Monads at the prompt?

Is it possible to interact with arbitrary Monad instances incrementally at the GHCi 开发者_JAVA百科prompt?

You can enter "do" commands interactively:

Prelude> x <- return 5

But as far as I can tell, everything is forced into the IO () Monad. What if I want to interact with an arbitrary Monad instead?

Am I forced to write the entire sequence of commands inside a giant do { ... } and/or use the infix operators directly? That's okay, but I'd much prefer to "enter" an arbitrary monad and interact with it a line at a time.

Possible?

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As things stand, the IO-specific behaviour relies on the way IO actions are a bit statelike and unretractable. So you can say things like

s <- readFile "foo.txt"

and get an actual value s :: String.

It's pretty clear that it takes more than just Monad structure to sustain that sort of interaction. It would not be so easy with

n <- [1, 2, 3]

to say what value n has.

One could certainly imagine adapting ghci to open a prompt allowing a monadic computation to be constructed do-style in multiple command-line interactions, delivering the whole computation when the prompt is closed. It's not clear what it would mean to inspect the intermediate values (other than to generate collections of printing computations of type m (IO ()), for the active monad m, of course).

But it would be interesting to ask whether what's special about IO that makes a nice interactive prompt behaviour possible can be isolated and generalized. I can't help sniffing a whiff of a comonadic value-in-context story about interaction at a prompt, but I haven't tracked it down yet. One might imagine addressing my list example by considering what it would mean to have a cursor into a space of possible values, the way IO has a cursor imposed on it by the here-and-now of the real world. Thank you for the food for thought.

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Sure you can. Just annotate your type.
e.g. for the Maybe Monad:

let x = return 5 :: Maybe Int

will result in

Just 5

Or take the list monad:

let x = return 5 :: [Int]

will result in

[5]

Of course you can also play around inside the monad:

let x = return 5 :: Maybe Int
x >>= return . (succ . succ)

which will result in Just 7