How to use "bind" keyword instead of ">>=" operator in Haskell

Question

I'm trying to understand Haskell monads and wrote this test program, which compiles and works as expected:

divide :: Int -> Int -> Either String Int
divide _ 0 = Left "Divide by zero error
divide numerator denom = Right (numerator `div` denom)
processNumsMonadically :: Int -> Int -> Either String Int
processNumsMonadically n d = divide n d >>= \q -> return (q+1)

When I try using the word bind instead of the >>= operator in the latter function definition:

processNumsMonadically n d = bind (divide n d) (\q -> return (q+1))

I get the error:

Not in scope: 'bind'

What is the correct way to use the word bind?

• sdcvvc: Note that bind is not a keyword. It's a name people use to refer to >>= operator. You can define bind = (>>=) if that helps.
• Ellen Spertus: Thank you for correcting me. I should have known it was a name, not a built-in keyword. I'll edit my post (and leave this comment if you leave yours).
• leftaroundabout: Indeed almost nothing is a keyword in Haskell (class, instance, type, newtype, data, case'of, do, if'then'else, let'in, where: those are pretty much all that can occur in the actual code part. Plus a few more which are only used in the module header, and a few from GHC extensions – the full list is here.) All other words are simply names of functions/actions, defined in some library (if not in your own code); you can use Hoogle or Hayoo to find them.

Answer 1

This isn't a part of Prelude; it resides in Control.Monad.Extra, a part of monad-extras package.

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However, you can call operators in prefix manner (like named functions) easily:

processNumsMonadically n d = (>>=) (divide n d) (\q -> return (q+1))

You could also just use do notation:

processNumsMonadically n d = do
 q <- divide n d
 return (q+1)

But while we're at it, I'd write using a Functor:

processNumsMonadically n d = fmap (+1) (divide n d)

or Applicative syntax:

processNumsMonadically n d = (+1) <$> divide n d

You could also lift the +1 to avoid the need for return and the lambda.

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As a personal style remark, bind used as a word isn't idiomatic, and IMHO you shouldn't use it.

• Ellen Spertus: Thank you for the answer and advice. FYI, I wasn't planning to use bind as a word in programs, just as an intermediate step when teaching monads. I appreciate the do syntax.
• Bartek Banachewicz: @espertus sorry, can't help it; I just keep seeing the more and more ways to write the function :)
• Ellen Spertus: Thanks for the improvements, although I'm just trying to stay one step ahead of my students. :-)