... for the same reason as this one doesn't get through: import Control.Monad.ST import Data.Array.ST main = print $ runST $ do return () ... but this one does: import Control.Monad.ST import Data.Array.ST main = print $ runST ( do return ()) it's all about rank-2 types; see SPJ's et al. paper on type inference for these types. However, I guess that the jury is still out, say this specific rank-2 behavior may be revised (and I also hope so). HTH Ralf
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- bounces@haskell.org] On Behalf Of Bulat Ziganshin Sent: Tuesday, January 03, 2006 2:28 AM To: haskell-cafe@haskell.org Subject: [Haskell-cafe] ST monad
Hello
the following code can't go through typechecking. can anyone help me to fix it or, better, let me know what i need to read to fix it myself? :)
import Control.Monad.ST import Data.Array.ST main = print $ runST $ do arr <- newArray (1,10) 127 a <- readArray arr 1 writeArray arr 1 216 b <- readArray arr 1 return (a,b)
PS: error message is
b.hs:4:15: Inferred type is less polymorphic than expected Quantified type variable `s' escapes Expected type: ST s a -> b Inferred type: (forall s1. ST s1 a) -> a In the first argument of `($)', namely `runST' In the second argument of `($)', namely `runST $ (do arr <- newArray (1, 10) 127 a <- readArray arr 1 writeArray arr 1 216 b <- readArray arr 1 return (a, b))'
-- Best regards, Bulat mailto:bulatz@HotPOP.com
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