| Copyright | (C) 2008-2015 Edward Kmett |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Stability | experimental |
| Portability | non-portable (fundeps, MPTCs) |
| Safe Haskell | Safe |
| Language | Haskell2010 |
Control.Monad.Free.Class
Description
Monads for free.
Documentation
class Monad m => MonadFree f m | m -> f where #
Monads provide substitution (fmap) and renormalization (join):
m>>=f =join(fmapf m)
A free Monad is one that does no work during the normalization step beyond simply grafting the two monadic values together.
[] is not a free Monad (in this sense) because smashes the lists flat.join [[a]]
On the other hand, consider:
data Tree a = Bin (Tree a) (Tree a) | Tip a
instanceMonadTree wherereturn= Tip Tip a>>=f = f a Bin l r>>=f = Bin (l>>=f) (r>>=f)
This Monad is the free Monad of Pair:
data Pair a = Pair a a
And we could make an instance of MonadFree for it directly:
instanceMonadFreePair Tree wherewrap(Pair l r) = Bin l r
Or we could choose to program with instead of Free PairTree
and thereby avoid having to define our own Monad instance.
Moreover, Control.Monad.Free.Church provides a MonadFree
instance that can improve the asymptotic complexity of code that
constructs free monads by effectively reassociating the use of
(>>=). You may also want to take a look at the kan-extensions
package (http://hackage.haskell.org/package/kan-extensions).
See Free for a more formal definition of the free Monad
for a Functor.
Methods
Add a layer.
wrap (fmap f x) ≡ wrap (fmap return x) >>= f
wrap :: (m ~ t n, MonadTrans t, MonadFree f n, Functor f) => f (m a) -> m a #
Add a layer.
wrap (fmap f x) ≡ wrap (fmap return x) >>= f
Instances
| (Functor f, MonadFree f m) => MonadFree f (ListT m) # | |
| (Functor f, MonadFree f m) => MonadFree f (MaybeT m) # | |
| Functor f => MonadFree f (Free f) # | |
| Functor f => MonadFree f (F f) # | |
| Monad m => MonadFree Identity (IterT m) # | |
| (Functor f, MonadFree f m) => MonadFree f (ExceptT e m) # | |
| (Functor f, MonadFree f m, Error e) => MonadFree f (ErrorT e m) # | |
| (Functor f, MonadFree f m) => MonadFree f (IdentityT * m) # | |
| (Functor f, MonadFree f m, Monoid w) => MonadFree f (WriterT w m) # | |
| (Functor f, MonadFree f m, Monoid w) => MonadFree f (WriterT w m) # | |
| (Functor f, MonadFree f m) => MonadFree f (StateT s m) # | |
| (Functor f, MonadFree f m) => MonadFree f (StateT s m) # | |
| (Functor f, Monad m) => MonadFree f (FreeT f m) # | |
| MonadFree f (FT f m) # | |
| (Functor f, MonadFree f m) => MonadFree f (ContT * r m) # | |
| (Functor f, MonadFree f m) => MonadFree f (ReaderT * e m) # | |
| (Functor f, MonadFree f m, Monoid w) => MonadFree f (RWST r w s m) # | |
| (Functor f, MonadFree f m, Monoid w) => MonadFree f (RWST r w s m) # | |