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How do you use recursive?

The trick to understanding the recursive codec is: pretend you are already done. When the function you pass to recursive is called the argument is the finished Codec.

An example may be worth a thousand words:

type Peano
    = Peano (Maybe Peano)


peanoCodec : Codec Peano
peanoCodec =
    Codec.recursive
        (\finishedCodec ->
            Codec.maybe finishedCodec
                |> Codec.map Peano (\(Peano p) -> p)
        )

Why does map take two opposite functions?

One is used for the encoder, the other for the decoder

How do I build Codecs for custom types?

You start building with custom which needs the pattern matcher for your type as an argument.

The pattern matcher is just the most generic case ... of possible for your type.

You then chain variantX calls for every alternative (in the same order as the pattern matcher).

You end with a call to buildCustom.

An example:

type Semaphore
    = Red Int String
    | Yellow Float
    | Green


semaphoreCodec : Codec Semaphore
semaphoreCodec =
    Codec.custom
        (\fred fyellow fgreen value ->
            case value of
                Red i s ->
                    fred i s

                Yellow f ->
                    fyellow f

                Green ->
                    fgreen
        )
        |> Codec.variant2 "Red" Red Codec.int Codec.string
        |> Codec.variant1 "Yellow" Yellow Codec.float
        |> Codec.variant0 "Green" Green
        |> Codec.buildCustom

What happens to existing Values if I change my Codecs?

Old Values will be parsed fine by new Codecs if you:

  • add new variants to custom types,
  • remove (from the end) parameters from variants,
  • change any Codec to a succeed one or
  • add optional fields (maybeField) to records.

New Values will be parsed fine by old Codecs if you:

  • remove variants from custom types,
  • append parameters to variants ,
  • change a succeed Codec to any other one or
  • remove fields from records.