Reader capability

An instance should fulfill the following laws. At this point these laws are not definitive, see https://github.com/haskell/mtl/issues/5.

k <*> ask @t = ask @t <**> k

ask @t *> m = m = m <* ask @t

local @t f (ask @t) = fmap f (ask @t)

local @t f . local @t g = local @t (g . f)

local @t f (pure x) = pure x

local @t f (m >>= \x -> k x) = local @t f m >>= \x -> local @t f (k x)

reader @t f = f <$> ask @t

ask @tag retrieves the environment of the reader capability tag.

asks @tag retrieves the image by f of the environment of the reader capability tag.

asks @tag f = f <$> ask @tag

local @tag f m runs the monadic action m in a modified environment e' = f e, where e is the environment of the reader capability tag. Symbolically: return e = ask @tag.

reader @tag act lifts a purely environment-dependent action act to a monadic action in an arbitrary monad m with capability HasReader.

It happens to coincide with asks: reader = asks.

Execute the given reader action on a sub-component of the current context as defined by the given transformer t, retaining arbitrary capabilities listed in cs.

See the similar zoom function for more details and examples.

This function is experimental and subject to change. See https://github.com/tweag/capability/issues/46.

Reified tag capability m

Defines the dictionary type for the methods of capability under tag in the monad m. Refer to interpret_ for an example use-case.

For example, the HasSink capability has the method yield :: a -> m (). The corresponding dictionary type is defined as follows.

>>> :{
  data instance Reified tag (HasSink tag a) m =
    ReifiedSink { _yield :: forall a. a -> m () }
  :}

Superclass dictionaries are represented as nested records. For example, the HasState capability has the superclasses HasSource and HasSink and the method state :: (s -> (a, s)) -> m a. The corresponding dictionary type is defined as follows.

>>> :{
  data instance Reified tag (HasState tag s) m =
    ReifiedState
      { _stateSource :: Reified tag (HasSource tag s) m,
        _stateSink :: Reified tag (HasSink tag s) m,
        _state :: forall a. (s -> (a, s)) -> m a
      }
  :}

Type synonym using the TypeOf type family to specify HasReader constraints without having to specify the type associated to a tag.

Type family associating a tag to the corresponding type. It is intended to simplify constraint declarations, by removing the need to redundantly specify the type associated to a tag.

It is poly-kinded, which allows users to define their own kind of tags. Standard haskell types can also be used as tags by specifying the Type kind when defining the type family instance.

Defining TypeOf instances for Symbols (typelevel string literals) is discouraged. Since symbols all belong to the same global namespace, such instances could conflict with others defined in external libraries. More generally, as for typeclasses, TypeOf instances should always be defined in the same module as the tag type to prevent issues due to orphan instances.

Example:

    import Capability.Reader

    data Foo
    data Bar
    type instance TypeOf Type Foo = Int
    type instance TypeOf Type Bar = String

    -- Same as: foo :: HasReader Foo Int M => …
    foo :: HasReader' Foo m => …
    foo = …

Derive HasSource from m's MonadReader instance.

Convert a pure state monad into a reader monad.

Pure meaning that the monad stack does not allow catching exceptions. Otherwise, an exception occurring in the action passed to local could cause the context to remain modified outside of the call to local. E.g.

local @tag (const r') (throw MyException)
`catch` \MyException -> ask @tag

returns r' instead of the previous value.

Note, that no MonadIO instance is provided, as this would allow catching exceptions.

See ReadState.

Convert a state monad into a reader monad.

Use this if the monad stack allows catching exceptions.

See ReadStatePure.