Barney Hilken
Tue, 05 Feb 2008 05:41:59 -0800
new record (x = 3,y = 4) subtraction \r -> ( x = 3 | r - x) replacement \r -> (x := 3 | r) (equivalent to the above) type (x::Int,y::Char) degenerate cases: empty record (|) subtracting a label (| r - x) a record can always be determined by the presence of a '|' within parenthesis.
One of the advantages of the systems with richer polymorphism and more predicates is that they need less syntax. It is possible (once you have solved the permutation/scoping problem) to use constructors as labels, and define all the basic operators on records as standard Haskell functions. With this approach you can even treat labels as "first-class citizens" and write polymorphic record zip:
labelZip :: ({n :: a} `Disjoint` {m :: b}) => n -> m -> [a] -> [b] -> [{n :: a, m :: b}]
labelZip n m = zipWith (\x y -> {n := x, m := y})
But no-one knows whether this extra expressive power has an
unacceptable cost in terms of extra complexity, because no-one has
implemented and used these systems seriously.
Barney. _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell