Opposite of Codec.invmap would be useful

[Choc13]

So I find myself writing the following quite a bit:

type VariableName = VariableName of string

module VariableName =
    let value (VariableName x) = x

module VariableNameCodec =
    let ofDomain = VariableName.value
    let toDomain = VariableName

    let encoder = ofDomain >> JsonEncode.string

    let decoder = JsonDecode.string >> Result.map toDomain

    let codec = decoder, encoder    

And then I saw Codec.invmap and assumed that would satisfy me and I could write JsonCodec.string |> Codec.invmap toDomain ofDomain. However invmap works in the other direction, mapping the JsonValue part of the codec rather than the VariableName part of the codec. Or alternatively, the input to the decoder and the output of the encoder.

I think it would be nice to add the following alongside invmap.

let inline contrainvmap f g (decoder, encoder) =
        map (map f) decoder, contramap g encoder

Although I'm not sure the name is quite correct because actually on the decoder it's mapping twice (once through the function and then through the result type it returns) and also because I'm not sure if "contrainvmap" is the correct category theory based name for this thing.

Thoughts on adding this (assuming I've not overlooked something that already exists which does this)?

[gusty]

I also thought about it, but it turns out it's quite easy to convert the isomorphism into a codec. Here's an example of mapping a codec from Map to list:

(Ok << Map.ofList, Map.toList)  |> Codec.compose c

As you can see, all we have to do is to add OK << to the first function and we have turned an iso-morphism into a codec, so now we can use compose and choose in which order it does compose with other codecs.

But I think your suggestion is still valid, I just wonder if there is a standard name for it.