Webb.Array

type WArray = Array

addAllFirst :: forall a f. Foldable f => f a -> Array a -> Array a

addAllLast :: forall a f. Foldable f => f a -> Array a -> Array a

addFirst :: forall a. a -> Array a -> Array a

addLast :: forall a. a -> Array a -> Array a

canFind :: forall a. (a -> Boolean) -> Array a -> Boolean

canFindEq :: forall a. Eq a => a -> Array a -> Boolean

empty :: forall a. Array a

findEq :: forall a. Eq a => a -> Array a -> Maybe a

first :: forall a. Array a -> Maybe a

isEmpty :: forall a. Array a -> Boolean

reject :: forall a. (a -> Boolean) -> Array a -> Array a

size :: forall a. Array a -> Int

zipWithA :: forall m a b c. Applicative m => (a -> b -> m c) -> Array a -> Array b -> m (Array c)

A generalization of zipWith which accumulates results in some Applicative functor.

sndChars = zipWithA (\a b -> charAt 2 (a <> b))
sndChars ["a", "b"] ["A", "B"] = Nothing -- since "aA" has no 3rd char
sndChars ["aa", "b"] ["AA", "BBB"] = Just ['A', 'B']

zipWith :: forall a b c. (a -> b -> c) -> Array a -> Array b -> Array c

Apply a function to pairs of elements at the same index in two arrays, collecting the results in a new array.

If one array is longer, elements will be discarded from the longer array.

For example

zipWith (*) [1, 2, 3] [4, 5, 6, 7] == [4, 10, 18]

zip :: forall a b. Array a -> Array b -> Array (Tuple a b)

Takes two arrays and returns an array of corresponding pairs. If one input array is short, excess elements of the longer array are discarded.

zip [1, 2, 3] ["a", "b"] = [Tuple 1 "a", Tuple 2 "b"]

updateAtIndices :: forall t a. Foldable t => t (Tuple Int a) -> Array a -> Array a

Change the elements at the specified indices in index/value pairs. Out-of-bounds indices will have no effect.

updates = [Tuple 0 "Hi", Tuple 2 "." , Tuple 10 "foobar"]

updateAtIndices updates ["Hello", "World", "!"] = ["Hi", "World", "."]

updateAt :: forall a. Int -> a -> Array a -> Maybe (Array a)

Change the element at the specified index, creating a new array, or returning Nothing if the index is out of bounds.

updateAt 1 "World" ["Hello", "Earth"] = Just ["Hello", "World"]
updateAt 10 "World" ["Hello", "Earth"] = Nothing

unzip :: forall a b. Array (Tuple a b) -> Tuple (Array a) (Array b)

Transforms an array of pairs into an array of first components and an array of second components.

unzip [Tuple 1 "a", Tuple 2 "b"] = Tuple [1, 2] ["a", "b"]

unsnoc :: forall a. Array a -> Maybe { init :: Array a, last :: a }

Break an array into its last element and all preceding elements.

unsnoc [1, 2, 3] = Just {init: [1, 2], last: 3}
unsnoc [] = Nothing

Running time: O(n) where n is the length of the array

unsafeIndex :: forall a. Partial => Array a -> Int -> a

Find the element of an array at the specified index.

unsafePartial $ unsafeIndex ["a", "b", "c"] 1 = "b"

Using unsafeIndex with an out-of-range index will not immediately raise a runtime error. Instead, the result will be undefined. Most attempts to subsequently use the result will cause a runtime error, of course, but this is not guaranteed, and is dependent on the backend; some programs will continue to run as if nothing is wrong. For example, in the JavaScript backend, the expression unsafePartial (unsafeIndex [true] 1) has type Boolean; since this expression evaluates to undefined, attempting to use it in an if statement will cause the else branch to be taken.

unionBy :: forall a. (a -> a -> Boolean) -> Array a -> Array a -> Array a

Calculate the union of two arrays, using the specified function to determine equality of elements. Note that duplicates in the first array are preserved while duplicates in the second array are removed.

mod3eq a b = a `mod` 3 == b `mod` 3
unionBy mod3eq [1, 5, 1, 2] [3, 4, 3, 3] = [1, 5, 1, 2, 3]

union :: forall a. Eq a => Array a -> Array a -> Array a

Calculate the union of two arrays. Note that duplicates in the first array are preserved while duplicates in the second array are removed.

