STArray and Lazy ST

Hal Daume III hdaume@ISI.EDU
Thu, 8 May 2003 08:44:25 -0700 (PDT)

> formats/Graph.hs:91:
>     Could not deduce (MArray (STArray s) Bool (ST s))
>         from the context (Ix v)
>     Probable fix:
>         Add (MArray (STArray s) Bool (ST s))
>         to the type signature(s) for `contains'
>         Or add an instance declaration for (MArray (STArray s) Bool (ST s))
>     arising from use of `readArray' at formats/Graph.hs:91
>     In the definition of `foo': readArray a b
> The thing is, there is already an instance MArray (STArray s) e (ST s)! So
> what is the compiler complaining about? The problem is, it's not the right
> ST type! In the error message the name ST means two different things which
> is REALLY confusing and it took me a while to figure it out. One ST comes
> from Control.Monad.ST.Lazy and the other from Data.Array.ST.

In the compiler's defense, it's not ever mentioning the Strict.ST
variant.  It's simply saying that your function has a constraint:

  MArray (STArray s) Bool (ST s)

for whichever ST is in scope.  In your case it's Lazy.ST.  It's the same
(style) error message as when you say:

> foo :: Show a => a -> a
> foo x = x + 2

which says:

    Could not deduce (Num a) from the context (Show a)
    Probable fix:
	Add (Num a) to the type signature(s) for `foo'
    arising from use of `+' at /nfs/isd/hdaume/Foo.hs:4
    In the definition of `foo': x + 2

You've said "Show a", but you need to also say "Num a".

As for the second question, you'd have to roll your own LazySTArray
module.  Probably the easiest way to do this would be to grab the source
for STArray from CVS (you can even use the web interface) and then change
it as necessary.

Someone else can probabaly answer this better, but I don't think
LazySTArrays are going to be very efficient.  I can't, of course, back
this up, but I think there's going to be either space or time leaks with
this.  Something you'll just have to try...

 - Hal