learning ELM Type System

constructor-list.elm

import Html exposing (text)
{-
type List a
 = Nil  -- []
  | Cons a (List a) -- ::
-} 
l1 = []
l2 = 1 :: []
l3 = 1 :: (2 :: [])
l4 = [1,2]
  
main =
   l3 == l4
   |> toString
   |> text

record-pipe-operator.elm

import Html exposing (text)
type alias Person = 
 {
   name:String, age: Int, occupation : String
 }
jem : Person
jem = {name="Jem", age=44, occupation= "Analyst"}
bread = {name="Bread", category ="carbs"}

promote: Person -> Person
promote p =
  {p | occupation = "Manager"}

main =
  --text (toString(promote jem)) --{ name = "Jem", age = 44, occupation = "Manager" }
   jem |> promote |> toString |> text --{ name = "Jem", age = 44, occupation = "Manager" }
   
   
   
----------
import Html exposing (text)
type alias Person = 
 {
   name:String, age: Int, occupation : String, dog: Dog, salary : Float
 }
 
type alias Dog = 
 {
   name:String, breed: String, sterilized : Bool
 }
 
joe : Person
joe = {name="Joe", age=44, occupation= "Analyst" , dog = fido, salary=10000}

fido: Dog
fido = {name="Fido", breed ="Husky", sterilized = False}

promote: Person -> Person
promote p =
  {p | occupation = "Manager" 
  , salary = p.salary*1.2}
  
sterilizePet : Person -> Person
sterilizePet p =
  let 
    dog = p.dog
  in
  {p | dog = {dog | sterilized = True}}

main =
  joe |> promote |> sterilizePet |> toString |> text

record-type-alias.elm

-- below greet function can take both bread or person
import Html exposing (text)

ram = {name="Ram", age=44}
bread = {name="Bread", category ="carbs"}

greet r = "Hello " ++ r.name

main =
  text (greet bread)
  
-----------------
import Html exposing (text)
type alias Person = 
 {
   name:String, age: Int
 }
ram : Person
ram = {name="Ram", age=44}
bread = {name="Bread", category ="carbs"}

greet : Person -> String
greet r = "Hello " ++ r.name

main =
  text (greet bread)

records.elm

-- Records in elm is just like javascript objects.
--http://elmrepl.cuberoot.in/  REPL
--http://elm-lang.org/examples/hello-html    example

ram = { name = "Ram", age = 44}

ram.name --output "Ram":String

--field accessor to fetch property value.
.name ram --output "Ram"
.age ram --output 40

sita = {name="Sita",age=30}

l1 = [ram, sita]
--[{ name = "Ram", age = 44 },{ name = "Sita", age = 43 }]

single-constructor.elm

import Html exposing (text, h1)
type EmailAddress = EmailAddress String  --union type
type alias Message = 
   {
     recipient : EmailAddress
     , body : String
   }
message : Message
message = 
  {
    recipient= EmailAddress "hello"
    , body="hello"
   }
   
validateAddress : String -> Result String EmailAddress
validateAddress s =
  if String.contains "@" s
  then Result.Ok (EmailAddress s)
  else Result.Err "Not a valid email address"

data : String
data = "[email protected]"

emailToString : EmailAddress ->String
emailToString (EmailAddress s) = s

main = 
    let content =
      data
      |> validateAddress
      |> Result.map emailToString
      |> Result.withDefault "invalid"
      
    in h1 [] [text content]

tuple.elm

import Html exposing (div,text,Html)
import Html.Attributes exposing (style)


type alias Rectangle = (Int, Int)

rect : Rectangle 
rect = (200,50)

render : Rectangle -> Html a
render (w,h) = 
      div
        [
          style 
            [("backgroundColor", "red"),
            ("width", (toString w)++"px"),
            ("height", (toString h)++"px")]
        ]
        [text "Hello,world"]
        
main = render rect

union-type.elm

import Html exposing (text)

data : Rating

data = Excellent 
 -- Excellent
 -- data  = Bad -- compile error

main = text (toString data)

type Rating = Excellent
            | Good
            | Poor
            
            
--------------------pattern matching
import Html exposing (text)

data : Rating

data = Other "Appalling" 
 -- Excellent
 -- data  = Bad -- compile error

main = text (render data)

type Rating = Excellent
            | Good
            | Poor
            | Other String
            
render : Rating -> String
render rating =
   case rating of 
     Other s -> s
     _ -> toString rating
     
------------