问题
http://hackage.haskell.org/package/free in Control.Monad.Free.Free
allows one to get access to the "free monad" for any given Functor
. It does not, however, have a MonadFix
instance. Is this because such an instance cannot be written, or was it just left out?
If such an instance cannot be written, why not?
回答1:
Consider the description of what mfix
does:
The fixed point of a monadic computation.
mfix f
executes the actionf
only once, with the eventual output fed back as the input.
The word "executes", in the context of Free
, means creating layers of the Functor
. Thus, "only once" means that in the result of evaluating mfix f
, the values held in Pure
constructors must fully determine how many layers of the functor are created.
Now, say we have a specific function once
that we know will always only create a single Free
constructor, plus however many Pure
constructors are needed to hold the leaf values. The output of 'once', then, will be only values of type Free f a
that are isomorphic to some value of type f a
. With this knowledge, we can un-Free
the output of once
safely, to get a value of type f a
.
Now, note that because mfix
is required to "execute the action only once", the result of mfix once
should, for a conforming instance, contain no additional monadic structure than once
creates in a single application. Thus we can deduce that the value obtained from mfix once
must also be isomorphic to a value of type f a
.
Given any function with type a -> f a
for some Functor
f
, we can wrap the result with a single Free
and get a function of type a -> Free f a
which satisfies the description of once
above, and we've already established that we can unwrap the result of mfix once
to get a value of type f a
back.
Therefore, a conforming instance (Functor f) => MonadFix (Free f)
would imply being able to write, via the wrapping and unwrapping described above, a function ffix :: (Functor f) => (a -> f a) -> f a
that would work for all instances of Functor
.
That's obviously not a proof that you can't write such an instance... but if it were possible, MonadFix
would be completely superfluous, because you could just as easily write ffix
directly. (And I suppose reimplement it as mfix
with a Monad
constraint, using liftM
. Ugh.)
回答2:
Well, inspired by the MonadFix
instance for Maybe
, I tried this one (using the following definitions of Free
):
data Free f a
= Pure a
| Impure (f (Free f a))
instance (Functor f) => Monad (Free f) where
return = Pure
Pure x >>= f = f x
Impure x >>= f = Impure (fmap (>>= f) x)
instance (Functor f) => MonadFix (Free f) where
mfix f = let Pure x = f x in Pure x
The laws are:
- Purity:
mfix (return . h) = return (fix h)
- Left shrinking:
mfix (\x -> a >>= \y -> f x y) = a >>= \y -> mfix (\x -> f x y)
- Sliding:
mfix (liftM h . f) = liftM h (mfix (f . h))
, for stricth
- Nesting:
mfix (\x -> mfix (f x)) = mfix (\x -> f x x)
Purity is easy to prove -- but I came across a problem when trying to prove left shrinking:
mfix (\x -> a >>= \y -> f x y)
= let Pure x = (\x -> a >>= \y -> f x y) x in Pure x
= let Pure x = a >>= \y -> f x y in Pure x
-- case a = Pure z
= let Pure x = Pure z >>= \y -> f x y in Pure x
= let Pure x = f x z in Pure x
= let Pure x = (\x -> f x z) x in Pure x
= mfix (\x -> f x z)
= Pure z >>= \y -> mfix (\x -> f x y)
-- case a = Impure t
= let Pure x = Impure t >>= \y -> f x y in Pure x
= let Pure x = Impure (fmap (>>= \y -> f x y) t) in Pure x
= Pure _|_
but
Impure t >>= \y -> mfix (\x -> f x y)
= Impure (fmap (>>= \y -> mfix (\x -> f x y)) t)
/= Pure _|_
So, at the very least, if the Pure
and Impure
constructors are distinguishable, then my implementation of mfix
does not satisfy the laws. I can't think of any other implementation, but that does not mean it does not exist. Sorry I couldn't illuminate further.
回答3:
No, it cannot be written in general, because not every Monad is an instance of MonadFix. Every Monad can be implemented using the FreeMonad underneath. If you could implement the MonadFix for Free, then you would be able to derive MonadFix from any Monad, which is not possible. But of course, you can define a FreeFix for the MonadFix class.
I think it might look somehow like this, but this is just a 3rd guess (still not tested):
data FreeFix m a = FreeFix { runFreeFix :: (forall r. (r -> m r) -> m r) -> m a }
instance (Monad m) => Monad (FreeFix m) where
return a = FreeFix $ \_-> do
return a
f >>= g = FreeFix $ \mfx -> do
x <- runFreeFix f mfx
runFreeFix (g x) mfx
instance (Monad m) => MonadFix (FreeFix m) where
mfix f = FreeFix $ \mfx -> do
mfx (\r->runFreeFix (f r) mfx)
The idea is that m is a Monad that lacks an implementation for mfix; so mfix needs to be a parameter when FreeFix will be reduced.
来源:https://stackoverflow.com/questions/14636048/is-it-possible-to-implement-monadfix-for-free