Re: [Haskell-cafe] Beginner problems with 'triple' code

27 Jul 2004


      Stu White  writes:
...
type Triple = Triple {
data Triple = Triple {	-- 'type' is for synonyms, 'data' for user-defined
...
entry 1 :: a;
    entry 2 :: a;
    entry 3 :: a;
entry_1 :: a;		-- field names must be a single identifier
      entry_2 :: a;
      entry_3 :: a;
...
sortTriple :: Triple a -> Triple a;
sortTriple :: (Ord a) => Triple a -> Triple a
				-- an ordering constraint on the 'a' is implied
...
} deriving (Show, Eq)
where
-- You can't have a 'where' clause on a datatype definition. Presumably you
	-- are trying to express that the 'sortTriple' component is invariant.  If
	-- so, then remove it from the datatype and write it at the toplevel.  If
	-- you rather mean that it has a default definition that can be overridden,
	-- then you can keep it in the datatype, but you will need to add the default
	-- functional value to each new Triple you create, perhaps with a 'smart'
	-- constructor:
	--   mkTriple x y z = Triple x y z sortTriple
...
sortTriple t@(Triple x y z)
        | (y
sortTriple t@(Triple x y z s)	-- you omitted the fourth component of the Triple
          | (y

Re: [Haskell-cafe] Beginner problems with 'triple' code

Malcolm Wallace