[Haskell-cafe] Getting my mind around UArray -> STUArray conversion

[Scott Michel]

On Fri, Jun 19, 2009 at 7:08 PM, Dan Doel<dan.doel at gmail.com> wrote:
> Oops, I replied too hastily.
>
> What I wrote in my first mail is a problem, as witnessed by the "ix" and "ix1"
> in the error message. However, it isn't the main error. The main error is that
> you have a monadic expression, with type something like:
>
>    ST s (UArray ix e)
>
> but the return type of your function is:
>
>    UArray ix e
>
> To make a no-op you need to add a runST, something like:
>
>    runST (unsafeThaw mem >>= unsafeFreeze)

Actually, I probably wanted runSTUArray. :-)

But even then, I can't manage to get wombat to compile correctly. I'm
starting to think that MArray is itself a WOMBAT (waste of money,
brains and time), for three reasons:

a) Overly restrictive Monads in which implementation is supported
(i.e., ST and IO)
b) The triviality of the examples gives no insight as to how they
could be used, other than a create array, do something completely
trivial and freeze.
c) They are evidently very hard to use in a general sense.

Even google-ing for examples just comes up with trivial examples of
MArray usage.

You might ask why I might need a MArray? I'm investigating the
feasibility of building a cycle accurate PPC750 emulator. Memory
emulates better as a mutable array. I'd like to be somewhat more
general because the various systems with which I deal aren't
necessarily 32-bit, sometimes they are 16-bit and sometimes they
aren't PPC750 (different systems hooked to a common bus.) Classes with
rank-n types looked like a good approach to solving this particular
design problem, with a default implementation.

I did try out your suggestions and here's what the code looks like now.
----
{-# LANGUAGE FlexibleContexts, RankNTypes, ScopedTypeVariables #-}
module Wombat where

import Control.Monad.ST;
import Data.Array.ST;
import Data.Array.Unboxed;
import Data.Array.MArray;

wombat :: forall e ix s. (IArray UArray e, Ix ix, MArray (STUArray s)
e (ST s)) => e -> ix -> UArray ix e -> UArray ix e
wombat val idx mem = runSTUArray (unsafeThaw mem >>= return)
----

GHCi says:
[1 of 1] Compiling Wombat           ( wombat.hs, interpreted )

wombat.hs:11:34:
    Could not deduce (MArray (STUArray s1) e (ST s1))
      from the context ()
      arising from a use of `unsafeThaw'
                   at wombat.hs:11:34-47
    Possible fix:
      add (MArray (STUArray s1) e (ST s1)) to the context of
        the polymorphic type `forall s. ST s (STUArray s ix e)'
      or add an instance declaration for (MArray (STUArray s1) e (ST s1))
    In the first argument of `(>>=)', namely `unsafeThaw mem'
    In the first argument of `runSTUArray', namely
        `(unsafeThaw mem >>= return)'
    In the expression: runSTUArray (unsafeThaw mem >>= return)