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

Elm math formula using functions , pipe and currrying

Simple math formula in elm using pipe and currying

import Html exposing (..)

main = view

view = div [][
  div[] [text "(a + b)2 = a2 + 2ab + b2"] 
  ,div[][text "LHS = RHS"]
  ,div[][text (calcuate 2 2)]
  ]    

-- (a + b)2 = a2 + 2ab + b2

calcuate x y = (left x y) ++ "=" ++ (right x y)

left x y = x |> add y |> sqrt |> toString 

right x y = (sqrt x |> add (2 |> multiply  (x |> multiply y))) |> add (sqrt y) |> toString 
--right x y =  toString (add (add (sqrt x) (multiply 2 (multiply x y))) (sqrt y))

multiply : Int -> Int -> Int 
multiply x y = x * y

add : Int -> Int -> Int
add x y = x + y

sqrt : Int -> Int 
sqrt x = x ^ 2

Pipe

Pipe is helpfull to reduce parentheses usage. They are aliases for function application

x |> multiplyWith10 |> divideBy10 

It is usefull when performing sequance of function call, first function result will be passed to second function and it continues to the until last function call reaches. (a + b)2 = a2 + 2ab + b2 This simple formula can be represented using functions in elm with the same way we do in other language ,but the only difference is the formula representation will not be as the same order as in mathamatical world.

           +  ( +  ( a2)   ( *       2 ( *       a b))) (b2)     
toString (add (add (sqrt x) (multiply 2 (multiply x y))) (sqrt y))

If we read this call, then toString of add of add of sqrt x and multiply with 2 of .....

If you are using pipe , it will be like

-- a2       +   2       *        a      *       b       +    b2
(sqrt x |> add (2 |> multiply  (x |> multiply y))) |> add (sqrt y) |> toString 

Currried out

x |> add y 

add function expect two parameter, that x and y

Using pipe we are currying add with y and passing x as the another parameter.

Elm basic snippet

Elm

If

If and else are something different here without curly brackets {}

if "10" == "10" then "Its ten" else "Not ten"

If Not equal has different symbol here,

if "10" /= "11" then "Its ten" else "Not ten"

String Util

 - toInt

If you want to parse String to Int , this is the right util. The below method will return True if the String is parsable to Int or else otherwise Generaly String.toInt will return Result String Int , Which holds the Value / Error. The below method (validateStringToInt) would be the sample to extract the result value

String.toInt "10"`
Ok 10 : Result.Result String Int

validateStringToInt x =   
  case (String.toInt x) of
    Ok value -> True
    Err msg -> False

String Contact

Use ++ instead of regular + operator to concat. Every thing value here (Immutable) when doing contact it will give your new value

a = "one " ++ "two"
"one two" : String
a ++ "end"
"one twoend" : String

Tuple

- Tuple

Tuples are simple structure which hold the data in a order to server.

(10,11)

Access Tuple

fist element of the typle

fst (10,11)

second element of the tuple

snd (10,11)

Lambda expression

Forward slash for the parameter and -> function body

(\x -> x * 2)

Below code is for maping list to double list

double list = List.map (\x -> x * 2) list
<function> : List number -> List number
double [2,3,4]
[4,6,8] : List number

In Short lambda can be written like for single element , that automaticaly taken as default parameter

double list = List.map ((*) 2) list

Elm-lang installation from Source

Recently I started to learn Elm-lang to get comfort in the functional world.

This post could be useful for people who try to install Elm-lang.

http://elm-lang.org/ site providing guide to use npm-installer , but this guide is some what different which try to install from the git source. Yes, It will download the raw source code and build the platform for you. It requires Haskel and cabal to make the Elm platfrom from the source.

sudo apt-get install ghc-7.10.1

sudo apt-get install cabal-install-1.22

cabal --version

ghc --version

which cabal

curl https://raw.githubusercontent.com/elm-lang/elm-platform/master/installers/BuildFromSource.hs > BuildFromSource.hs

runhaskell BuildFromSource.hs 0.17

export PATH=/home/ranjithrajd/rspace/software/elm/Elm-Platform/0.15.1/.cabal-sandbox/bin:$PATH

cd /home/ranjithrajd/rspace/tutorial/elm/project

Compile your elm code elm-make

Start the server elm-reactor

Install elm html module to the project elm package install evancz/elm-html