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): a ||| b | isDefaultValue a = b | otherwise = a a &&& b | isDefaultValue a = defaultValue | otherwise = b -- Class of types having default value: class Defaults a where defaultValue :: a instance Defaults () where defaultValue = () instance Defaults Bool where defaultValue = False instance Defaults [a] where defaultValue = [] instance Defaults (a->a) where defaultValue = id instance Defaults (Maybe a) where defaultValue = Nothing instance Defaults (a->IO a) where defaultValue = return instance Defaults a => Defaults (IO a) where defaultValue = return defaultValue instance Defaults Int where defaultValue = 0 instance Defaults Integer where defaultValue = 0 instance Defaults Double where defaultValue = 0 -- Class of types that can be tested for default value: class TestDefaultValue a where isDefaultValue :: a -> Bool instance TestDefaultValue Bool where isDefaultValue = not instance TestDefaultValue [a] where isDefaultValue = null instance TestDefaultValue Int where isDefaultValue = (==0) instance TestDefaultValue Integer where isDefaultValue = (==0) instance TestDefaultValue Double where isDefaultValue = (==0) usage examples: return$ (isDir &&& addTrailingPathSeparator) filespec openWebsite$ (isWindows &&& windosifyPath) file let options = (compressionEnabled &&& cvt "-m" compressionMethod')++ (encryptionEnabled &&& cvt "-ae=" encryptionMethod')++encryptionOptions++ (protectionEnabled &&& cvt "-rr" protectionMethod') let another_msg = ciphers ||| "not encrypted" let msg = ftLocked footer &&& "yes" ||| "no" -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com