Thanks all of you.

So there is no trick that can make extensible records O(1) for field access, like the native haskell records?. I didn´t know that all the extensible records have O(n) or O(log n) at most.

That is not better than my State monad with a Data.Map. It is not possible to use HList-like records like the one that Adam mentioned since the type signature must not change when a new field is added.

2015-05-05 22:20 GMT+02:00 Marcin Mrotek <marcin.jan.mrotek@gmail.com>:

Hello,

I'm not sure if this is what you're looking for, but vinyl + lens state monad combinators let you write something like this:

foo :: State (Rec Foo [B,C,F]) Bar
foo = ...

bar = State (Rec Foo [A,B,C,D,E,F]
bar = do
...
x <- zoom rsubset foo
...
rlens SA .= 3
...

Unfortunately Vinyl has O(n) lookup (unless it gets optimized away by sufficiently glorious haskell compiler, I guess, but I have no idea whether it actually can happen). But I'm not sure if the speed impact is noticeable, compared to using monad transformer stacks, for example.

http://hackage.haskell.org/package/vinyl
http://hackage.haskell.org/package/vinyl-0.5.1/docs/Data-Vinyl-Lens.html
http://hackage.haskell.org/package/lens
https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/a-little-lens-starter-tutorial

Best regards,
Marcin Mrotek

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Alberto.