[Haskell-cafe] Extensible states

[Marcin Mrotek]

Okay, perhaps I'm too newbie to understand the big picture, but it
seems to me you can get either:

a) O(1) access to any, arbitrarily selected (at runtime) field
b) O(1) append

I guess option a) is better performance-wise, as appending is usually
done less often than selecting (an O(1) slice is already possible with
independently typed regular Haskell records) but
dependently-typed-list-based implementation, or at the very least
Vinyl (I haven't ever used HList) has the advantage of being dead
simple in both implementation and usage. I mean, with Vinyl, you can
write manual recursion over Rec's like:

foo :: Rec ... -> Rec ...
foo RNil = ...
foo (r :& rs) = ...

whenever GHC's typechecker gives up and goes on a strike; and I dare
to say, with commonly used record sizes (have you ever used a record
with more than, let's say, 10 fields?) the speed tradeoff is not
noticeable.

Don't get me wrong, I'm all in for cutting edge solutions like the one
mentioned by Marcos, but I do think that rejecting the existing ones
based on the non-constant time lookup is just premature optimisation.

Best regards,
Marcin Mrotek