Re: [Haskell-cafe] guards in applicative style

Lorenzo is correct, but actually for the wrong reason. =) The *type* of guard is a historical accident, and the fact that it requires MonadPlus doesn't really tell us anything. Let's take a look at its implementation: guard :: (MonadPlus m) => Bool -> m () guard True = return () guard False = mzero 'return' is not specific to Monad; we could just as well use 'pure'. 'mzero' is a method of 'MonadPlus' but there is no reason we can't use 'empty' from the 'Alternative' class. So we could define guardA :: Alternative f => Bool -> f () guardA True = pure () guardA False = empty (As another example, consider the function 'sequence :: Monad m => [m a] -> m [a]'. Actually this function does not need Monad at all, it only needs Applicative.) However, guardA is not as useful as guard, and it is not possible to do the equivalent of the example shown using a list comprehension with a guard. The reason is that whereas monadic computations can make use of intermediate computed values to decide what to do next, Applicative computations cannot. So there is no way to generate values for x and y and then pass them to 'guardA' to do the filtering. guardA can only be used to conditionally abort an Applicative computation using information *external* to the Applicative computation; it cannot express a condition on the intermediate values computed by the Applicative computation itself. -Brent On Wed, Sep 12, 2012 at 03:52:03PM +0100, Lorenzo Bolla wrote:

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 wrote:

...

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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