[Haskell-cafe] Extensible states

[Marcin Mrotek]

Just as an afterthought about the O(n) lookup issue - extensible
records are dependently typed versions of old, well-known data
structures, so you always have a trade off between lookup and
appending. In Haskell it's relatively easy to implement dependently
typed lists (Vinyl, HList, etc), or maps (?) (apparently, Extensible
linked by Adam, but I admit I haven't looked into the code).
Dependently typed arrays are trickier. I guess it could be, for
example, hacked around a (Vector Dynamic), then using (fromJust .
fromDynamic) when it's certain at compile time that a there's a value
of a certain type hidden in there, but I'm not sure if anyone has
actually implemented it. They would necessarily have O(n) append,
though.

Best regards,
Marcin Mrotek