Servant.Server.Internal.Context

Contexts are used to pass values to combinators. (They are not meant to be used to pass parameters to your handlers, i.e. they should not replace any custom ReaderT-monad-stack that you're using with hoistServer.) If you don't use combinators that require any context entries, you can just use serve as always.

If you are using combinators that require a non-empty Context you have to use serveWithContext and pass it a Context that contains all the values your combinators need. A Context is essentially a heterogeneous list and accessing the elements is being done by type (see getContextEntry). The parameter of the type Context is a type-level list reflecting the types of the contained context entries. To create a Context with entries, use the operator (:.):

>>> :type True :. () :. EmptyContextTrue :. () :. EmptyContext :: Context '[Bool, ()]

Append two type-level lists.

Hint: import it as

import Servant.Server (type (.++))

Append two contexts.

This class is used to access context entries in Contexts. getContextEntry returns the first value where the type matches:

>>> getContextEntry (True :. False :. EmptyContext) :: BoolTrue

If the Context does not contain an entry of the requested type, you'll get an error:

>>> getContextEntry (True :. False :. EmptyContext) :: String......No instance for ...HasContextEntry '[] [Char]......

Normally context entries are accessed by their types. In case you need to have multiple values of the same type in your Context and need to access them, we provide NamedContext. You can think of it as sub-namespaces for Contexts.

descendIntoNamedContext allows you to access NamedContexts. Usually you won't have to use it yourself but instead use a combinator like WithNamedContext.

This is how descendIntoNamedContext works:

>>> :set -XFlexibleContexts>>> let subContext = True :. EmptyContext>>> :type subContextsubContext :: Context '[Bool]>>> let parentContext = False :. (NamedContext subContext :: NamedContext "subContext" '[Bool]) :. EmptyContext>>> :type parentContextparentContext :: Context '[Bool, NamedContext "subContext" '[Bool]]>>> descendIntoNamedContext (Proxy :: Proxy "subContext") parentContext :: Context '[Bool]True :. EmptyContext