[Haskell-beginners] cascade of if statements

[Rustom Mody]

On Wed, Oct 31, 2012 at 10:44 PM, Daniel Trstenjak <
daniel.trstenjak at gmail.com> wrote:

>
> (<||>) :: Monad m => m Bool -> m Bool -> m Bool
> (<||>) m1 m2 = do
>    r1 <- m1
>    if r1 then return True else m2
>
>
We can do some algebraic simplification, using the law
*if P  then True else Q  *is same as *P || Q* :

(<||>) m1 m2 = do
   r1 <- m1
   r2 <- m2
   return (r1 || r2)

Which reminds me of the definition of liftM2
(from Control.Monad):

liftM2 f m1 m2          = do { x1 <- m1; x2 <- m2; return (f x1 x2) }

 And so we can golf down to:
(<||>) = liftM2 (||)

Now, instead of using liftM2 from Control.Monad, I could as well use liftA2
from Control.Applicative :

(<||>) = liftA2 (||)

So my questions to the experts:

1> Are these equivalent or am I missing something?
2> Any thumb rules on when to use Control.Monad and when to use
Control.Applicative?

Rusi
-- 
http://blog.languager.org
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