Universum.Unsafe

Unsafe functions to work with lists and Maybe. Sometimes unavoidable but better don't use them. This module is intended to be imported qualified and it's not even included in default prelude exports.

import qualified Universum.Unsafe as Unsafe

foo :: [a] -> a
foo = Unsafe.head

Documentation

head :: HasCallStack => [a] -> a

\(\mathcal{O}(1)\). Extract the first element of a list, which must be non-empty.

>>> head [1, 2, 3]
1
>>> head [1..]
1
>>> head []
*** Exception: Prelude.head: empty list

WARNING: This function is partial. You can use case-matching, uncons or listToMaybe instead.

tail :: HasCallStack => [a] -> [a]

\(\mathcal{O}(1)\). Extract the elements after the head of a list, which must be non-empty.

>>> tail [1, 2, 3]
[2,3]
>>> tail [1]
[]
>>> tail []
*** Exception: Prelude.tail: empty list

WARNING: This function is partial. You can use case-matching or uncons instead.

init :: HasCallStack => [a] -> [a]

\(\mathcal{O}(n)\). Return all the elements of a list except the last one. The list must be non-empty.

>>> init [1, 2, 3]
[1,2]
>>> init [1]
[]
>>> init []
*** Exception: Prelude.init: empty list

WARNING: This function is partial. You can use reverse with case-matching or uncons instead.

last :: HasCallStack => [a] -> a

\(\mathcal{O}(n)\). Extract the last element of a list, which must be finite and non-empty.

>>> last [1, 2, 3]
3
>>> last [1..]
* Hangs forever *
>>> last []
*** Exception: Prelude.last: empty list

WARNING: This function is partial. You can use reverse with case-matching, uncons or listToMaybe instead.

at :: Int -> [a] -> a

Similar to !! but with flipped arguments.

(!!) :: HasCallStack => [a] -> Int -> a infixl 9

List index (subscript) operator, starting from 0. It is an instance of the more general genericIndex, which takes an index of any integral type.

>>> ['a', 'b', 'c'] !! 0
'a'
>>> ['a', 'b', 'c'] !! 2
'c'
>>> ['a', 'b', 'c'] !! 3
*** Exception: Prelude.!!: index too large
>>> ['a', 'b', 'c'] !! (-1)
*** Exception: Prelude.!!: negative index

WARNING: This function is partial. You can use atMay instead.

fromJust :: HasCallStack => Maybe a -> a

The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing.

Examples

Basic usage:

>>> fromJust (Just 1)
1

>>> 2 * (fromJust (Just 10))
20

>>> 2 * (fromJust Nothing)
*** Exception: Maybe.fromJust: Nothing
...

WARNING: This function is partial. You can use case-matching instead.

foldr1 :: Foldable t => (a -> a -> a) -> t a -> a

A variant of foldr that has no base case, and thus may only be applied to non-empty structures.

This function is non-total and will raise a runtime exception if the structure happens to be empty.

Examples

Basic usage:

>>> foldr1 (+) [1..4]
10

>>> foldr1 (+) []
Exception: Prelude.foldr1: empty list

>>> foldr1 (+) Nothing
*** Exception: foldr1: empty structure

>>> foldr1 (-) [1..4]
-2

>>> foldr1 (&&) [True, False, True, True]
False

>>> foldr1 (||) [False, False, True, True]
True

>>> foldr1 (+) [1..]
* Hangs forever *

foldl1 :: Foldable t => (a -> a -> a) -> t a -> a

A variant of foldl that has no base case, and thus may only be applied to non-empty structures.

This function is non-total and will raise a runtime exception if the structure happens to be empty.

foldl1 f = foldl1 f . toList

Examples

Basic usage:

>>> foldl1 (+) [1..4]
10

>>> foldl1 (+) []
*** Exception: Prelude.foldl1: empty list

>>> foldl1 (+) Nothing
*** Exception: foldl1: empty structure

>>> foldl1 (-) [1..4]
-8

>>> foldl1 (&&) [True, False, True, True]
False

>>> foldl1 (||) [False, False, True, True]
True

>>> foldl1 (+) [1..]
* Hangs forever *

minimum :: (Foldable t, Ord a) => t a -> a

The least element of a non-empty structure.

This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the minimum in faster than linear time.

Examples

Basic usage:

>>> minimum [1..10]
1

>>> minimum []
*** Exception: Prelude.minimum: empty list

>>> minimum Nothing
*** Exception: minimum: empty structure

WARNING: This function is partial for possibly-empty structures like lists.

Since: base-4.8.0.0

maximum :: (Foldable t, Ord a) => t a -> a

The largest element of a non-empty structure.

This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the maximum in faster than linear time.

Examples

Basic usage:

>>> maximum [1..10]
10

>>> maximum []
*** Exception: Prelude.maximum: empty list

>>> maximum Nothing
*** Exception: maximum: empty structure

WARNING: This function is partial for possibly-empty structures like lists.

Since: base-4.8.0.0

minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a

The least element of a non-empty structure with respect to the given comparison function.

Examples

Basic usage:

>>> minimumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]
"!"

WARNING: This function is partial for possibly-empty structures like lists.

maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a

The largest element of a non-empty structure with respect to the given comparison function.

Examples

Basic usage:

>>> maximumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]
"Longest"

WARNING: This function is partial for possibly-empty structures like lists.