Hi, Greg Wever wrote:
What is the use case for changing the type of a record field on update?
Lots. For example, I often use in the context of abstract syntax parametrised on the type of some kind of "annotation". A transformation may then take an abstract syntax tree with one type of annotation onto an abstract syntax tree with a different type of annotation. And if I opted to use named fields for my abstract syntax tree nodes, as I tend to do, polymorphic record update is exactly what I need.
This seems to make things harder on the implementer and harder on users to reason about the types in their program.
Can't comment on the former, but I don't find the latter compelling. I find it completely natural and in a well-documented program there will be enough explicit type information around to make it pretty clear what is going on.
I think it would be easier on everyone to just remove this capability
I'd say that would be very unfortunate.
or have an explicit syntax for changing the type to make it easier for both implementer (to distinguish) and user (to declare intent).
Also sounds unfortunate, and might lead to very repetitive code? I have not followed this thread close enough to be able to say for sure, but if the upshot is that code like the one I am alluding to above would need an annotation wherever there is a polymorphic record update today, then that would not be very nice. For all its flaws, the present Haskell "record" system is far from as useless as it sometimes is made out to be, and it would be a pity if the price for fixing some of those flaws were to lose something as useful as update of polymorphic record fields. Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham nhn@cs.nott.ac.uk