Hello, the following code doesn't compile
<snip>
module Matrix
where
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
type Block s = STUArray s Int Double
data MMatrix s = MMatrix Int Int (Block s)
newMatrix_ :: Int -> Int -> ST s (MMatrix s)
newMatrix_ m n = do b <- newArray_ (1, m*n)
return (MMatrix m n b)
runMatrix = do _A <- newMatrix_ 3 3
_B <- newMatrix_ 3 3
matMul _A _B
return "Success"
main = show $ runST runMatrix
matMul :: MMatrix s -> MMatrix s -> ST s (MMatrix s)
--matMul a b = do let foo = 2*5
--return a
matMul a b = do { let foo = 2*5; return a }
</snip>
under ghc 6.4.1, yielding the error message:
question.hs:25:41: parse error on input `<-'
Failed, modules loaded: none.
The offending is line the one containing "let foo = 2*5", which is a little
test I've done of let-clauses. Now, suppose instead that for the last function,
matMul, I replace the version that's commented out. No errors!
Could someone enlighten me as to why? I'm a bit confused, as I thought the two
forms are equivalent save for formatting...
This is on the back of a email discussion that I was reading about let-clauses,
in which someone declared that they where better than where clauses for monadic
code. If anyone could comment on this, I'd appreciate it as well.
Many thanks in advance,
Martin
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