[Haskell-cafe] Bool as type class to serve EDSLs.

[Nicolas Pouillard]

Excerpts from Bulat Ziganshin's message of Thu May 28 15:07:02 +0200 2009:
> Hello Lennart,
> 
> Thursday, May 28, 2009, 11:57:09 AM, you wrote:
> 
> > -- | Generalization of the 'Bool' type.  Used by the generalized 'Eq' and 'Ord'.
> > class Boolean bool where
> >     (&&)  :: bool -> bool -> bool   -- ^Logical conjunction.
> >     (||)  :: bool -> bool -> bool   -- ^Logical disjunction.
> 
> i use another approach which imho is somewhat closer to interpretation
> of logical operations in dynamic languages (lua, ruby, perl):

I tend to prefer Monoid:

(|||) :: (Monoid a, Eq a) => a -> a -> a
a ||| b | a == mempty = b
        | otherwise   = a

(&&&) :: (Monoid a, Eq a, Monoid b) => a -> b -> b
a &&& b | a == mempty = mempty
        | otherwise   = b

In particular I also like this one (however I would like another name):

mapNonEmpty :: (Eq a, Monoid a, Monoid b) => (a -> b) -> a -> b                                       
mapNonEmpty f x | x == mempty = mempty                                                                
                | otherwise   = f x                                                                   

Best regards,

-- 
Nicolas Pouillard
http://nicolaspouillard.fr