Create Custom JSON Decoders in Elm 0.18
You’ve modeled your data exactly how it should be, and everything’s working fine. Now it’s time to finish your JSON Decoder, but certain fields are strings where in your Elm code they’re complex data types! This happens most often with dates, but tagged unions have this problem too.
In 0.17 we had
customDecoder, which could turn any
Result String a into a
Decoder a, but it went away in 0.18.
So… what do we do?
Rolling Our Own Custom Decoder
What we need to do, essentially, is create a function that converts some input data to a
Let’s use date decoding as an example.
JSON doesn’t have a way to express dates, so we have to encode dates in strings.
For example, today’s date would be:
That’s ISO8601 format, designed to be unambiguous.
But we have to put it in a string.
Good thing we have
Date.fromString, and can convert from this string to this result:
Date.fromString "2017-01-13T09:00:00-05:00" -- Ok <Wed Jan 11 2017 09:00:00 GMT-0500 (EST)>
But that’s not exactly a JSON Decoder, is it?
For that, we’ll need to lift it into Decoder land by using
import Date exposing (Date) import Json.Decode exposing (Decoder, string, andThen, succeed, fail) date : Decoder Date date = let convert : String -> Decoder Date convert raw = case Date.fromString raw of Ok date -> succeed date Err error -> fail error in string |> andThen convert
succeed returns a decoder that always succeeds with the given value, and
fail returns a decoder that always fails with the given error.
We can map from
Result String a to
Decoder a that way.
Once you have this, you can use the
date decoder just like any other decoder!
In Real Life: Use
Of course, this is a lot of code to write for something fairly minor.
Going the long way around lets you customize to your heart’s content, but if you only need to map from
Result String a to
Decoder a there’s a simpler option.
Json.Decode.Extra exposes a function
fromResult that does this mapping for you.
The decoder above could be rewritten like this:
import Json.Decode.Extra exposing (fromResult) date : Decoder Date date = string |> andThen (Date.fromString >> fromResult)
So if all you need is to map a result, use
And finally, a caveat: if you actually need to parse a date,
Json.Decode.Extra also has a
No need to reinvent the wheel.
Now you know:
- If you need to convert from any data type to a decoder, use
- If you need to convert a
Result String a, use
- If you need a date, use