# Set default values while decoding JSON

**URL:** <https://discourse.elm-lang.org/t/set-default-values-while-decoding-json/6387>\
**Category:** Learn\
**Created:** [October 9, 2020, 9:06pm UTC](https://discourse.elm-lang.org/t/set-default-values-while-decoding-json/6387 "2020-10-09T21:06:45Z")\
**Posts on this page:** 1\
**Showing post:** 5

<div class="post-metadata">

**Author:** ![lydell](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.elm-lang.org/lydell/32/178_2.png) [@lydell](https://discourse.elm-lang.org/u/lydell)\
**Post date:** [October 10, 2020, 9:26am UTC](https://discourse.elm-lang.org/t/set-default-values-while-decoding-json/6387/5 "2020-10-10T09:26:21Z")

</div>

> I think I will go with the Dec.succeed, because it keeps the name of the type in the function

Just for full understanding, you can keep “the name of the type” in the function in the `map2` situation too:

```auto
decodeLocations : Decoder (List Location)
decodeLocations =
    Dec.field "locations"
        (Dec.list
            (Dec.map2 (\id name -> Location id name True)
                (Dec.field "id" Dec.int)
                (Dec.field "name" Dec.string)
            )
        )

```

The `map3` version can also be written that way:

```auto
decodeLocations : Decoder (List Location)
decodeLocations =
    Dec.field "locations"
        (Dec.list
            (Dec.map3 (\id name checked -> Location id name checked)
                (Dec.field "id" Dec.int)
                (Dec.field "name" Dec.string)
                (Dec.succeed True)
            )
        )

```

However, directly calling `Location` like that is usually frowned upon. It’s much more clear to just “build the record instead”: `{ id = id, name = name, checked = checked }`. But it’s good to know that when you make a `type alias Foo` for a record (yes, only for records!), you get not only a _type_ named `Foo`, Elm also auto-generates a _function_ called `Foo` that takes all the stuff in the record in order and builds such a record. It is that auto-generated function that you use in the body of `decodeLocations`, while the type with the same name is used in the `decodeLocations : Decoder (List Location)` type annotation.

Yet one way of doing it is exploiting the fact that the auto-generated function takes the record contents in a specific order. So you _could_ move `checked : Bool` _first_ in the record, then you could do (but I’m not saying it’s a good solution!):

```auto
type alias Location =
  { checked : Bool
  , name : String
  , id : Int
  }

decodeLocations : Decoder (List Location)
decodeLocations =
    Dec.field "locations"
        (Dec.list
            (Dec.map2 (Location True)
                (Dec.field "id" Dec.int)
                (Dec.field "name" Dec.string)
            )
        )

```

You don’t need to understand all of those details right now, but it might be handy some time! Either way, I like @Atlewee’s solution best too.

> Any suggestions on a good beginners tutorial on Json decoding?

This blog post was enlightening for me at least:

> **[Getting Unstuck with Elm JSON Decoders](https://thoughtbot.com/blog/getting-unstuck-with-elm-json-decoders)**
>
> Solving a Maybe problem is often easier than solving a Decode one.

---

_[View the full topic](https://discourse.elm-lang.org/t/set-default-values-while-decoding-json/6387)._