Running time: O(n^2)

union [1, 2, 1, 1] [3, 3, 3, 4] = [1, 2, 1, 1, 3, 4]

uncons :: forall a. Array a -> Maybe { head :: a, tail :: Array a }

Break an array into its first element and remaining elements.

Using uncons provides a way of writing code that would use cons patterns in Haskell or pre-PureScript 0.7:

f (x : xs) = something
f [] = somethingElse

Becomes:

f arr = case uncons arr of
  Just { head: x, tail: xs } -> something
  Nothing -> somethingElse

transpose :: forall a. Array (Array a) -> Array (Array a)

The 'transpose' function transposes the rows and columns of its argument. For example,

transpose 
  [ [1, 2, 3]
  , [4, 5, 6]
  ] == 
  [ [1, 4]
  , [2, 5]
  , [3, 6]
  ]

If some of the rows are shorter than the following rows, their elements are skipped:

transpose 
  [ [10, 11]
  , [20]
  , [30, 31, 32]
  ] == 
  [ [10, 20, 30]
  , [11, 31]
  , [32]
  ]

toUnfoldable :: forall f. Unfoldable f => Array ~> f

Convert an Array into an Unfoldable structure.

takeWhile :: forall a. (a -> Boolean) -> Array a -> Array a

Calculate the longest initial subarray for which all element satisfy the specified predicate, creating a new array.

takeWhile (_ > 0) [4, 1, 0, -4, 5] = [4, 1]
takeWhile (_ > 0) [-1, 4] = []

takeEnd :: forall a. Int -> Array a -> Array a

Keep only a number of elements from the end of an array, creating a new array.

letters = ["a", "b", "c"]

takeEnd 2 letters = ["b", "c"]
takeEnd 100 letters = ["a", "b", "c"]

take :: forall a. Int -> Array a -> Array a

Keep only a number of elements from the start of an array, creating a new array.

letters = ["a", "b", "c"]

take 2 letters = ["a", "b"]
take 100 letters = ["a", "b", "c"]

tail :: forall a. Array a -> Maybe (Array a)

Get all but the first element of an array, creating a new array, or Nothing if the array is empty

tail [1, 2, 3, 4] = Just [2, 3, 4]
tail [] = Nothing

Running time: O(n) where n is the length of the array

splitAt :: forall a. Int -> Array a -> { after :: Array a, before :: Array a }

Splits an array into two subarrays, where before contains the elements up to (but not including) the given index, and after contains the rest of the elements, from that index on.

>>> splitAt 3 [1, 2, 3, 4, 5]
{ before: [1, 2, 3], after: [4, 5] }

Thus, the length of (splitAt i arr).before will equal either i or length arr, if that is shorter. (Or if i is negative the length will be 0.)

splitAt 2 ([] :: Array Int) == { before: [], after: [] }
splitAt 3 [1, 2, 3, 4, 5] == { before: [1, 2, 3], after: [4, 5] }

span :: forall a. (a -> Boolean) -> Array a -> { init :: Array a, rest :: Array a }

Split an array into two parts:

1. the longest initial subarray for which all elements satisfy the specified predicate
2. the remaining elements

span (\n -> n % 2 == 1) [1,3,2,4,5] == { init: [1,3], rest: [2,4,5] }

Running time: O(n).

sortWith :: forall a b. Ord b => (a -> b) -> Array a -> Array a

Sort the elements of an array in increasing order, where elements are sorted based on a projection. Sorting is stable: the order of elements is preserved if they are equal according to the projection.

sortWith (_.age) [{name: "Alice", age: 42}, {name: "Bob", age: 21}]
   = [{name: "Bob", age: 21}, {name: "Alice", age: 42}]

sortBy :: forall a. (a -> a -> Ordering) -> Array a -> Array a

Sort the elements of an array in increasing order, where elements are compared using the specified partial ordering, creating a new array. Sorting is stable: the order of elements is preserved if they are equal according to the specified partial ordering.

compareLength a b = compare (length a) (length b)
sortBy compareLength [[1, 2, 3], [7, 9], [-2]] = [[-2],[7,9],[1,2,3]]

sort :: forall a. Ord a => Array a -> Array a

Sort the elements of an array in increasing order, creating a new array. Sorting is stable: the order of equal elements is preserved.

sort [2, -3, 1] = [-3, 1, 2]

some :: forall f a. Alternative f => Lazy (f (Array a)) => f a -> f (Array a)

Attempt a computation multiple times, requiring at least one success.

