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.zigans...@gmail.com
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