Hello haskell-cafe, just one more nice snippet of code: class Defaults a where defaultValue :: a instance Defaults Bool where defaultValue = False instance Defaults [a] where defaultValue = [] instance Defaults (a->a) where defaultValue = id instance Num a => Defaults a where defaultValue = 0 class TestDefaultValue a where isDefaultValue :: a -> Bool instance TestDefaultValue Bool where isDefaultValue = not instance TestDefaultValue [a] where isDefaultValue = null instance Num a => TestDefaultValue a where isDefaultValue = (==0) -- This function is useful for defining default values, -- such as (directory ||| ".") or (memsize ||| 10*mb) a ||| b | isDefaultValue a = b | otherwise = a -- Useful for short-cutting boolean expressions, like this: -- putStr$ (need_separator &&& "\n\n") ++ (need_attention &&& map toUpper) str a &&& b | isDefaultValue a = defaultValue | otherwise = b -- |Apply function to value only if it is not empty unlessNull f xs = xs &&& f xs -- |Recursive list processing until list end, -- for example 'recursive (splitAt n)' splits list -- into the chuunks of n elements recursive :: ([a]->(b,[a])) -> [a] -> [b] recursive f list = list &&& (x:recursive f xs) where (x,xs) = f list -- |Split list into parts whose size defined by calling len_f -- at the yet unprocessed parts of list splitByLen :: ([a]->Int) -> [a] -> [[a]] splitByLen len_f = recursive (\xs -> splitAt (len_f xs) xs) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com