[Haskell-cafe] Extensible states
Alberto G. Corona
agocorona at gmail.com
Tue May 5 21:03:20 UTC 2015
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 at gmail.com>:
> 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.
> Best regards,
> Marcin Mrotek
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
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