State Transformer

Ashley Yakeley
Fri, 11 Jan 2002 23:31:55 -0800

At 2002-01-11 06:18, Jorge Adriano wrote:

>The whole problem is with passing the 'r' as a parameter, which is precisly 
>what I'm trying to avoid.

You could always pass it implicitly (using -fglasgow-exts):

testfunc = do
           r <- newSTRef ('x',0)
            with ?ref = r)
           (c,n) <- readSTRef r
           return n

foo :: (Num a,?ref :: STRef s (Char,a)) => ST s ()
foo = do
        (c,n) <- readSTRef ?ref
        writeSTRef ?ref ('a', n+1)

bar :: (Num a,?ref :: STRef s (t,a)) => ST s ()
bar = do
        (c,n) <- readSTRef ?ref
        writeSTRef ?ref (c,n+2)

tryTestFunc = runST testfunc

Curiously, GHC isn't smart enough to infer the types of foo and bar by 

Ashley Yakeley, Seattle WA