That’s the Kleisli composition operator for monads. It allows you to compose functions with signatures like 'a -> M<'b> and 'b -> M<'c'> where M is monadic: in your case the Result<'t> from the linked article.
>=> is really just a function composition, but >> wouldn’t work here since the return type of the first function isn’t the argument of the second one – it is wrapped in a Result<'t> and needs to be unwrapped, which is exactly what >=> implementation does.
It could be defined in terms of >>= as well:
let (>=>) f1 f2 arg =
f1 arg >>= f2
It seems that Haskell’s Control.Monad package uses this definition. The full type signature might also be helpful (taken from here):
-- | Left-to-right Kleisli composition of monads.
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c)
f >=> g = \x -> f x >>= g
Yet another fun fact is that Kleisli composition makes the three monad laws easier to express by only using functions (and in my opinion it makes them much clearer):
- Left identity:
return >=> g ≡ g - Right identity:
f >=> return ≡ f - Associativity:
(f >=> g) >=> h ≡ f >=> (g >=> h)