Implementation of partitionBy function in Elm.

PartitionBy.elm

import Html exposing ( text, div, span )
import Html.Attributes exposing ( .. )
import Maybe exposing ( withDefault )
import String
import Array exposing ( Array )

main =  
  partitionBy String.length ["aa", "a", "b", "b", "cc", "cc", "a", "ab", "bb"]
  |> toString
  |> text


example = 
  Array.fromList
    [ Array.fromList ["a1", "b2"]
    , Array.fromList ["a2", "b2"]
    , Array.fromList ["a3", "b3"]
    ]


nestArray2: a -> Array (Array a)
nestArray2 x = 
  Array.fromList 
    [ Array.fromList 
        [x] 
    ]
    

partitionBy: (a -> b) -> List a -> List (List a)
partitionBy f list = 
  case list of
    [] -> 
      []
    (h::t) -> 
      List.foldl 
        (partitionHelper f)
        (nestArray2 h, f h)
        t
      |> fst
      |> Array.toList
      |> List.map Array.toList
      
      
partitionHelper: (a -> b) -> a -> (Array (Array a), b) -> (Array (Array a), b)
partitionHelper f value (arrays, prior) = 
  let fx = f value
  in 
    if fx == prior
      then (appendToLastSubarray value arrays, fx)
      else (Array.push (Array.fromList [ value ]) arrays, fx)
      

appendToLastSubarray: a -> Array (Array a) -> Array (Array a)
appendToLastSubarray value arrays = 
  let (firstArrays, lastArray') = splitPop arrays
      lastArray = lastArray' |> Maybe.withDefault Array.empty
  in 
    Array.push (Array.push value lastArray) firstArrays
  

splitPop: Array a -> (Array a, Maybe a)
splitPop array = 
  let last = Array.length array - 1
  in 
    ( Array.slice 0 -1 array
    , Array.get last array
    )