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Keeping decoders simple

Jasper Woudenberg on December 06, 2018

It can take a bit of time to wrap your head around how JSON decoders work. Especially functions like Json.Decode.andThen take a bit of practice to ...
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Tony Gu • • Edited

Great write up! Thank you very much for sharing your experience with us.

I cached two errors when trying your code in Ellie.

  1. The individualDecoder should be like this: 
individualDecoder : Decoder Individual
individualDecoder =
    Decode.map4
        (\id species name owner -> Individual id species name owner)
        (Decode.field "id" Decode.int)
        (Decode.field "species" Decode.string)
        (Decode.field "name" Decode.string)
        (Decode.maybe <| Decode.field "owner" Decode.int)
  1. The addIfPet function should be like this: 
addIfPet : Individual -> Dict Int (List Pet) -> Dict Int (List Pet)
addIfPet individual petsCaredFor =
    case pet individual of
        Nothing ->
            petsCaredFor

        Just newPet ->
            case individual.owner of
                Just ownerid ->
                    Dict.update ownerid (Just << addOrInit newPet) petsCaredFor

                _ ->
                    petsCaredFor

I made the change accordingly in my Ellie and print some debug info on screen to visualize the decoding and converting process

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Jasper Woudenberg •

Amazing, thank you! Would you be okay if I linked your Ellie instead of mine from the post? I'll attribute it to you of course!

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digitalsatori profile image
Tony Gu •

Sure, That's totally fine. I'm glad it helped.

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Piotr Małecki-Jurek •

Hi, the article is great, and explains so much.
Did you thought about replacing

Decode.map4
    (\id species name owner -> Individual id species name owner)
    ...

with

Decode.map4
    Individual
    ...

I think it would be nice to just stress that out. It clears the function a bit more.

Cheers :)

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Jasper Woudenberg •

Hey, thanks for reading! Yeah, your way is what I usually do in code I write. I was a bit on the fence about the style to use in this post, and ended up deciding for the lambda because I thought that might be more accessible, although that is entirely debatable :).