Move `HasServer (NamedRoutes routes)` instance
The instance has been moved to `Servant.Server.Internal`, as the
instances for other combinators. It is necessary so that the instance
can be re-exported from `Servant.Server` without circular imports.
Otherwise, users have to import `Servant.Server.Generic` manually ;
forgetting to do so will produce confusing error messages about the
missing instance.
Move `HasClient (NamedRoutes routes)` instance
Moved so that the instance is made available when importing
`Servant.Client`, avoiding possibly confusing errors when
`Servant.Client.Generic` isn't imported.
QuantifiedConstraints isn't available for GHC 8.4 (where our GHCJS
version is still stuck).
We may need to take a drastic decision for GHCJS at some point.
We define `ServerT (NamedRoutes api) m` as `api (AsServerT m)`, so that
the server of an record-defined API is a record of handlers.
The implementation piggy backs on the instance for “vanilla” servant
types with `(:<|>)`, using the `GServantProduct` for converting backd
and forth between the record / vanilla servers.
The main difficulty is that GHC needs to know that this operation is
legit, which can be expressed as the fact that:
```
GToServant (Rep (ServerT (NamedRoutes api))) m ~
ServerT (GToServant (Rep (api AsApi))) m
```
plus a few additional constraints.
This is easy enough for `route`, as we know that `m ~ Handler`. But in
the case of `hoistServerWithContext`, the two involved monads are
unknown ; in other words, this constraint needs to hold `forall m.`
Switching `-XQuantifiedConstraints` on is not sufficient, as our
constraints involve type families (`Rep` and `ServerT`). Our trick is to
use an intermediary typeclass, `GServer`, as a provider of evidence (in
the form of a `Dict`) that our constraints are indeed satisfied for a
particular monad.
The only instance of `GServer` is defined along with it, so it is
practically invisible to users.
Currently there is no way for Servant users to customize formatting of
error messages that arise when combinators can't parse URL or request
body, apart from reimplementing those combinators for themselves or
using middlewares.
This commit adds a possibility to specify custom error formatters
through Context.
Fixes#685
This commit adds an explanation and a link to the Servant Cookbook
to `genericServerT`.
Moreover, the `genericServer` and `genericServe`'s haddocks are
slightly edited to add a missing 'a'.
