Re: Proposal: FirstClassFieldUpdates

28 Jul 2009


      Jon Fairbairn wrote:
...
Isaac Dupree

writes:
...
Jon Fairbairn wrote:
...
Parenthesis around updates would make them into functions, ie
({a=1,b=2,...}) would mean the same as (\d -> d{a=1,b=2,...}), but be
more concise.
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:
oh maybe I got confused.  (My confusion also could have been the result 
of a bug that was recently fixed in GHC that affected the type of some 
cases like that where there are multiple constructors...)

-Isaac