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Homework Week 3

maybeToList

maybeToList : Maybe a -> List a
maybeToList x = Debug.todo ""

> maybeToList (Just 3)
[3]
> maybeToList Nothing
[]

updateList

Change or remove element if it matches the shouldChange test.

updateList : (a -> Bool) -> (a -> Maybe a) -> List a -> List a
updateList shouldChange f xs = Debug.todo ""

> updateList (\x -> x == 3) (\v -> Just (v + 1)) [1,3,5]
[1,4,5] : List number
> updateList (\x -> x == 3) (\v -> Nothing) [1,3,5]
[1,5] : List number

find

find : (a -> Bool) -> List a -> Maybe a
find f xs = Debug.todo ""

> find (\x -> x == 2) [1,3,5,2]
Just 2 : Maybe number
> find (\x -> x == 2) [1,3,5]
Nothing : Maybe number

Implement updateListKv

updateListKv :
  List (k, v)
  -> k
  -> (v -> Maybe v)
  -> List (k, v)
updateListKv old k f = Debug.todo ""

> updateListKv [("foo", 1), ("bar", 2)] "foo" (\x -> Just (x + 1))
[("foo", 2), ("bar", 2)]
> updateListKv [("foo", 1), ("bar", 2)] "foo" (\x -> Nothing)
[("bar", 2)]

keepOks

keepOks : List (Result a b) -> List b
keepOks xs = Debug.todo ""

> keepOks [Ok 1, Err "bad", Ok 2]
[1,2] : List number

mapOk

mapOk : (b -> c) -> Result a b -> Result a c
mapOk f res = Debug.todo ""

> mapOk (\x -> x + 1) (Ok 2)
Ok 3 : Result a number
> mapOk (\x -> x + 1) (Err "str")
Err "str" : Result String number

either

either : (a -> c) -> (b -> c) -> Result a b -> c
either fa fb res = Debug.todo

> either (\x -> x + 1) (\x -> x - 1) (Ok 1)
0 : number
> either (\x -> x + 1) (\x -> x - 1) (Err 1)
2 : number

Implement parseDate

Do elm install justinmimbs/date to get the package. Use Date.fromIsoString.

import Date exposing (Date)

parseDate : Maybe String -> Maybe Date
parseDate v =
    Debug.todo ""

Timer

Implement "Timer" from http://eugenkiss.github.io/7guis/tasks/

Some additional notes:

  • see html at https://guide.elm-lang.org/interop/ used together with elm make --output=out/YourApp.js to compile to JS and be able to include CSS like bootstrap in your timer app

  • use the Time module from elm/time package to get current time and subscribe to periodic updates https://package.elm-lang.org/packages/elm/time/1.0.0/Time

  • use Task.perform function to convert a Task into a command. For example:

    import Time

type Msg
  ...
  | GotInitialTime Time.Posix

update : Msg -> Model -> ( Model, Cmd Msg )
update =
  ...
  ( model
  , Task.perform GotInitialTime Time.now
  )
  ...

    See https://package.elm-lang.org/packages/elm/core/latest/Task

  • use subscription mechanism to subscribe to periodic time changes. For example:

    ...

type Msg =
  ...
  | TimeUpdate Time.Posix

...

subscriptions : Model -> Sub Msg
subscriptions model =
    Time.every 200.0 TimeUpdate

main : Program () Model Msg
main =
    Browser.element
        { init = init
        , view = view
        , update = update
        , subscriptions = subscriptions
        }

(optional) Graceful Labeling

Graceful Labeling from https://johncrane.gitbooks.io/ninety-nine-elm-problems/content/p/p92.html