This is obvious and all, but it suddenly clicked why you can’t choose different paths inside of Applicative. What I mean is that things of this form aren’t possible:

something = do p' <- getABool
               if p then putStrLn "it was true"
                    else putStrLn "it wasn't"

However, I couldn’t think why. Well, the reason is that the methods of Applicative are pure and (<*>) (obviously), which are like return and ap (also obviously). But! Note that with (<*>) (and ap) you can only work inside the functor:

mf `ap` mx ≡ mf >>= \f -> mx >>= \x -> f x

On other words, you would have to evaluate both x and y in something, somewhat like this:

something' p x y = do p' <- p
                      x' <- x
                      y' <- y
                      return (if p then x' else y')
 
-- or
somethingA p x y = f <$> p <*> x <*> y
    where f p x y = if p then x else y

…evaluating both x and y and (in IO) performing both their side effects.

I’m sure this explanation is terrible, but it helped me at least?

This entry was posted on Friday, August 29th, 2008 at 6.14 am and is filed under Haskell. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.