[Haskell-cafe] Data creation pattern?

[Stephen Tetley]

Hi Eugene

Is something like this close to what you want:

For example this builds an object with ordered strings...

makeOrdered :: String -> String -> String -> Object
makeOrdered a b c = let (s,t,u) = sort3 (a,b,c) in Object s t u


Alternatively you could build the with the Strings as they appear in
the input then use an Object 'transformer' - a function from Object to
Object (fun :: Object -> Object), e.g

orderObject :: Object -> Object
orderObject (Object a b c) = Object s t u
  where
    s = min3 a b c
    t = med3 a b c
    u = max3 a b c


demo1 = orderObject (Object "4" "6" "1")



Support code


-- inefficient, but I had max3, etc to hand...
sort3 :: Ord a => (a,a,a) -> (a,a,a)
sort3 (a,b,c) = (s,t,u)
  where
    s = min3 a b c
    t = med3 a b c
    u = max3 a b c

-- Or without the intermediate triple:
--
-- sort3 :: Ord a => a -> a -> a -> (a,a,a)
-- sort3 a b c = (s,t,u)
--   where
--    s = min3 a b c
--    t = med3 a b c
--    u = max3 a b c

-- | max of 3
max3 :: Ord a => a -> a -> a -> a
max3 a b c = max (max a b) c

-- | min of 3
min3 :: Ord a => a -> a -> a -> a
min3 a b c = min (min a b) c


-- | median of 3
med3 :: Ord a => a -> a -> a -> a
med3 a b c = if c <= x then x else if c > y then y else c
  where
    (x,y)                 = order a b
    order p q | p <= q    = (p,q)
              | otherwise = (q,p)