Hypothetical messaging primitive for Objective-Haskell

gistfile1.hs

mutableStringWithString :: Id -> IO Id
mutableStringWithString str =
    getClass "NSMutableString" <.> stringWithString str

arrayWithObjects :: [Id] -> IO Id
arrayWithObjects objs = do
    getClass "NSArray" <.> arrayWithObjectsAndCount objs (length objs)

Furthermore, return x >>= blah is usually a pretty clear sign that you shouldn't be using monadic binding in the first place! In fact, what you want for that instance is just flip ($). So, dig this:

(@.) = flip ($)

concatNSStrings :: Id -> Id -> IO Id
concatNSStrings a b = 
    a @. stringByAppendingString b

Right, but the @ was to solve the problem of a variadic application. Also, it could be written (@) if need be (to get around the infix restriction).

Yeah, still not necessary if you're going to put the receiver at the end. So, you could do

concatNSStrings :: Id -> Id -> IO Id
concatNSStrings a b = (@) a stringByAppendingString b

if you want, but it doesn't really make a difference. (@) is still just flip ($), with no bind in site. With currying, we have stopped caring about variadic nonsense.

I'm talking about this case:

f obj a b =
    (@) obj >>= somethingWithTwoArguments a b

Unless I'm missing something, flip ($) can't do that.

Ah, gotcha. You still won't be able to use (@) as an alias for return, since infix operators have to take two arguments.

And flip ($) will work fine, because its function argument is partially applied; remember that function application binds tighter than infix operators. So, the following works:

(@.) = flip ($)

putFourStrings :: String -> String -> String -> String -> IO ()
putFourStrings a b c d = putStrLn a >> putStrLn b >> putStrLn c >> putStrLn d

"fourth" @. putFourStrings "first" "second" "third"
-- first
-- second
-- third
-- fourth

(In addition: the very fact that return x >>= f a b c can do what you want it to indicates that x @. f a b c can do the same. In fact, the two are provably equivalent).

Hmm, interesting. I'll make sure to add that. Thanks!

So, this works for the example you gave, and for any Id -> … function, but it doesn't seem to work for IO Id -> …:

mutableStringWithString :: Id -> IO Id
mutableStringWithString str = (getClass "NSMutableString") @. stringWithString str

Is there some way to make an <@.> operator for lifting + applying + joining?

Yep! But you don't need to make a new operator. It's called (>>=). ;-)

mutableStringWithString str = 
    getClass "NSMutableString" >>= stringWithString str

That appears to drop str into the self position of stringWithString, though.

Nope. Remember, we're putting the receiver last.

You're right. Sorry, I've been hopping back and forth between a few different mental models today.

;) no worries