The Lazy constraint is used to generate the result lazily, to ensure termination.

snoc :: forall a. Array a -> a -> Array a

Append an element to the end of an array, creating a new array.

snoc [1, 2, 3] 4 = [1, 2, 3, 4]

slice :: forall a. Int -> Int -> Array a -> Array a

Extract a subarray by a start and end index.

letters = ["a", "b", "c"]
slice 1 3 letters = ["b", "c"]
slice 5 7 letters = []
slice 4 1 letters = []

singleton :: forall a. a -> Array a

Create an array of one element

singleton 2 = [2]

scanr :: forall a b. (a -> b -> b) -> b -> Array a -> Array b

Fold a data structure from the right, keeping all intermediate results instead of only the final result. Note that the initial value does not appear in the result (unlike Haskell's Prelude.scanr).

scanr (+) 0 [1,2,3] = [6,5,3]
scanr (flip (-)) 10 [1,2,3] = [4,5,7]

scanl :: forall a b. (b -> a -> b) -> b -> Array a -> Array b

Fold a data structure from the left, keeping all intermediate results instead of only the final result. Note that the initial value does not appear in the result (unlike Haskell's Prelude.scanl).

scanl (+) 0  [1,2,3] = [1,3,6]
scanl (-) 10 [1,2,3] = [9,7,4]

reverse :: forall a. Array a -> Array a

Reverse an array, creating a new array.

reverse [] = []
reverse [1, 2, 3] = [3, 2, 1]

replicate :: forall a. Int -> a -> Array a

Create an array containing a value repeated the specified number of times.

replicate 2 "Hi" = ["Hi", "Hi"]

range :: Int -> Int -> Array Int

Create an array containing a range of integers, including both endpoints.

range 2 5 = [2, 3, 4, 5]

partition :: forall a. (a -> Boolean) -> Array a -> { no :: Array a, yes :: Array a }

Partition an array using a predicate function, creating a set of new arrays. One for the values satisfying the predicate function and one for values that don't.

partition (_ > 0) [-1, 4, -5, 7] = { yes: [4, 7], no: [-1, -5] }

null :: forall a. Array a -> Boolean

Test whether an array is empty.

null [] = true
null [1, 2] = false

nubEq :: forall a. Eq a => Array a -> Array a

Remove the duplicates from an array, creating a new array.

This less efficient version of nub only requires an Eq instance.

nubEq [1, 2, 1, 3, 3] = [1, 2, 3]

nubByEq :: forall a. (a -> a -> Boolean) -> Array a -> Array a

Remove the duplicates from an array, where element equality is determined by the specified equivalence relation, creating a new array.

This less efficient version of nubBy only requires an equivalence relation.

mod3eq a b = a `mod` 3 == b `mod` 3
nubByEq mod3eq [1, 3, 4, 5, 6] = [1, 3, 5]

nubBy :: forall a. (a -> a -> Ordering) -> Array a -> Array a

Remove the duplicates from an array, where element equality is determined by the specified ordering, creating a new array.

nubBy compare [1, 3, 4, 2, 2, 1] == [1, 3, 4, 2]

nub :: forall a. Ord a => Array a -> Array a

Remove the duplicates from an array, creating a new array.

nub [1, 2, 1, 3, 3] = [1, 2, 3]

notElem :: forall a. Eq a => a -> Array a -> Boolean

Returns true if the array does not have the given element.

modifyAtIndices :: forall t a. Foldable t => t Int -> (a -> a) -> Array a -> Array a

Apply a function to the element at the specified indices, creating a new array. Out-of-bounds indices will have no effect.

indices = [1, 3]
modifyAtIndices indices toUpper ["Hello", "World", "and", "others"]
   = ["Hello", "WORLD", "and", "OTHERS"]

modifyAt :: forall a. Int -> (a -> a) -> Array a -> Maybe (Array a)

Apply a function to the element at the specified index, creating a new array, or returning Nothing if the index is out of bounds.

