What is the idiomatic way to have default params in APIs?

[Batou99]

I am currently defining an API with:

type DeviceApi = 
           "v2/devices/catalog/search" :> Header "X-HEADER" String
                                             :> QueryParam "name" String
                                             :> QueryParam "description" String
                                             :> QueryParam "page" Int
                                             :> QueryParam "limit" Int
                                             :> QueryParam "tags" [Tag]
                                             :> QueryParam "serial" [Tag]
                                             :> QueryParam "dir" SortDir
                                             :> QueryParam "sort" SortBy
                                             :> ReqBody '[JSON] FilterReqBody
                                             :> Get '[JSON] DevicePaginatedListing

And using this to generate a function to call the endpoint

catalogSearch :: AuthHeader -> Maybe String -> Maybe String -> Maybe Int -> Maybe Int -> Maybe [Tag] -> Maybe [Tag] -> Maybe SortDir -> Maybe SortBy -> FilterReqBody -> ClientM DevicePaginatedListing

The issue is that the function signature is quite ugly, I'd like to define a record type and default params to be able to generate the function like this:

data CatalogSearchParams = CatalogSearchParams { deviceName :: Maybe String
                                               , desc :: Maybe String
                                               , page :: Maybe Int
                                               , limit :: Maybe Int
                                               , tags :: Maybe [Tag]
                                               , serial :: Maybe [Tag]
                                               , dir :: Maybe SortDir
                                               , sort :: Maybe SortBy
                                               , reqBody :: FilterReqBody}

defaultCatalogSearchParams = CatalogSearchParams { deviceName = Nothing
                                                 , desc = Nothing
                                                 , page = Just 1
                                                 , limit = Just 1000
                                                 , tags = Nothing
                                                 , serial = Nothing
                                                 , dir = Just Asc
                                                 , sort = Just Name
                                                 , reqBody = FilterReqBody [] }

catalogSearch :: AuthHeader -> CatalogSearchParams -> ClientM DevicePaginatedListing
catalogSearch = client api

Which of course does not work.
How do you normally deal with endpoints that accept a lot of query params? I am sure this is something not uncommon.

[phadej]

I has almost working prototype, but it depends on yet unreleased generics-sop-0.3.1.0. I hope I'll complete it during ZuriHac.

[phadej]

See haskell-servant/servant-contrib#4

I won't say it's idiomatic, as I just wrote it; but that's something I could imagine using myself. (Required parameters will complicate the story)

[fizruk]

I think there was an issue somewhere (can't find it now) to add combinator(s) that'd use ToForm/FromForm to parse FormURLEncoded request bodies and/or QueryParams.

I think that would be more "idiomatic" than ManyQueryParams, since it would use normal Haskell records (together with Generic-based deriving).

@haskell-servant/maintainers what do you think?

[3noch]

Would it be difficult to add a combinator that could masquerade as a QueryParam but provide a default?

type Api = "test" :> (QueryParam "value" Int :?> 10) :> Get '[JSON] Value

where QueryParam "value" Int :?> 10 is itself a QueryParam that wraps another one.

If there is/were a way to customize what the Nothing case means when passing data to the handler, then this could be easy.

This would be also very handy when generating docs since defaults are an important part of documentation that is not currently captured.

[dmjio]

I'd shy away from specifying the value at the type level. Suppose the query param took a Double.

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[3noch]

You can always add defaults in your handler. The question is: how do you capture that information in the API specification? I think we have two options, specify the default in the type itself, or use type classes to allow the default to be at the value level. But the type-class approach would be very boilerplate heavy. Perhaps you could combine them to get the best of both worlds.

[fizruk]

@3noch I agree with @dmjio. Unfortunately DataKinds can't yet lift any (even constant) terms to the type level.

Besides, I don't like defaults parameter values on the handler side.
If you don't have a value for a query parameter, but you need it — throw 400 (or use RequiredQueryParam if/when it lands in Servant 😉).

AFAIK default parameters are mostly useful when you have many parameters and would like to omit few. But then I'd rather pack those parameters in a record data type and have a smart constructor (maybe more than one) for it with some defaults.
That is roughly the same idea @Batou99 originally posted.

[3noch]

@fizruk I use Servant often to write specifications for other APIs. They don't tend to have the same values as Haskellers and they use defaults liberally. Not being able to easily encode them in the specification is a problem, IMO. It's even more annoying when they're not documented, and servant, currently, has no way to generate docs for defaults.

[fizruk]

@3noch if you're a client of such API then I guess you can treat any QueryParam with default value as RequiredQueryParam and always pass values explicitly. No need to encode defaults.
Note that you can still copy those default values for client functions.

[3noch]

@fizruk Right, yes. I should have been clearer. I actually hope to use servant specs as the engine for generating docs for such APIs.

[phadej]

@fizruk the ManyQueryParams is a show-off example. It says in the type which query arguments it uses, where FromForm would hide that inside term-value instance. I think both cases have their uses, but definitely FromForm based approach is simpler and probably good enough for most (almost all) cases.

Similar reasoning applies to default values. Sometimes you want them shown at type-level, but you can bake them into an instance as well.

The drawback with FromForm approach, is that you need to repeat yourself more. Especially with default values, it's hard to use Generics to derive proper instance.

Side-note: IIRC swagger supports default values?

[Batou99]

The issue here (for me) it's less about mandatory vs optional params and more about easy to call functions.

In the example I show above the function catalogSearch it's awful to call. It has 8 possible QueryParams and the ReqBody, that makes calling the function very cumbersome.

If I declare a record to wrap the QueryParams and declare a wrapper function to pass the record instead it's way less cumbersome, but it forces me to declare the records and the wrapper for each function defined in the API.

I think this is a common enough case to merit a standard idiomatic solution. I just don't know if there is already one (does not look like it) or if we need something for it.

[coderfromhere]

This is still highly desired for mapping onto real-world APIs out there