RIO.List.Partial · rio-0.1.22.0

Basic functions

4 declarations

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

This is a partial function, it throws an error on empty lists. Use pattern matching, uncons or listToMaybe instead. Consider refactoring to use Data.List.NonEmpty.

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

To disable the warning about partiality put {-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-} at the top of the file. To disable it throughout a package put the same options into ghc-options section of Cabal file. To disable it in GHCi put :set -Wno-x-partial -Wno-unrecognised-warning-flags into ~/.ghci config file. See also the migration guide.

Examples

Example1 expression

>>> head [1, 2, 3]1

Example1 expression

>>> head [1..]1

Example1 expression

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

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

#

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

WARNING: This function is partial. Consider using unsnoc instead.

Examples

Example1 expression

>>> last [1, 2, 3]3

Example1 expression

>>> last [1..]* Hangs forever *

Example1 expression

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

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

#

This is a partial function, it throws an error on empty lists. Replace it with drop 1, or use pattern matching or uncons instead. Consider refactoring to use Data.List.NonEmpty.

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

To disable the warning about partiality put {-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-} at the top of the file. To disable it throughout a package put the same options into ghc-options section of Cabal file. To disable it in GHCi put :set -Wno-x-partial -Wno-unrecognised-warning-flags into ~/.ghci config file. See also the migration guide.

Examples

Example1 expression

>>> tail [1, 2, 3][2,3]

Example1 expression

>>> tail [1][]

Example1 expression

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

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

#

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

WARNING: This function is partial. Consider using unsnoc instead.

Examples

Example1 expression

>>> init [1, 2, 3][1,2]

Example1 expression

>>> init [1][]

Example1 expression

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

Reducing lists (folds)

3 declarations

methodfoldl1 :: (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:

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

valuefoldl1' :: HasCallStack => (a -> a -> a) -> [a] -> a

#

A strict version of foldl1.

methodfoldr1 :: (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:

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Example1 expression

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

Special folds

methodmaximum :: 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:

Example1 expression

>>> maximum [1..10]10

Example1 expression

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

Example1 expression

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

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

methodminimum :: 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:

Example1 expression

>>> minimum [1..10]1

Example1 expression

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

Example1 expression

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

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

valuemaximumBy :: 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:

Example1 expression

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

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

valueminimumBy :: 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:

Example1 expression

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

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

Building lists

0 declarations

Scans

valuescanl1 :: (a -> a -> a) -> [a] -> [a]

#

\mathcal{O}(n). scanl1 is a variant of scanl that has no starting value argument:

scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]

Examples

Example1 expression

>>> scanl1 (+) [1..4][1,3,6,10]

Example1 expression

>>> scanl1 (+) [][]

Example1 expression

>>> scanl1 (-) [1..4][1,-1,-4,-8]

Example1 expression

>>> scanl1 (&&) [True, False, True, True][True,False,False,False]

Example1 expression

>>> scanl1 (||) [False, False, True, True][False,False,True,True]

Example1 expression

>>> take 10 (scanl1 (+) [1..])[1,3,6,10,15,21,28,36,45,55]

Example1 expression

>>> take 1 (scanl1 undefined ('a' : undefined))"a"

valuescanr1 :: (a -> a -> a) -> [a] -> [a]

#

\mathcal{O}(n). scanr1 is a variant of scanr that has no starting value argument.

Examples

Example1 expression

>>> scanr1 (+) [1..4][10,9,7,4]

Example1 expression

>>> scanr1 (+) [][]

Example1 expression

>>> scanr1 (-) [1..4][-2,3,-1,4]

Example1 expression

>>> scanr1 (&&) [True, False, True, True][False,False,True,True]

Example1 expression

>>> scanr1 (||) [True, True, False, False][True,True,False,False]

Example1 expression

>>> force $ scanr1 (+) [1..]*** Exception: stack overflow

Indexing lists

1 declaration

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

#

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

WARNING: This function is partial, and should only be used if you are sure that the indexing will not fail. Otherwise, use !?.

WARNING: This function takes linear time in the index.

Examples

Example1 expression

>>> ['a', 'b', 'c'] !! 0'a'

Example1 expression

>>> ['a', 'b', 'c'] !! 2'c'

Example1 expression

>>> ['a', 'b', 'c'] !! 3*** Exception: Prelude.!!: index too large

Example1 expression

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