Capability.Accessors

Coerce the type in the context m to to.

Example:

newtype MyInt = MyInt Int
newtype MyReader a = MyReader (Reader Int a)
  deriving (HasReader "a" MyInt) via
    Coerce MyInt (MonadReader (Reader Int))

Converts the Capability.Reader.HasReader "a" Int instance of Capability.Reader.MonadReader (Reader Int) to a Capability.Reader.HasReader "a" MyInt instance using Coercible Int MyInt.

Rename the tag.

Example:

newtype MyReader a = MyReader (Reader Int a)
  deriving (HasReader "foo" Int) via
    Rename "bar" (MonadReader (Reader Int))

Converts the Capability.Reader.HasReader "bar" Int instance of Capability.Reader.MonadReader (Reader Int) to a Capability.Reader.HasReader "foo" Int instance by renaming the tag.

Note, that Capability.Reader.MonadReader itself does not fix a tag, and Rename is redundant in this example.

See Pos below for a common use-case.

Access the record field field in the context m.

Example:

data Foo = Foo { foo :: Int }
newtype MyReader a = MyReader (Reader Foo a)
  deriving (HasReader "foo" Int) via
    Field "foo" () (MonadReader (Reader Foo))

Converts the Capability.Reader.HasReader () Foo instance of Capability.Reader.MonadReader (Reader Foo) to a Capability.Reader.HasReader "foo" Int instance by focusing on the field foo in the Foo record.

See Rename for a way to change the tag.

Access the value at position pos in the context m.

Example:

newtype MyReader a = MyReader (Reader (Int, Bool) a)
  deriving (HasReader 1 Int) via
    Pos 1 () (MonadReader (Reader (Int, Bool)))

Converts the Capability.Reader.HasReader () (Int, Bool) instance of Capability.Reader.MonadReader (Reader (Int, Bool)) to a Capability.Reader.HasReader 1 Int instance by focusing on the first element of the tuple.

The implied number tag can be renamed to a more descriptive name using the Rename combinator:

newtype MyReader a = MyReader (Reader (Int, Bool) a)
  deriving (HasReader "foo" Int) via
    Rename 1 (Pos 1 () (MonadReader (Reader (Int, Bool))))

Choose the given constructor in the sum-type in context m.

Example:

data MyError = ErrA String | ErrB String
newtype MyExcept a = MyExcept (ExceptT MyError Identity a)
  deriving (HasThrow "ErrB" String) via
    Ctor "ErrB" () (MonadError (ExceptT MyError Identity))

Converts the HasThrow () "MyError" instance of MonadError (ExceptT MyError Identity) to a HasThrow "ErrB" String instance by wrapping thrown Strings in the ErrB constructor.

Skip one level in a monad transformer stack.

Note, that instances generated with this strategy can incur a performance penalty.

Example:

newtype MyStates a = MyStates (StateT Int (State Bool) a)
  deriving (HasState "foo" Bool) via
    Lift (StateT Int (MonadState (State Bool)))

Uses the MonadTrans instance of StateT Int to lift the Capability.State.HasState "foo" Bool instance of the underlying Capability.State.MonadState (State Bool) over the StateT Int monad transformer.

Compose two accessors.

This is not necessary in deriving via clauses, but in places where a transformer is expected as a type argument. E.g. HasError.wrapError.