Control.Ether.Tagged

Type-level machinery for tag manipulation.

Documentation

class Taggable m

The Taggable class defines the type families to manage tags in monad transformer stacks. Its kind is restricted to * -> * to prevent incorrect instances.

Associated Types

type Tag m :: Maybe *

The Tag type family equals Nothing for most types, but for tagged monad transformers it equals Just tag.

type Inner m :: Maybe (* -> *)

The Inner type family equals Nothing for most types, but for monad transformers with inner monad m it equals Just m.

Instances

Taggable IO

Taggable Identity

Taggable STM

Taggable First

Taggable Last

Taggable Maybe

Taggable ((->) r)

Taggable (Either e)

Taggable (ST s)

Taggable (ST s)

Taggable (Proxy *)

Taggable (ListT m)

Taggable (MaybeT m)

Taggable (IdentityT m)

Taggable (ContT r m)

Taggable (ReaderT r m)

Taggable (StateT s m)

Taggable (StateT s m)

Taggable (ExceptT e m)

Taggable (WriterT w m)

Taggable (WriterT w m)

Taggable (ReaderT tag r m)

Taggable (WriterT tag w m)

Taggable (StateT tag s m)

Taggable (StateT tag s m)

Taggable (ExceptT tag e m)

class (Taggable m, Tag m ~ Just tag) => Tagged m tag | m -> tag where

The Tagged type class establishes a relationship between a tagged monad transformer and its untagged counterpart.

Minimal complete definition

Nothing

Associated Types

type Untagged m :: * -> *

Methods

tagged :: proxy tag -> Untagged m a -> m a

untagged :: proxy tag -> m a -> Untagged m a

Instances

Tagged (ReaderT tag r m) tag

Tagged (WriterT tag w m) tag

Tagged (StateT tag s m) tag

Tagged (StateT tag s m) tag

Tagged (ExceptT tag e m) tag

type Tags m = MaybeToList (Tag m) ++ ListMapTag (Inners m)

The Tags type function returns a type-level list of all tags in a monad transformer stack. Requires Taggable instances.

type Inners m = Inners' (Inner m)

The Inners type function recursively applies Inner and returns the results in a type-level list.

class UniqueTag a

The main purpose of the UniqueTag class is to provide clear error messages when the tag uniqueness property is violated. You should never write instances for it unless you know what you're doing.

type family UniqueTags m :: Constraint

The UniqueTags constraint placed on a type variable representing a monad transformer stack ensures that every tag in the stack appears only once.

Equations

UniqueTags m = Unique (Tags m)

ensureUniqueTags :: UniqueTags m => m a -> m a

Type-restricted version of id that adds a UniqueTags constraint to the provided monadic value.