# Json2elm: generate pipeline and applicative decoders

**URL:** <https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374>\
**Category:** Show and Tell\
**Created:** [May 18, 2021, 7:00pm UTC](https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374 "2021-05-18T19:00:26Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![alexkorban](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/alexkorban/32/3242_2.png) [@alexkorban](https://discourse.elm-lang.org/u/alexkorban)\
**Post date:** [May 18, 2021, 7:00pm UTC](https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374/1 "2021-05-18T19:00:26Z")

</div>

I’ve added a few features to [json2elm](https://discourse.elm-lang.org/elm/json2elm).

In addition to plain `elm/json` decoders, it can now also generate applicative and pipeline decoders. It also generates the required imports based on the chosen decoder type. You can also choose between noun-based and verb-based function names for decoders and encoders.

Applicative decoders take advantage of the `andMap` function from `elm-community/json-extra` and look like this:

```elm
accountDecoder : Json.Decode.Decoder Account
accountDecoder = 
    Json.Decode.succeed Account
        |> Json.Decode.Extra.andMap (Json.Decode.field "id" Json.Decode.int)
        |> Json.Decode.Extra.andMap (Json.Decode.field "prefs" accountPrefsDecoder)
        |> Json.Decode.Extra.andMap (Json.Decode.field "user" accountUserDecoder)

```

Pipeline decoders rely on the `NoRedInk/elm-json-decode-pipeline` package and look like this:

```elm
accountDecoder : Json.Decode.Decoder Account
accountDecoder = 
    Json.Decode.succeed Account
        |> Json.Decode.Pipeline.required "id" Json.Decode.int
        |> Json.Decode.Pipeline.required "prefs" accountPrefsDecoder
        |> Json.Decode.Pipeline.required "user" accountUserDecoder

```

I’ve also added imports to the generated code, so for example for pipeline decoders you will get a preamble like this:

```elm
import Json.Decode
import Json.Encode
import Json.Decode.Pipeline

-- Required packages:
-- * elm/json
-- * NoRedInk/elm-json-decode-pipeline

```

Finally, there are two naming styles you can choose from for the functions: noun-based and verb-based. If you use nouns, you will get names like `accountDecoder` and `encodedAccount` versus `decodeAccount` and `encodeAccount` when using verbs. I am partial to noun function names for the reason that in functional programs, we are describing what the transformations of the data are, rather than how they are done.

---

<div class="post-metadata">

**Author:** ![francescortiz](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/francescortiz/32/1721_2.png) [@francescortiz](https://discourse.elm-lang.org/u/francescortiz)\
**Post date:** [May 19, 2021, 7:29am UTC](https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374/2 "2021-05-19T07:29:50Z")

</div>

Thanks for keeping this project alive, I think it is a good tool, specially when you start learning Elm. Personally, I name decoders `xxxxDecoder` and encoders `encodeXxxx` because in the case of the decoders you describe the work to be done but you need an additional decode call in order to get the work done, so you are actually creating the description of the work, AKA, decoder, while encoders are regular functions that transform an input into a `Value`.

---

<div class="post-metadata">

**Author:** ![alexkorban](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/alexkorban/32/3242_2.png) [@alexkorban](https://discourse.elm-lang.org/u/alexkorban)\
**Post date:** [May 19, 2021, 8:41am UTC](https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374/3 "2021-05-19T08:41:32Z")

</div>

I guess I’m unlikely to satisfy everyone’s preferences when it comes to naming functions.

I tend to view all pure functions as a description of what needs to be done, similar to decoders, so I prefer noun names for that reason, and also because otherwise I have to come up with largely pointless verb prefixes like “get” and “calc” and “generate”.

The nomenclature happens to be very confused. For example, why do we have `view` (noun) and `Browser.element` (noun) and `List.singleton` (noun) but also `List.map` (verb) and `List.filter` (verb)? I don’t see a rationale other than historical precedent.

---

<div class="post-metadata">

**Author:** ![system](https://canada1.discourse-cdn.com/flex035/uploads/elm_lang/original/1X/50a05e53677a2c3b47776d7abd0f113eb50193a1.png) [@system](https://discourse.elm-lang.org/u/system)\
**Post date:** [May 29, 2021, 8:42am UTC](https://discourse.elm-lang.org/t/json2elm-generate-pipeline-and-applicative-decoders/7374/4 "2021-05-29T08:42:23Z")

</div>

This topic was automatically closed 10 days after the last reply. New replies are no longer allowed.