PairDict

Lookup value-pairs from the left or the right.

Let's compare

normal Dict

you want 🔑 1?

    ( 🔑 0, 🌳 )
  → ( 🔑 1, 🍐 )
    ( 🔑 2, 🍐 )

Going through while comparing your key... Ah! Here it is:

    🍐

PairDict

you want the left value of 🗝️ 1 and the right value of 🔑 0?

  → ( 🔑 0, 🗝️ 2 )
    ( 🔑 2, 🗝️ 0 )
    ( 🔑 1, 🗝️ 1 ) ←

Going through while checking, if your key is equal... Ah! Here they are:

    🔑 1 and 🗝️ 2

👍 How to PairDict

Example: cased letters

lowerUppercaseLetters=
  empty
  |>insert ( 'a', 'A' )
  |>insert ( 'b', 'B' )
  |>insert ( 'c', 'C' )

upperCase char=
  rightOf char lowerUppercaseLetters

try in the ellie for the example cased letters

Example: periodic table

type Element=
  Hydrogen
  | Helium

elementPair { element, atomicNumber }=
  ( element, protonCount )

elementAtomicNumberPairdict=
  empty
  |>insert ({ element= Hydrogen, atomicNumber= 1 } |>elementPair)
  |>insert ({ element= Helium, atomicNumber= 2 } |>elementPair)

atomicNumberByElement=
  toDict elementAtomicNumberPairdict

Example: brackets

You have pairs that belong together:

brackets=
  empty
  |>insert ( '(', ')' )
  |>insert ( '{', '}' )

typeChar character=
  case leftOf character brackets of
    Just opening->
      String.fromList [ opening, character ]

    Nothing->
      case rightOf character brackets of
        Just closing->
          String.fromList [ character, closing ]

        Nothing->
          String.fromChar character

"Typing (: " ++(typeChar '(') ++". Even }: " ++(typeChar '}')

👎 How not to PairDict

Example: automatic answers

answers=
  fromList
    [ ( "Hi", "Hi there!" )
    , ( "Bye", "Ok, have a nice day and spread some love.")
    , ( "How are you", "I don't have feelings :(" )
    , ( "Are you a robot", "I think the most human answer is 'Haha... yes'" )
    ]

please use a Dict where it is more appropriate: Dicts are for one-way access

Example: translation, synonymes...

englishGerman=
  fromList
    [ ( "elm", "Ulme" )
    , ( "git", "Schwachkopf" )
    ]

A right → left and backwards relationship is only fitting, when left or right don't have multiple translations.

Please take a look at elm-bidict

Example: partners, opposites...

partners=
  empty
  |>insert ( "Ann", "Alan" )
  |>insert ( "Alex", "Alastair" )
  |>insert ( "Alan", "Ann" ) --wait, this is no duplicate and gets inserted?

A PairDict ony makes sense, when the left & right sides describe something different.