Type safe generation of internal links.

Given an API with a few endpoints:

>>> :set -XDataKinds -XTypeFamilies -XTypeOperators
>>> import Servant.API
>>> import Servant.Utils.Links
>>> import Data.Proxy
>>> 
>>> 
>>> 
>>> type Hello = "hello" :> Get '[JSON] Int
>>> type Bye   = "bye"   :> QueryParam "name" String :> Delete '[JSON] ()
>>> type API   = Hello :<|> Bye
>>> let api = Proxy :: Proxy API

It is possible to generate links that are guaranteed to be within API with safeLink. The first argument to safeLink is a type representing the API you would like to restrict links to. The second argument is the destination endpoint you would like the link to point to, this will need to end with a verb like GET or POST. Further arguments may be required depending on the type of the endpoint. If everything lines up you will get a URI out the other end.

You may omit QueryParams and the like should you not want to provide them, but types which form part of the URL path like Capture must be included. The reason you may want to omit QueryParams is that safeLink is a bit magical: if parameters are included that could take input it will return a function that accepts that input and generates a link. This is best shown with an example. Here, a link is generated with no parameters:

>>> let hello = Proxy :: Proxy ("hello" :> Get '[JSON] Int)
>>> print (safeLink api hello :: URI)
hello

If the API has an endpoint with parameters then we can generate links with or without those:

>>> let with = Proxy :: Proxy ("bye" :> QueryParam "name" String :> Delete '[JSON] ())
>>> print $ safeLink api with "Hubert"
bye?name=Hubert

>>> let without = Proxy :: Proxy ("bye" :> Delete '[JSON] ())
>>> print $ safeLink api without
bye

If you would like create a helper for generating links only within that API, you can partially apply safeLink if you specify a correct type signature like so:

>>> :set -XConstraintKinds
>>> :{
>>> let apiLink :: (IsElem endpoint API, HasLink endpoint)
>>> => Proxy endpoint -> MkLink endpoint
>>> apiLink = safeLink api
>>> :}

Attempting to construct a link to an endpoint that does not exist in api will result in a type error like this:

>>> let bad_link = Proxy :: Proxy ("hello" :> Delete '[JSON] ())
>>> safeLink api bad_link
...
    Could not deduce (Or
                        (IsElem' (Delete '[JSON] ()) (Get '[JSON] Int))
                        (IsElem'
                           ("hello" :> Delete '[JSON] ())
                           ("bye" :> (QueryParam "name" String :> Delete '[JSON] ()))))
      arising from a use of ‘safeLink’
    In the expression: safeLink api bad_link
    In an equation for ‘it’: it = safeLink api bad_link

This error is essentially saying that the type family couldn't find bad_link under api after trying the open (but empty) type family IsElem' as a last resort.

Note that URI is Network.URI.URI from the network-uri package.