modifyAt 1 toUpper ["Hello", "World"] = Just ["Hello", "WORLD"]
modifyAt 10 toUpper ["Hello", "World"] = Nothing

mapWithIndex :: forall a b. (Int -> a -> b) -> Array a -> Array b

Apply a function to each element in an array, supplying a generated zero-based index integer along with the element, creating an array with the new elements.

prefixIndex index element = show index <> element

mapWithIndex prefixIndex ["Hello", "World"] = ["0Hello", "1World"]

mapMaybe :: forall a b. (a -> Maybe b) -> Array a -> Array b

Apply a function to each element in an array, keeping only the results which contain a value, creating a new array.

parseEmail :: String -> Maybe Email
parseEmail = ...

mapMaybe parseEmail ["a.com", "hello@example.com", "--"]
   = [Email {user: "hello", domain: "example.com"}]

many :: forall f a. Alternative f => Lazy (f (Array a)) => f a -> f (Array a)

Attempt a computation multiple times, returning as many successful results as possible (possibly zero).

The Lazy constraint is used to generate the result lazily, to ensure termination.

length :: forall a. Array a -> Int

Get the number of elements in an array.

length ["Hello", "World"] = 2

last :: forall a. Array a -> Maybe a

Get the last element in an array, or Nothing if the array is empty

Running time: O(1).

last [1, 2] = Just 2
last [] = Nothing

intersperse :: forall a. a -> Array a -> Array a

Inserts the given element in between each element in the array. The array must have two or more elements for this operation to take effect.

intersperse " " [ "a", "b" ] == [ "a", " ", "b" ]
intersperse 0 [ 1, 2, 3, 4, 5 ] == [ 1, 0, 2, 0, 3, 0, 4, 0, 5 ]

If the array has less than two elements, the input array is returned.

intersperse " " [] == []
intersperse " " ["a"] == ["a"]

intersectBy :: forall a. (a -> a -> Boolean) -> Array a -> Array a -> Array a

Calculate the intersection of two arrays, using the specified equivalence relation to compare elements, creating a new array. Note that duplicates in the first array are preserved while duplicates in the second array are removed.

mod3eq a b = a `mod` 3 == b `mod` 3
intersectBy mod3eq [1, 2, 3] [4, 6, 7] = [1, 3]

intersect :: forall a. Eq a => Array a -> Array a -> Array a

Calculate the intersection of two arrays, creating a new array. Note that duplicates in the first array are preserved while duplicates in the second array are removed.

intersect [1, 1, 2] [2, 2, 1] = [1, 1, 2]

intercalate :: forall a. Monoid a => a -> Array a -> a

insertBy :: forall a. (a -> a -> Ordering) -> a -> Array a -> Array a

Insert an element into a sorted array, using the specified function to determine the ordering of elements.

invertCompare a b = invert $ compare a b

insertBy invertCompare 10 [21, 20, 2, 1] = [21, 20, 10, 2, 1]

insertAt :: forall a. Int -> a -> Array a -> Maybe (Array a)

Insert an element at the specified index, creating a new array, or returning Nothing if the index is out of bounds.

insertAt 2 "!" ["Hello", "World"] = Just ["Hello", "World", "!"]
insertAt 10 "!" ["Hello"] = Nothing

insert :: forall a. Ord a => a -> Array a -> Array a

Insert an element into a sorted array.

insert 10 [1, 2, 20, 21] = [1, 2, 10, 20, 21]

init :: forall a. Array a -> Maybe (Array a)

Get all but the last element of an array, creating a new array, or Nothing if the array is empty.

init [1, 2, 3, 4] = Just [1, 2, 3]
init [] = Nothing

Running time: O(n) where n is the length of the array

index :: forall a. Array a -> Int -> Maybe a

This function provides a safe way to read a value at a particular index from an array.

sentence = ["Hello", "World", "!"]

index sentence 0 = Just "Hello"
index sentence 7 = Nothing

head :: forall a. Array a -> Maybe a

Get the first element in an array, or Nothing if the array is empty

Running time: O(1).

head [1, 2] = Just 1
head [] = Nothing

groupBy :: forall a. (a -> a -> Boolean) -> Array a -> Array (NonEmptyArray a)

Group equal, consecutive elements of an array into arrays, using the specified equivalence relation to determine equality.

groupBy (\a b -> odd a && odd b) [1, 3, 2, 4, 3, 3]
   = [NonEmptyArray [1, 3], NonEmptyArray [2], NonEmptyArray [4], NonEmptyArray [3, 3]]

groupAllBy :: forall a. (a -> a -> Ordering) -> Array a -> Array (NonEmptyArray a)

