (or equivalent) and value type string. Insert "Hello" at position (42, 5).
``````type Point = (Integer, Integer)
m :: Map.Map Point String
m = Map.empty
m' = Map.insert (42, 5) "Hello" m``````
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
``a = replicate n 0``
286
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
``forM_ (zip [0 ..] s) (\(i, c) -> putStrLn \$ "Char " ++ show i ++ " is " ++ [c])``
288
Set the boolean b to true if the set x contains the element e, false otherwise.
``b = e `member` x``
289
Create the string s by concatenating the strings a and b.
``s = a ++ b``
Alternative implementation:
``s = a <> b``
299

This line will not be compiled or executed.
``-- This is a comment``
301
Compute the Fibonacci sequence of n numbers using recursion.

Note that naive recursion is extremely inefficient for this task.
``````fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib n-1 + fib n-2``````
340
Assign to c the value of the last character of the string s.

Explain the type of c, and what happens if s is empty.

Make sure to properly handle multi-bytes characters.
``````lastChar :: String -> Char
lastChar s
| s=="" = ""
| otherwise = s !! (length s - 1)``````