Hello, Ralf!
In fact, he mentions that values can be retrieved by either type or
index, while only the former is discussed in detail, if I am right. To
me it seems, he would also need to end up using dependant-type
encoding of naturals, say data Zero ... and data Succ ..., for
look-up. If I
Hi Oleg, I like the polymorphic list indexed by Ints... there
do seem to be a couple of differences between this and the list
indexed by natural numbers.
The list indexed by integers cannot determine the type of the
return value through induction on the class... in other words
it cannot
MR K P SCHUPKE wrote:
Hi Oleg, I like the polymorphic list indexed by Ints... there
do seem to be a couple of differences between this and the list
indexed by natural numbers.
...
Agreed with these differences.
Another difference: it is initially a heterogeneous set
rather than list!
This is the latest varient. Let me know whether you guys find this
interesting, as I am coding what I need for a practical application,
it may be tedious for you... I am still of the opinion that some
kind of polygenious list (or maybe heteromorphic?) would be good in
the standard libraries,
Hi Kean,
looks cool.
I get your point about static typing.
I guess that adding a fold operator would make your implementation more
complete.
Oleg has also encountered some of your operations (as you probably know):
http://www.haskell.org/pipermail/haskell/2003-August/012355.html
(Oleg also
I did not know about Oleg's posting, as I originally said, I based my implementation on
a paper by Conor McBride. Oleg is addressing the question of type safe casting, rather
than generic storage, so his code is a bit different. Infact his class:
class TypeSeq t s where
type_index:: t - s -
I have written a first attempt at a fold function for the heterogenious list:
class RFold i r where
rFold :: (forall a . a - i - i) - i - r - i
instance RFold i RNil where
rFold f i RNil = i
instance RFold i r = RFold i (a `RCons` r) where
rFold f i (x `RCons` xs) = f x (rFold f i xs)
I have written a first attempt at a fold function for the heterogenious list:
class RFold i r where
rFold :: (forall a . a - i - i) - i - r - i
instance RFold i RNil where
rFold f i RNil = i
instance RFold i r = RFold i (a `RCons` r) where
rFold f i (x `RCons` xs) = f x (rFold f i xs)
Though I haven't tried it, the explicit 'Sat' dictionary representation
would probably work here, something like:
data ShowD a = ShowD { showD :: a - String }
-- our explicit dictionary for show, would need one of
-- these for each class we care about
-- the satisfaction class:
class
Ok... After playing with these types, I could not get it to work with
the satFold
below. However it did inspire me to try something else, and this seems
to work
quite well.
First redefine the RFold function to use RFoldFn class as its operator.
Then create
instances of RFoldFn to do what you
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
Didn't know If I should post it straight away... its quite long and I dont do
attachments (well not If I can help it. I am aware Dynamic can model heterogenious
lists
(thanks for correct terminology) - but I need static typing. Thats the clever thing
about
this code - the list is heterogenious
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