Question

Is it possible to interact with arbitrary Monad instances incrementally at the GHCi 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?

Answer 1

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