Is there a way to decode a field in source object into another field in target object about decoders

[nvie]

There is no direct way of doing this without cluttering up the API and using the field() construct in many other places.

A way that is currently possible is to decode the “original” fields from the source object first, and then “map” the successfully decoded result by a transformer function to the intended output structure. For example:

type Output = {
  a: string,
  b: number,
}

const decoder: Decoder<Output> = map(
  object({
    a: string,
    c: number,
  }),
  obj => ({ ...obj, b: obj.c }),
);

[nvie]

The API you suggested is something I thought about, but it would require special knowledge about the field decoder inside the object decoder (so it has implications for composability). It also has implications for annotating error messages inline, which would then need special treatment. I don’t think it’s impossible, but it would definitely be harder to implement. That’s why I haven’t found a better solution for this yet.

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