Re: Proposal: FirstClassFieldUpdates

Hi all, Jon Fairbairn wrote:

Isaac Dupree wrote:

...

yes it is, however field updates are occasionally slightly annoying, since they can't change something's type at all, IIRC. Say, data C nx ny = C { x :: nx, y :: ny } x_set :: nx2 -> C nx1 ny -> C nx2 ny --x_set x2 c = c {x = x2} --type error

If that were the case, it would be a serious wart that needed to be addressed. However, implementation would be fairly straightforward as an "extension" is already present in ghc:

let x_set x2 c = c{x=x2} :t x_set x_set :: nx -> C t ny -> C nx ny

;-) (or did I misunderstand you?)

Indeed, field update can change the type, which I guess is the point Jon is making. And Isaac's example above compiles without any problems or extensions. H98 as far as I'm aware. However, there are cases where the current field update facility arguably isn't good enough, namely when there are more than one constructor, some of which do not have certain of the fields. See http://hackage.haskell.org/trac/haskell-prime/wiki/ExistingRecords for examples, in particular "Polymorphic Record Update take II". As an aside, having revisited that discussion, I still think the idea of being able to do an update for constructors that do NOT have certain fields makes a lot of sense. The record wildcard extension, that was mentioned as a possible workaround, seems far less appealing and flexible. I don't even know what it would mean if it's used more than once in a scope, e.g. what about the following: f (C1 {a = 1, ..}) (C2 {b = 2, ..}) = (C1 {a = 10, ..}, C2 {b = 20, ..}) ??? Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham nhn@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.