Robert Dockins wrote:
data Tuple a b = Tuple a !b
-- (a,b) === Tuple a (Tuple b ())-- (a,b,c) === Tuple a (Tuple b (Tuple c ())) -- etc...
A problem with this is that there's no way of supporting partially-applied tuple type constructors without some sort of type system extension. But maybe there's no reason to support this. Are there any situations where people use "(,) a" where they couldn't simply define a new product type? I rather like the following solution for traversable tuples: Introduce unboxed pairs (# a,b #) and unboxed unit (# #) into the language. Allow datatypes to be specialized at unboxed types. Define data Tuple a = Tuple a and let (a,b) = Tuple (# a,(# b,(# #)#)#) (a,b,c) = Tuple (# a,(# b,(# c,(# #)#)#)#) ... Then allow class instances to be declared at unboxed types and type parameters to be instantiated at unboxed types by doing compile-time specialization (like C++ templates), and voila. No problem of ordinary tuple access becoming less efficient, though some care would be needed to prevent typeclass-based traversal from taking O(n^2) time. Of course, the compile-time specialization part is a bit tricky. -- Ben