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