Add LenientData

[phadej]

Motivated by servant making parameter parsing fail.

Is it ok to define ToHttpApiData instance with toQueryParam = either (const "") toQueryParam . getLenientData, i.e. serialising Left to an empty string?

[fizruk]

Ok, what options do we have here? So far I see these:

1. toQueryParam = either (const "") toQueryParam . getLenientData.
2. toQueryParam = either (error "Can't encode") toQueryParam . getLenientData.
3. No ToHttpApiData instance.

I like option 3 the most. Because LenientData makes sense only for decoding. However this means that we won't be able to use client with QueryParam "param" (LenientData MyParam).

Option 2 is bad because of error and option 1 is bad because it silently ignores Left (which may or may not lead to problems in real code).

What I would do is rather use Lenient as a parameter of QueryParam (much like Required). So that we could have

type API = LenientQueryParam "param" MyParam

This would allow us to have Maybe (LenientData MyParam) in handler while using just Maybe MyParam on the client side. If combined with Required this might even be

type API = RequiredLenientQueryParam "param" MyParam

Which would result in LenientData MyParam on server and MyParam on client.

If Servant were to implement QueryParam (and other combinators) with Lenient option we won't have to ever use ToHttpApiData instance for LenientData a since on client we will always have just a.

[phadej]

I'm 👍 with your plan, so the QueryParam will be parametrised by requiredness and leniency of parsing. That makes sense

[fizruk]

I'm otherwise ok with this PR, are you going to add anything else here or should I merge it already?

[phadej]

I'll add few instances, Foldable and Traversable could be useful

[phadej]

@fizruk,I'd say this is ready. Even we won't need ToHttpApiData instance, theres no downside of having it.

[fizruk]

Why don't you just derive Functor, Foldable and Traversable?

[phadej]

autoderivied all of them

[fizruk]

Why don't you also derive Data?

[fizruk]

Nice, thanks!