I'm no expert at all, but I would say "no".
"guard" type is:
guard :: MonadPlus m => Bool -> m ()
and "MonadPlus" is a monad "plus" (ehm...) mzero and mplus
(http://en.wikibooks.org/wiki/Haskell/MonadPlus).
On the other hand Applicative is "less" than a monad
(http://www.haskell.org/haskellwiki/Applicative_functor), therefore
"guard" as is cannot be defined.
But, in your specific example, with lists, you can always use "filter":
filter (uncurry somePredicate) ((,) <$> list1 <*> list2 (somePredicate ???))
hth,
L.
On Wed, Sep 12, 2012 at 3:40 PM, felipe zapata
Hi Haskellers,
Suppose I have two list and I want to calculate the cartesian product between the two of them, constrained to a predicate. In List comprehension notation is just
result = [ (x, y) | x <- list1, y <-list2, somePredicate x y ]
or in monadic notation
result = do x <- list1 y <- list2 guard (somePredicate x y) return $ (x,y)
Then I was wondering if we can do something similar using an applicative style
result = (,) <$> list1 <*> list2 (somePredicate ???)
The question is then, there is a way for defining a guard in applicative Style?
Thanks in advance,
Felipe Zapata.
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