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 [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
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