John Meacham writes:
This was brought up in passing in a recent conversation on haskell-cafe
http://www.haskell.org//pipermail/haskell-cafe/2005-March/009321.html
It certainly seems like an interesting idea, Would type inference still work okay?
The other problem mentioned is that they are not quite isomorphic, since HLists are equivalant to (a,(b,c)) rather than (a,b,c), but changing HCons so that it is strict in its second argument makes them behave the same I think..
Since most of the HList functionality is defined using type classes, we
could probably declare a new type, TCons, make it strict in the second
argument, and use it alongside HCons.
data TCons a b = TCons a !b
One way t make tuples into sugar for HLists would be to effectively have
a series of declarations like these:
type (a,b) = TCons a (TCons b HNil)
type (a,b,c) = TCons a (TCons b (TCons c HNil))
But then we can't use tuples in instance declarations. That is, there
isn't any way to desugar 'instance Functor ((,) a)' without using a type
lambda.
On the other hand, using HLists for tuples means we can have projection
functions that work on any tuple arity, which would be nice.
--
David Menendez