Malcolm Wallace wrote:
Yes, I find it interesting that consecutive updates are not equivalent to a combined update. I believe this is largely because named fields are defined as sugar - they behave in some sense like textual macros in other languages, which can often turn out to have unexpected properties.
Christian Maeder
I share your characterization "like textual macros". The question is, should this be eliminated?
Although the behaviour is initially surprising, I suppose it is easily explained. After all, an equivalent expression using lambdas would have the same typing problem: data A a = A { one, two :: a } update v@(A{}) = v { one = Void } { two = Void } update' v@(A{}) = (\A one two -> A one Void) $ (\A one two -> A Void two) $ v Because of this, I would be reluctant to define that consecutive field updates can be semantically amalgamated into a single update.
Would it be cleaner, though less convenient some times, if the types of updated values must not change?
I believe this is a very different question from the consecutive update one. By analogy with the lambda equivalent, I can see no reason to outlaw a type change, where all the relevant types change at the same time: update_ok v@(A{}) = v { one=Void, two=Void } update_ok' v@(A{}) = (\A one two -> A Void Void) v Regards, Malcolm