[Haskell-beginners] cascade of if statements
Rustom Mody
rustompmody at gmail.com
Sat Nov 3 09:37:21 CET 2012
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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