[Haskell-cafe] Extensible states

[Marcin Mrotek]

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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