Group equal elements of an array into arrays, using the specified comparison function to determine equality.

groupAllBy (comparing Down) [1, 3, 2, 4, 3, 3]
   = [NonEmptyArray [4], NonEmptyArray [3, 3, 3], NonEmptyArray [2], NonEmptyArray [1]]

groupAll :: forall a. Ord a => Array a -> Array (NonEmptyArray a)

Group equal elements of an array into arrays.

groupAll [1, 1, 2, 2, 1] == [NonEmptyArray [1, 1, 1], NonEmptyArray [2, 2]]

group :: forall a. Eq a => Array a -> Array (NonEmptyArray a)

Group equal, consecutive elements of an array into arrays.

group [1, 1, 2, 2, 1] == [NonEmptyArray [1, 1], NonEmptyArray [2, 2], NonEmptyArray [1]]

fromFoldable :: forall f. Foldable f => f ~> Array

Convert a Foldable structure into an Array.

fromFoldable (Just 1) = [1]
fromFoldable (Nothing) = []

foldr :: forall a b. (a -> b -> b) -> b -> Array a -> b

foldl :: forall a b. (b -> a -> b) -> b -> Array a -> b

foldRecM :: forall m a b. MonadRec m => (b -> a -> m b) -> b -> Array a -> m b

foldMap :: forall a m. Monoid m => (a -> m) -> Array a -> m

foldM :: forall m a b. Monad m => (b -> a -> m b) -> b -> Array a -> m b

Perform a fold using a monadic step function.

foldM (\x y -> Just (x + y)) 0 [1, 4] = Just 5

fold :: forall m. Monoid m => Array m -> m

findMap :: forall a b. (a -> Maybe b) -> Array a -> Maybe b

Find the first element in a data structure which satisfies a predicate mapping.

findLastIndex :: forall a. (a -> Boolean) -> Array a -> Maybe Int

Find the last index for which a predicate holds.

findLastIndex (contains $ Pattern "b") ["a", "bb", "b", "d"] = Just 2
findLastIndex (contains $ Pattern "x") ["a", "bb", "b", "d"] = Nothing

findIndex :: forall a. (a -> Boolean) -> Array a -> Maybe Int

Find the first index for which a predicate holds.

findIndex (contains $ Pattern "b") ["a", "bb", "b", "d"] = Just 1
findIndex (contains $ Pattern "x") ["a", "bb", "b", "d"] = Nothing

find :: forall a. (a -> Boolean) -> Array a -> Maybe a

Find the first element for which a predicate holds.

find (contains $ Pattern "b") ["a", "bb", "b", "d"] = Just "bb"
find (contains $ Pattern "x") ["a", "bb", "b", "d"] = Nothing

filterA :: forall a f. Applicative f => (a -> f Boolean) -> Array a -> f (Array a)

Filter where the predicate returns a Boolean in some Applicative.

powerSet :: forall a. Array a -> Array (Array a)
powerSet = filterA (const [true, false])

filter :: forall a. (a -> Boolean) -> Array a -> Array a

Filter an array, keeping the elements which satisfy a predicate function, creating a new array.

filter (_ > 0) [-1, 4, -5, 7] = [4, 7]

elemLastIndex :: forall a. Eq a => a -> Array a -> Maybe Int

Find the index of the last element equal to the specified element.

elemLastIndex "a" ["a", "b", "a", "c"] = Just 2
elemLastIndex "Earth" ["Hello", "World", "!"] = Nothing

elemIndex :: forall a. Eq a => a -> Array a -> Maybe Int

Find the index of the first element equal to the specified element.

elemIndex "a" ["a", "b", "a", "c"] = Just 0
elemIndex "Earth" ["Hello", "World", "!"] = Nothing

elem :: forall a. Eq a => a -> Array a -> Boolean

Returns true if the array has the given element.

dropWhile :: forall a. (a -> Boolean) -> Array a -> Array a

Remove the longest initial subarray for which all element satisfy the specified predicate, creating a new array.

dropWhile (_ < 0) [-3, -1, 0, 4, -6] = [0, 4, -6]

dropEnd :: forall a. Int -> Array a -> Array a

Drop a number of elements from the end of an array, creating a new array.

