Useful list functions in Haskell

p0.md

Useful list functions in Haskell

head, last

• Header:

  head, last :: [a] -> a

• Description:

  • head xs is the first element of the xs list.
  • last xs is the last element of the xs list.

  Error if xs is empty.

• Examples:

  head [1 .. 4]
  👉 1
  last [1 .. 4]
  👉 4

tail, init

• Header:

  tail, init :: [a] -> [a]

• Description:

  • tail xs is the xs list without its first element.
  • init xs is the xs list without its last element.

  Error if xs is empty.

• Examples:

  tail [1..4]
  👉 [2, 3, 4]
  init [1..4]
  👉 [1, 2, 3]

reverse

• Header:

  reverse :: [a] -> [a]

• Description:

  reverse xs is the xs list in the reverse order.

• Examples:

  reverse [1..4]
  👉 [4, 3, 2, 1]

length

• Header:

  length :: [a] -> Int

• Description:

  length xs is the number of elements elements in the xs list.

null

• Header:

  null :: [a] -> Bool

• Description:

  null xs tells whether the xs list is empty.

elem

• Header:

  elem :: (Eq a) => a -> [a] -> Bool

• Description:

  elem x xs tells whether x is contained in the xs list.

(!!)

• Header:

  (!!) :: [a] -> Int -> a

• Description:

  xs !! is the i-th element of the xs` list (starting from zero).

maximum, minimum

• Header:

  maximum, minimum :: (Ord a) => [a] -> a

• Description:

  • maximum xs is the bigger element from the xs list (not empty!).
  • minimum xs is the smaller element from the xs list (not empty!).

and, or

• Header:

  and, or :: [Bool] -> Bool

• Description:

  • and bs is the conjuntion of the bs list of bolleans.
  • or bs is the disjuntion of the bs list of bolleans.

sum, prod

• Header:

  sum, prod :: [Int] -> Int

• Description:

  • sum xs is the sum of the elements in the xs list.
  • prod xs is the product of the elements in the xs list.

• Examples:

  fact n = prod [1 .. n]
  
  fact 5
  👉 120

take, drop

• Header:

  take, drop :: Int -> [a] -> [a]

• Description:

  • take n xs is the prefix of length n of the xs list.
  • drop n xs is the sufix of the xs list when the first n elements are removed.

• Examples:

  take 3 [1 .. 7]
  👉 [1, 2, 3]
  drop 3 [1 .. 7]
  👉 [4, 5, 6, 7]

zip

• Header:

  zip :: [a] -> [b] -> [(a, b)]

• Description:

  zip xs ys is the list that joins, in ordee, each pair of elements from xs and ys. If some list lacks elements, the result is truncated.

• Examples:

  zip [1, 2, 3] ['a', 'b', 'c']
  👉 [(1, 'a'), (2, 'b'), (3, 'c')]
  zip [1 .. 10] [1 .. 3]
  👉 [(1, 1), (2, 2), (3, 3)]

repeat

• Header:

  repeat :: a -> [a]

• Description:

  repeat x is the infinite list whose element are always x.

• Examples:

  repeat 3
  👉 [3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, ...]
  take 4 (repeat 3)
  👉 [3, 3, 3, 3]

concat

• Header:

  concat :: [[a]] -> [a]

• Description:

  concat xs is the list that concatenates all lists in xs.

• Examples:

  concat [[1, 2, 3], [], [3], [1, 2]]
  👉 [1, 2, 3, 3, 1, 2]