Extract a date using Date.fromString

Can be helpful when decoding large objects incrementally.

See the andMap docs for an explanation of how andMap works and how to use it.

Infix version of andMap that makes for a nice DSL when decoding objects.

See the (|:) docs for an explanation of how (|:) works and how to use it.

This function turns a list of decoders into a decoder that returns a list.

The returned decoder will zip the list of decoders with a list of values, matching each decoder with exactly one value at the same position. This is most often useful in cases when you find yourself needing to dynamically generate a list of decoders based on some data, and decode some other data with this list of decoders. There are other functions that seem similar:

• Json.Decode.oneOf, which will try every decoder for every value in the list, might be too lenient (e.g. a 4.0 will be interpreted as an Int just fine).
• Json.Decode.tuple1-8, which do something similar, but have a hard-coded length. As opposed to these functions, where you can decode several different types and combine them, you'll need to manually unify all those types in one sum type to use sequence.

Note that this function, unlike List.map2's behaviour, expects the list of decoders to have the same length as the list of values in the JSON.

type FloatOrInt
    = I Int
    | F Float


-- we'd like a list like [I, F, I] from this
-- fairly contrived example, but data like this does exist!
json = "[1, 2.0, 3]"


intDecoder = Decode.map I Decode.int
floatDecoder = Decode.map F Decode.float


decoder : Decoder (List FloatOrInt)
decoder =
    sequence [ intDecoder, floatDecoder, intDecoder ]


decoded = Decode.decodeString decoder json
-- Ok ([I 1,F 2,I 3]) : Result String (List FloatOrInt)

Extract a set.

Extract a dict using separate decoders for keys and values.

Try running the given decoder; if that fails, then succeed with the given fallback value.

-- If this field is missing or malformed, it will decode to [].
field "optionalNames" (list string)
  |> (withDefault [])

Transform a result into a decoder

Sometimes it can be useful to use functions that primarily operate on Result in decoders. An example of this is Json.Decode.Extra.date. It uses the built-in Date.fromString to parse a String as a Date, and then converts the Result from that conversion into a decoder which has either already succeeded or failed based on the outcome.

date : Decoder Date
date =
    string |> andThen (Date.fromString >> fromResult)