[Haskell-cafe] vector recycling

Ben midfield at gmail.com
Sat Apr 17 18:07:39 EDT 2010

On Fri, Apr 16, 2010 at 11:19 PM, Roman Leshchinskiy <rl at cse.unsw.edu.au> wrote:
> That said, it would be quite possible to provide something like the following:
> fold_inplace :: Vector v a => (v a -> b -> v a) -> v a -> [b] -> v a

as far as i understand there would be two ways of writing such a
function : 1) to use mutable vectors monadically underneath and hide
them inside some kind of unsafeX, or 2) to give a specialized fold
with sufficient hints to the compiler to use the rewriting framework.

i attempted to start 1, but it seems like this function can not have
an entirely pure interface and still avoid copying.  more
specifically, the signature for the update function

(v a -> b -> v a)

seems like it necessarily creates a copy, unless i misunderstand?
wouldn't you need some kind of monadic update function

(v a -> b -> m () )

which modifies the vector in place?

> This could use the recycling framework to safely do as much in-place as possible while still preserving a purely functional interface. I have to think about it. Really, this looks like just a poor man's substitute for linear types.

although i am supposed to know something about category theory, since
my training is in math, i don't know about girard's later work.  is
there a short precis you can give (or a pointer?)

thanks and best regards, ben

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