Safe Haskell | None |
---|---|
Language | Haskell2010 |
- data Context contextTypes where
- EmptyContext :: Context `[]`
- (:.) :: x -> Context xs -> Context (x : xs)
- class HasContextEntry context val where
- getContextEntry :: Context context -> val
- data NamedContext name subContext = NamedContext (Context subContext)
- descendIntoNamedContext :: forall context name subContext. HasContextEntry context (NamedContext name subContext) => Proxy (name :: Symbol) -> Context context -> Context subContext
Documentation
data Context contextTypes where Source
Context
s 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 Enter
.) 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 heterogenous
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 :. () :. EmptyContext
True :. () :. EmptyContext :: Context '[Bool, ()]
EmptyContext :: Context `[]` | |
(:.) :: x -> Context xs -> Context (x : xs) infixr 5 |
class HasContextEntry context val where Source
This class is used to access context entries in Context
s. getContextEntry
returns the first value where the type matches:
>>>
getContextEntry (True :. False :. EmptyContext) :: Bool
True
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]) ...
getContextEntry :: Context context -> val Source
HasContextEntry ((:) * val xs) val Source | |
HasContextEntry xs val => HasContextEntry ((:) * notIt xs) val Source |
support for named subcontexts
data NamedContext name subContext Source
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
Context
s.
NamedContext (Context subContext) |
descendIntoNamedContext :: forall context name subContext. HasContextEntry context (NamedContext name subContext) => Proxy (name :: Symbol) -> Context context -> Context subContext Source
descendIntoNamedContext
allows you to access NamedContext
s. 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 subContext
subContext :: Context '[Bool]>>>
let parentContext = False :. (NamedContext subContext :: NamedContext "subContext" '[Bool]) :. EmptyContext
>>>
:type parentContext
parentContext :: Context '[Bool, NamedContext "subContext" '[Bool]]>>>
descendIntoNamedContext (Proxy :: Proxy "subContext") parentContext :: Context '[Bool]
True :. EmptyContext