Question

Given the following:

> (liftM2 fromMaybe) (ioError $ userError "OOPS") (return $ Just "ok")

ghci gives me

*** Exception: user error (OOPS)

Of course, fromMaybe is working correctly:

> (liftM2 fromMaybe) (return $ "not me") (return $ Just "ok")
"ok"

But it seems that the IO operation is being carried out and then discarded:

> (liftM2 fromMaybe) (putStrLn "computing.." >> "discarded") (return $ Just "ok")
computing..
"ok"

Why is this happening? Is there any way to make the IO monad lazier?

Specifically, given value :: IO (Maybe a) what’s a (clean, concise) way to say

result <- (liftM2 fromMaybe) err value

and have it unpack result or throw an IOError accordingly?

Answer 1

I don’t know that making IO lazier is the right direction here. What you seem to want to do is first get at the Maybe, then eliminate it. This can be written several ways, here’s one option:

test :: IO (Maybe a) -> IO a
test = (>>= maybe (ioError $ userError "oops") return)