Elm: 99 questions

question1.elm

-- q1: myLast
import Html exposing (text)

myLast : List a -> Result String a
myLast list = 
    Result.fromMaybe ("Can't get head of empty.")
    <| List.head
    <| List.reverse list

main =
  case myLast [] of
    Ok list -> text (toString list)
    Err err -> text err

question2.elm

-- q2: myButLast 
import Html exposing (text)

myButLast : List a -> Result String a
myButLast aList = 
  case aList of
    [] -> Err "List is empty"
    [_] -> Err "List is too short"
    x -> Result.fromMaybe (toString x) 
         <| List.head
         <| List.drop ((List.length x) - 2) x

main =
  case myButLast [1, 3, 4] of
    Ok list -> text (toString list)
    Err err -> text err

question3.elm

-- q3: elementAt
import Html exposing (text)

elementAt : List a -> Int -> Result String a
elementAt aList i = 
  if i < 1 || i > List.length aList then
    Err "Invalid Index"
  else
    case i of
      1 -> Result.fromMaybe "Failed to get head" (List.head aList)
      _ -> elementAt (List.drop 1 aList) (i - 1)

main =
  case elementAt ["asdf", "qwer"] 2 of
    Ok element -> text <| toString element
    Err err -> text err

question4.elm

-- q4: myLength
import Html exposing (text)

myLength : List a -> Int
myLength aList =
  case aList of
    [] -> 0
    [x] -> 1
    x::xs -> 1 + myLength xs

main =
  text <| toString <| myLength [1,2,3,4,5]

question5.elm

-- q5: myReverse
import Html exposing (text)

myReverse : List a -> List a
myReverse aList = List.foldl (::) [] aList

main =
  text <| toString <| myReverse [1,2,3]

questions6.elm

-- q6: myReverse
import Html exposing (text)

isPalindrome  : List a -> Bool
isPalindrome aList = aList == List.reverse aList

main =
  text <| toString <| isPalindrome [1,2,3,2,1]