Added insertAt, rewrote splitAt to use new helper

src/List/Extra.elm

@@ -23,6 +23,7 @@ module List.Extra
  , updateIf
  , updateAt
  , updateIfIndex
+ , insertAt
  , singleton
  , removeAt
  , filterNot
@@ -81,7 +82,7 @@ module List.Extra
 {-| Convenience functions for working with List
 
 # Basics
-@docs last, init, getAt, (!!), uncons, maximumBy, minimumBy, andMap, andThen, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, replaceIf, setAt, remove, updateIf, updateAt, updateIfIndex, singleton, removeAt, filterNot, swapAt, stableSortWith
+@docs last, init, getAt, (!!), uncons, maximumBy, minimumBy, andMap, andThen, takeWhile, dropWhile, unique, uniqueBy, allDifferent, allDifferentBy, replaceIf, setAt, remove, updateIf, updateAt, updateIfIndex, insertAt, singleton, removeAt, filterNot, swapAt, stableSortWith
 
 # List transformations
 @docs intercalate, transpose, subsequences, permutations, interweave
@@ -487,6 +488,15 @@ updateIfIndex predicate update list =
  list
 
 
+{-| Insert an element at given index
+-}
+insertAt : Int -> a -> List a -> List a
+insertAt index value list =
+ splitAtWithReversedInit index [] list
+ |> Tuple.mapSecond ((::) value)
+ |> uncurry appendUsingReversedInit
+
+
 {-| Remove the first occurrence of a value from a list.
 -}
 remove : a -> List a -> List a
@@ -886,7 +896,8 @@ unfoldr f seed =
 -}
 splitAt : Int -> List a -> ( List a, List a )
 splitAt n xs =
- ( take n xs, drop n xs )
+ splitAtWithReversedInit n [] xs
+ |> Tuple.mapFirst List.reverse
 
 
 {-| Take elements from the right, while predicate still holds.
@@ -1271,3 +1282,30 @@ greedyGroupsOfWithStep size step xs =
  group :: greedyGroupsOfWithStep size step xs_
  else
  []
+
+
+
+-- Helpers
+
+
+splitAtWithReversedInit : Int -> List a -> List a -> ( List a, List a )
+splitAtWithReversedInit n reversedInit tail =
+ if n <= 0 then
+ ( reversedInit, tail )
+ else
+ case tail of
+ head :: nextTail ->
+ splitAtWithReversedInit (n - 1) (head :: reversedInit) nextTail
+
+ [] ->
+ ( reversedInit, tail )
+
+
+appendUsingReversedInit : List a -> List a -> List a
+appendUsingReversedInit reversedInit tail =
+ case reversedInit of
+ head :: remainingReversedInit ->
+ appendUsingReversedInit remainingReversedInit (head :: tail)
+
+ [] ->
+ tail

tests/Tests.elm

@@ -2,8 +2,9 @@ module Tests exposing (..)
 
 import Test exposing (..)
 import Expect
-import Fuzz exposing (list, int, tuple3)
+import Fuzz exposing (char, list, int, intRange, tuple3)
 import List exposing (map, range)
+import Random
 import Tuple exposing (first)
 import List.Extra exposing (..)
 
@@ -89,6 +90,32 @@ all =
  \() ->
  Expect.equal (findIndices (\x -> x % 2 == 0) [ 1, 2, 4 ]) [ 1, 2 ]
  ]
+ , describe "insertAt" <|
+ [ fuzz
+ (list char |> Fuzz.andThen (\xs -> tuple3 ( intRange 0 (List.length xs), char, Fuzz.constant xs )))
+ "0 <= index <= length"
+ (\( n, x, xs ) ->
+ Expect.equal
+ (List.take n xs ++ (x :: List.drop n xs))
+ (insertAt n x xs)
+ )
+ , fuzz
+ (tuple3 ( intRange Random.minInt -1, char, list char ))
+ "index < 0"
+ (\( n, x, xs ) ->
+ Expect.equal
+ (x :: xs)
+ (insertAt n x xs)
+ )
+ , fuzz
+ (list char |> Fuzz.andThen (\xs -> tuple3 ( intRange (List.length xs + 1) Random.maxInt, char, Fuzz.constant xs )))
+ "index > length"
+ (\( n, x, xs ) ->
+ Expect.equal
+ (xs ++ [ x ])
+ (insertAt n x xs)
+ )
+ ]
  , describe "intercalate" <|
  [ test "computes example" <|
  \() ->
@@ -370,7 +397,8 @@ all =
  \listA listB ->
  not (List.Extra.isPrefixOf listA listB)
  || not (List.Extra.isPrefixOf listB listA)
- || listA == listB
+ || listA
+ == listB
  |> Expect.true "Expected exactly one to be prefix of the other."
  , fuzz3 (list int) (list int) (list int) "transitivity" <|
  \listA listB listC ->
@@ -392,7 +420,8 @@ all =
  \listA listB ->
  not (List.Extra.isSuffixOf listA listB)
  || not (List.Extra.isSuffixOf listB listA)
- || listA == listB
+ || listA
+ == listB
  |> Expect.true "Expected exactly one to be suffix of the other."
  , fuzz3 (list int) (list int) (list int) "transitivity" <|
  \listA listB listC ->