I would like to see your code indeed ... it seems the attachment was missing. Anyway, I am not sure if it obvious or not, but heterogenously typed lists can be nicely modelled with Data.Typeable (!!!) I guess we should add something like this to the module? See http://www.cs.vu.nl/boilerplate/testsuite/hlist.hs or the inlined code below Regards, Ralf -- Heterogeneously typed lists data HList = HNil | forall a. Typeable a => HCons a HList -- The empty list initHList :: HList initHList = HNil -- Add an entry addHList :: Typeable a => a -> HList -> HList addHList a l = HCons a l -- Test for an empty list nullHList :: HList -> Bool nullHList HNil = True nullHList (HCons _ _) = False -- Retrieve head by type case headHList :: Typeable a => HList -> Maybe a headHList HNil = Nothing headHList (HCons a _) = cast a -- Retrieve head by type case tailHList :: HList -> HList tailHList HNil = error "tailHList" tailHList (HCons _ l) = l -- Access per index; starts at 1 nth1HList :: Typeable a => Int -> HList -> Maybe a nth1HList i l | i < 1 || i == 0 && nullHList l = error "nth1HList" nth1HList 1 l = headHList l nth1HList i l = nth1HList (i-1) (tailHList l) ---------------------------------------------------------------------------- -- A demo list mylist = addHList (1::Int) $ addHList (True::Bool) $ addHList ("42"::String) $ initHList -- Main function for testing main = print ( show (nth1HList 1 mylist :: Maybe Int) -- shows Maybe 1 , ( show (nth1HList 1 mylist :: Maybe Bool) -- shows Nothing , ( show (nth1HList 2 mylist :: Maybe Bool) -- shows Maybe True , ( show (nth1HList 3 mylist :: Maybe String) -- shows Maybe "42" )))) MR K P SCHUPKE wrote:

I needed a list which could handle items of different types for the database code I am writing. I have written a module implementing such a list based on dependant types (from Conor McBride: Faking It; Simulating Depandant Types in Haskell). Although McBride does not mention lists/vectors with items of differing types, the solution to implementing them came from his 'nthFront' function for re-arranging the order of arguments to a function.

Any type can be inserted into the list, which supports head/tail/init/last, as well as indexed lookup, and a cartesian-product (concatenating two lists together). I have included fromTuple/toTuple as well.

This seems quite a useful construct, and if there is nothing similar in the standard libraries at the moment, do you think this is worth including?

Regards, Keean Schupke. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users