[Elm] Functions in Elm

Functions are an important building block in Elm. In this lesson we will review stateless functions, function composition, anonymous functions, Currying, and more.

Write Pure function:

import Htrml exposing (text)

main = 
    view "Hello World"

view message = 
    text message

We created a function call 'view', and pass the value ("Hello World") to 'message' param.

And inside view function, we output message value.

Add type:

import Htrml exposing (text)

main = 
    view "Hello World"

view: String -> Html msg
view message = 
    text message

For view function, we add ':String' to say the input value should be a string type.

If we change type to "Int":

It will output the error message on the screen.

Function execute from inside to outside:

import Html exposing (Html, text)
import String

main = 
 view "Hello World!!!"

view: String -> Html msg
view message =
 text (String.repeat 3((String.toUpper message)))

Output:

HELLO WORLD!!!HELLO WORLD!!!HELLO WORLD!!!

Forward:

import Html exposing (Html, text)
import String

main = 
 view "Hello World!!!"

view: String -> Html msg
view message =
 --text (String.repeat 3((String.toUpper message)))
 message
  |> String.toUpper
  |> String.repeat 3
  |> text

'|>' fowards symbol, so the message will go thought 'toUpper' --> 'repeat 3' --> text.

The 'repeat 3': Since we only provide the first value, it returns a function instead of a value. When two upper is evaluated, the message is being passed in. Finally, a value is returned and passed to the next line. This is known as currying. 

Then later, we can provide the rest of the parameters. This really helps us to build new functions from others.

For example, let's replace repeat with a function of our own creation called triple.

import Html exposing (Html, text)
import String

main = 
 view "Hello World!!!"

triple: String -> String
triple =
 -- repeat : Int --> String -> String  repeat function take int and string param and return string
 String.repeat 3

view: String -> Html msg
view message =
 --text (String.repeat 3((String.toUpper message)))
 message
  |> String.toUpper
  |> triple
  |> text

Let's add an anonymous function to add a comma and a space between each repetition. Anonymous functions begin with a back slash. 

We define str to take the value being passed in from the previous line. We use the double plus or string concatenation operator to add a comma and a space.

import Html exposing (Html, text)
import String

main =
 view "Hello World"

triple: String -> String
triple =
 -- repeat : Int --> String -> String  repeat function take int and string param and return string
 String.repeat 3

view: String -> Html msg
view message =
 --text (String.repeat 3((String.toUpper message)))
 message
  |> String.toUpper
  |> str -> str ++ ", "
  |> triple
  |> text