Explain Monad using Elm - WIP

Explain Monad using Elm.md

Explain Monad using Elm

Try to guess what will the code display before before clicking Details.

Inspired by https://janiczek.github.io/silent-teacher/.

text

main =
    Html.text "Hello"

Details

Displayed: Hello

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toString

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (Debug.toString maybeHead)

Details

Displayed: Just "one"

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (Debug.toString maybeHead)

Details

Displayed: Nothing

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case

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (case maybeHead of
            Just head ->
                head

            Nothing ->
                ""
        )

Details

Displayed: one

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (case maybeHead of
            Just head ->
                head

            Nothing ->
                ""
        )

Details

Displayed:

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withDefault

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "" maybeHead)

Details

Displayed: one

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "" maybeHead)

Details

Displayed:

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "Empty list" maybeHead)

Details

Displayed: Empty list

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map

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "Empty list" (Maybe.map (\head -> "Head: " ++ head) maybeHead))

Details

Displayed: Head: one

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "Empty list" (Maybe.map (\head -> "Head: " ++ head) maybeHead))

Details

Displayed: Empty list

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list =
    []


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "Empty list" (Maybe.map (\head -> "Head: " ++ head) maybeHead))

Details

Displayed: Empty list

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andThen

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.withDefault "Empty list" (Maybe.andThen (\head -> Just ("Head: " ++ head)) maybeHead))

Details

Displayed: Head: one

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|>

list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (Maybe.map (\head -> "Head: " ++ head) maybeHead |> Maybe.withDefault "Empty list")

Details

Displayed: Head: one

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list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    Html.text
        (maybeHead |> Maybe.map (\head -> "Head: " ++ head) |> Maybe.withDefault "Empty list")

Details

Displayed: Head: one

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list =
    [ "one", "two" ]


maybeHead =
    List.head list


main =
    maybeHead |> Maybe.map (\head -> "Head: " ++ head) |> Maybe.withDefault "Empty list" |> Html.text

Details

Displayed: Head: one

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