[Haskell-cafe] When you don't want to restrict the type of array...

Patrick Chilton chpatrick at gmail.com
Fri Aug 12 08:35:24 UTC 2016

First of all, the current standard for arrays is the vector
<https://hackage.haskell.org/package/vector> package, array is pretty
obsolete. Secondly, are you sure you want your functions to take both
immutable and mutable arrays? You could just make them take immutable ones,
and convert using (unsafe)freeze/thaw
Also, since you know that you'll only be storing Face and Vertex, you might
as well make an Unbox (or Storable) instance for them and use Unboxed (or
Storable) vectors.

Then you end up with:

import qualified Data.Vector.Unboxed as VU

data Surface = Surface { vertices :: VU.Vector Vertex, faces :: VU.Vector
Face }

render :: Surface -> IO ()


On Fri, Aug 12, 2016 at 7:12 AM, Christopher Howard <ch.howard at zoho.com>

> Hi. I recently learned how to use Haskell IArrays and MArrays, and they
> are quite convenient. I find myself often using some type of MArray for
> performance reasons, but also some IArray type for array data that is
> static in a set of computations.
> Then I started wanting to use arrays inside other kinds of data types,
> and so quickly ran into the question: what do you do if you want a data
> type to contain an array, but you don't know in advance what array type
> you'll be working with (immutable vs. mutible, boxed vs. unboxed).
> My exploration of this question led to discovering something called
> GADTs, which allowed me to do definitions like so
> data Surface a where
>   ISurface :: IArray a e => (a Int Vertex) -> (a Int Face) -> Surface a
>   MSurface :: MArray a e m => (a Int Vertex) -> (a Int Face) -> Surface a
> Am I heading down the right track here, or is there some better approach?
> --
> https://qlfiles.net
> My PGP public key ID is 0x340EA95A (pgp.mit.edu).
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