letters = ["a", "b", "c", "d"]

dropEnd 2 letters = ["a", "b"]
dropEnd 10 letters = []

drop :: forall a. Int -> Array a -> Array a

Drop a number of elements from the start of an array, creating a new array.

letters = ["a", "b", "c", "d"]

drop 2 letters = ["c", "d"]
drop 10 letters = []

difference :: forall a. Eq a => Array a -> Array a -> Array a

Delete the first occurrence of each element in the second array from the first array, creating a new array.

difference [2, 1] [2, 3] = [1]

Running time: O(n*m), where n is the length of the first array, and m is the length of the second.

deleteBy :: forall a. (a -> a -> Boolean) -> a -> Array a -> Array a

Delete the first element of an array which matches the specified value, under the equivalence relation provided in the first argument, creating a new array.

mod3eq a b = a `mod` 3 == b `mod` 3
deleteBy mod3eq 6 [1, 3, 4, 3] = [1, 4, 3]

deleteAt :: forall a. Int -> Array a -> Maybe (Array a)

Delete the element at the specified index, creating a new array, or returning Nothing if the index is out of bounds.

deleteAt 0 ["Hello", "World"] = Just ["World"]
deleteAt 10 ["Hello", "World"] = Nothing

delete :: forall a. Eq a => a -> Array a -> Array a

Delete the first element of an array which is equal to the specified value, creating a new array.

delete 7 [1, 7, 3, 7] = [1, 3, 7]
delete 7 [1, 2, 3] = [1, 2, 3]

Running time: O(n)

cons :: forall a. a -> Array a -> Array a

Attaches an element to the front of an array, creating a new array.

cons 1 [2, 3, 4] = [1, 2, 3, 4]

Note, the running time of this function is O(n).

concatMap :: forall a b. (a -> Array b) -> Array a -> Array b

Apply a function to each element in an array, and flatten the results into a single, new array.

concatMap (split $ Pattern " ") ["Hello World", "other thing"]
   = ["Hello", "World", "other", "thing"]

concat :: forall a. Array (Array a) -> Array a

Flatten an array of arrays, creating a new array.

concat [[1, 2, 3], [], [4, 5, 6]] = [1, 2, 3, 4, 5, 6]

catMaybes :: forall a. Array (Maybe a) -> Array a

Filter an array of optional values, keeping only the elements which contain a value, creating a new array.

catMaybes [Nothing, Just 2, Nothing, Just 4] = [2, 4]

any :: forall a. (a -> Boolean) -> Array a -> Boolean

Returns true if at least one array element satisfies the given predicate, iterating the array only as necessary and stopping as soon as the predicate yields true.

any (_ > 0) [] = False
any (_ > 0) [-1, 0, 1] = True
any (_ > 0) [-1, -2, -3] = False

alterAt :: forall a. Int -> (a -> Maybe a) -> Array a -> Maybe (Array a)

Update or delete the element at the specified index by applying a function to the current value, returning a new array or Nothing if the index is out-of-bounds.

alterAt 1 (stripSuffix $ Pattern "!") ["Hello", "World!"]
   = Just ["Hello", "World"]

alterAt 1 (stripSuffix $ Pattern "!!!!!") ["Hello", "World!"]
   = Just ["Hello"]

alterAt 10 (stripSuffix $ Pattern "!") ["Hello", "World!"] = Nothing

all :: forall a. (a -> Boolean) -> Array a -> Boolean

Returns true if all the array elements satisfy the given predicate. iterating the array only as necessary and stopping as soon as the predicate yields false.

all (_ > 0) [] = True
all (_ > 0) [1, 2, 3] = True
all (_ > 0) [-1, -2, -3] = False

Operator alias for Data.Array.difference (non-associative / precedence 5)

Operator alias for Data.Array.cons (right-associative / precedence 6)

An infix alias for cons.

1 : [2, 3, 4] = [1, 2, 3, 4]

Note, the running time of this function is O(n).

Operator alias for Data.Array.range (non-associative / precedence 8)

An infix synonym for range.

2 .. 5 = [2, 3, 4, 5]

Operator alias for Data.Array.index (left-associative / precedence 8)

An infix version of index.

sentence = ["Hello", "World", "!"]

sentence !! 0 = Just "Hello"
sentence !! 7 = Nothing