Dict - Elm Datastructure

Elm Dict Package /elm-lang/core/1.0.0/Dict

The Dict api is useful when situation comes to store key , pair value.It is Similar to Map / Dictionary in some other language.

Here you can find some basic function to play with Dict Api.

Use elm rector to execute this samples.

Dict.insert

Kind of put operation to store key , value pair in Dict

import Html exposing (Html,text)
import Dict exposing (Dict)

main = text (toString (insertIntoDict "Apple" 1 Dict.empty))

insertIntoDict: String -> Int -> Dict String Int -> Dict String Int
insertIntoDict key input myMap = 
  let      
    newMap =  Dict.insert key input myMap      
  in  
    newMap

Repl

> import Dict
> Dict.insert "A" 1 Dict.empty
Dict.fromList [("A",1)] : Dict.Dict String number

Dict.insert function expection three parameter

1. Key
2. Value
3. Dict (This dict can be empty dict for the first time)

Dict.update

Update need a handler to check before performing the update action, That check function is Higher order function that too accpet Maybe type parameter.

import Html exposing (Html,text)
import Dict exposing (Dict)

main = text (toString  view)

view = 
  let
    appleDict = insertIntoDict "Apple" 1 Dict.empty
  in
    insertIntoDict "Apple" 2 appleDict

process : Int -> Maybe Int -> Maybe Int
process x y = case y of 
    Just val -> Just x
    Nothing -> Just 1

insertIntoDict: String -> Int -> Dict String Int ->  Dict String Int
insertIntoDict key input myMap= 
  let      
    newMap = case (Dict.get key myMap) of
      Nothing -> Dict.insert key input myMap
      Just val -> Dict.update key (process input) myMap
  in  
    newMap

Dict.update key (process input) myMap

Dict Update function expect second parameter to be a higer order function and That should have signature of

Maybe a -> Maybe a

Int this process function we are updating the newValue = 2 to the "Apple" key which already exist in Dict with oldValue = 1.

This process function acctualy expecting first paramter as Int which is new value that to be updated on Dict. This Int value curried out and applied as new value (clousure) to the higher order function. Find the process Signature

  process : Int -> Maybe Int -> Maybe Int

Dict.get

Get function to read the value based on the key from Dict

 import Html exposing (Html,text)
import Dict exposing (Dict)

main = text (toString view)

view = 
  let
    appleDict = insertIntoDict "Apple" 1 Dict.empty
    appleDict2 = insertIntoDict "Apple" 2 appleDict
  in    
    readFromDict "Apple" appleDict2

process : Int -> Maybe Int -> Maybe Int
process x y = case y of 
    Just val -> Just x
    Nothing -> Just 0

insertIntoDict: String -> Int -> Dict String Int ->  Dict String Int
insertIntoDict key input myMap= 
  let      
    newMap = case (Dict.get key myMap) of
      Nothing -> Dict.insert key input myMap
      Just val -> Dict.update key (process input) myMap
  in  
    newMap

readFromDict : String -> Dict String Int -> Int
readFromDict key map =   
  case (Dict.get key map) of
    Nothing -> 0
    Just v -> v

Dict.get function return the Maybe. Which need to be further processed to extract the value. This sample use the pattern matching (case) to process the Int value.

case (Dict.get key map) of
    Nothing -> 0
    Just v -> v