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Studying Elm

Studying Elm

I'm finding it pretty difficult to learn, so I'm writing a blog post.

The example

I am going to study on the zip-codes example. I hope this will allow me (and hopefully someone else) to understand the concepts. At the bottom of this post you can find all the resurces I've used to find the needed information.

The code

I am going to post small chunks of code and comment them, I cannot guarantee though that the explanation is correct (or that I've explained everything), I can guarantee though that I made all efforts possible to do so.

Feel free to open issues or fork to point out / fix errors.


Three different ways of importing a module:

import Char
-- is importing Char as itself so you can write Char.isDigit
import Html exposing (..)
{ -
is adding all functions in Html in the current scope so you can
write div [] [ text "oh hai" ]
import Html.Attributes as Attr exposing (..)
import Html.Events exposing (..)
import Http
import Json.Decode as Json exposing ((:=))
is allowing to call Json.Decode as Json, the last bit exposes := from
Json, you need ( and ) because it's a symbol
import String
import Task exposing (..)
  • := is explained here.


-- Result abstracts an operation that could succeed or fail. It is defined as
type Result error value
    = Ok value
    | Err error

#### View

The error value will be a single message, while the
succeed value will be a list of strings.
view : String -> Result String (List String) -> Html
view string result =
  let field =
          [ placeholder "Zip Code"
          , value string
          , on "input" targetValue (Signal.message query.address)
          , myStyle

      messages =
        case result of
          Err msg ->
              [ div [ myStyle ] [ text msg ] ]

          Ok cities ->
     (\city -> div [ myStyle ] [ text city ]) cities
      div [] (field :: messages)


myStyle returns an Attribute which is formed by an array of tuples
representing CSS styles
myStyle : Attribute
myStyle =
    [ ("width", "100%")
    , ("height", "40px")
    , ("padding", "10px 0")
    , ("font-size", "2em")
    , ("text-align", "center")


main gets run when the program starts, whenver the value of query.signal or results.signal changes they are mapped to view using Signal.map2, which produces the HTML that is ultimately shown on the page.

main =
  Signal.map2 view query.signal results.signal

Query Signal

Whenever the value represented by either query.signal or results.signal changes it is mapped to
view, using Signal.map2
query : Signal.Mailbox String
query =
  Signal.mailbox ""

#### Results Signal

results is a Mailbox that contains a Result, which will be a String in case of error
or a List String in case of success.
results : Signal.Mailbox (Result String (List String))
results =
  Signal.mailbox (Err "A valid US zip code is 5 numbers.")

Explanations of what a Mailbox is:

#### Port

Ports let you communicate with JavaScript.

port requests : Signal (Task x ())
{- in this case we are communicating *to* JavaScript. -}
port requests = lookupZipCode query.signal
  produces a Signal of the Tasks that interrogate the server, whenever query.signal
  changes lookupZipCode gets called
    |> (\task -> Task.toResult task `andThen` Signal.send results.address)
    |> is the forward function application, you could use it as follows:

    double a =
      a * 2

    add a b =
      a + b

    six =
      add 2 1
        |> double

    instead of double (add 2 1), you can see how it's useful with lots of function calls.

    Task.toResult ensures that the task will never fail by joining the error and the
    result of the HTTP query (what the docs above call x and a)
    Result will then be sent to results.address (remember that results is the mailbox)

#### HTTP call

lookupZipCode : String -> Task String (List String)
lookupZipCode will return                   ^ List of String, town names
                                  ^ Error of type String

So basically it performs the HTTP call returning a result.
lookupZipCode query =
  let toUrl =
        if String.length query == 5 && String.all Char.isDigit query
          then succeed ("" ++ query)
          -- succeed is part of Task, succeeds when run
          else fail "Give me a valid US zip code!"
          -- fail is part of Task, fails when run
      toUrl `andThen` (mapError (always "Not found :(") << Http.get places)

JSON Decoder

Json.Decode turns JSON values into Elm values.

places : Json.Decoder (List String)
places =
  let place =
        Json.object2 (\city state -> city ++ ", " ++ state)
        -- extracts the following two fields from the object
          ("place name" := Json.string)
          ("state" := Json.string)
      "places" := Json.list place
      -- := applies the Decoder to "places"


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