On Wed, 2004-06-30 at 12:54, Duncan Coutts wrote:
To restate the question for Haskell-Cafe readers: Is it possible to return an arbitrary unboxed array that was constructed in the ST monad (as an STUArray)?
The issue is that you end up with a MArray class constraint that involves the state thread's 's' parameter, but this type variable gets 'hidden' by runST which universally quantifies over it. runST :: forall a. (forall s. ST s a) -> a
I'm inclined to believe it is impossible. The author of ghc's mutable array libraries (probably Simon M) notes in a comment in the code: -- I don't know how to write a single rule for listUArrayST, because -- the type looks like constrained over 's', which runST doesn't -- like. In fact all MArray (STUArray s) instances are polymorphic -- wrt. 's', but runST can't know that. For that reason the modules contains rewrite rules for each unboxable type rather than a single polymorphic rule: {-# RULES "listArray/UArray/Bool" listArray = \lu (es :: [Bool]) -> runST (listUArrayST lu es >>= unsafeFreezeSTUArray) "listArray/UArray/Char" listArray = \lu (es :: [Char]) -> runST (listUArrayST lu es >>= unsafeFreezeSTUArray) "listArray/UArray/Int" listArray = \lu (es :: [Int]) -> runST (listUArrayST lu es >>= unsafeFreezeSTUArray) etc... Duncan