fix vs. ArrowLoop

问题

Description of loop from Control.Arrow:

The loop operator expresses computations in which an output value is fed back as input, although the computation occurs only once. It underlies the rec value recursion construct in arrow notation.

Its source code, and its instantiation for (->):

class Arrow a => ArrowLoop a where
    loop :: a (b,d) (c,d) -> a b c

instance ArrowLoop (->) where
    loop f b = let (c,d) = f (b,d) in c

This immediately reminds me of fix, the fixpoint combinator:

fix :: (a -> a) -> a
fix f = let x = f x in x

So my question is:

1. Is it possible to implement that particular loop via fix?
2. How are their functionalities different?

回答1:

1. Well, of course. Every recursive definition can be written with fix:

   loop f b = let (c, d) = f (b, d) in c
   loop f b = fst $ let (c, d) = f (b, d) in (c, d)
   loop f b = fst $ let x = f (b, d) in x
   loop f b = fst $ let x = f' x in x
     where f' (_, d) = f (b, d)
   loop f b = fst $ fix $ f . (b,) . snd
   

   And it works the other way around:

   fix f = loop (join (,) . f . snd) ()
   

2. The above should convince you that loop and fix are equivalently powerful when talking about (->). Why, then, if arrows are meant to generalize functions, is ArrowLoop not defined like so?

   class Arrow a => ArrowLoop a where
     fix :: a b b -> b
   

   Arrows also generalize the notion of "process": when Arrow a, a b c is a way to calculate a c from a b. If ArrowLoop was defined to directly generalize fix, then it would be severely crippled. fix would have to "execute" the process without any context and directly produce a value of type b, which means the "process" a b b cannot e.g. perform IO. Or, consider the arrow

   newtype LT i o = LT { runLT :: [i] -> [o] }
   

   You’d like it if fix would produce a [b] from a LT b b, but it doesn’t.

   loop is a way around these restrictions. It takes a process as argument and produces a process as result. In a sense, all the context associated with the first process can be survived in the second, which would not be possible if loop were more like fix.

   Note that I can implement an analogue of fix for ArrowLoops:

   -- resulting process ignores its input
   fix' :: ArrowLoop a -- taking an impl of loop as argument
        => a b b -> a u b
   fix' f = loop ((id &&& id) . f . arr snd)
   -- take off the outer application to () (application means (->), after all)
   -- and arrowify: join (,) = id &&& id; snd = arr snd; (Prelude..) = (Control.Category..)
   -- but the RHSs are more general
   

   But I don't believe

   loop' :: Arrow a => (forall x u. a x x -> a u x) -- taking an impl of fix' as argument
         -> a (b, d) (c, d) -> a b c
   

   is implementable, so we can’t base ArrowLoop on fix' either.

来源：https://stackoverflow.com/questions/51758376/fix-vs-arrowloop