#8334: unexpected type inference failure in main ------------------------------------+------------------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I'm not sure if this is a failure of me understanding the semantics of type families, or something where inference should work! Or maybe its just me failing to understand the rules / convention for main. {{{ {-# LANGUAGE MagicHash, UnboxedTuples #-} module PrefetchTest where import GHC.Prim import Data.Vector.Storable.Mutable -- import Foreign.ForeignPtr -- (unsafeForeignPtrToPtr) import Foreign.Ptr import GHC.ST import Data.Primitive.ByteArray import Control.Monad.Primitive -- newByteArray :: PrimMonad m => Int -> m (MutableByteArray (PrimState m)) -- sameMutableByteArray :: MutableByteArray s -> MutableByteArray s -> Bool --unsafeFreezeByteArray :: PrimMonad m => MutableByteArray (PrimState m) -> m ByteArray -- unsafeThawByteArray :: PrimMonad m => ByteArray -> m (MutableByteArray (PrimState m)) --main :: IO () main = do mv1 <- newByteArray 17 v1 <- unsafeFreezeByteArray mv1 return () }}} i get {{{ refetch.hs:42:5: No instance for (Monad m0) arising from a do statement The type variable ‛m0’ is ambiguous Relevant bindings include main :: m0 () (bound at prefetch.hs:41:1) Note: there are several potential instances: instance Monad (ST s) -- Defined in ‛GHC.ST’ instance Monad ((->) r) -- Defined in ‛GHC.Base’ instance Monad IO -- Defined in ‛GHC.Base’ ...plus one other In a stmt of a 'do' block: mv1 <- newByteArray 17 In the expression: do { mv1 <- newByteArray 17; v1 <- unsafeFreezeByteArray mv1; return () } In an equation for ‛main’: main = do { mv1 <- newByteArray 17; v1 <- unsafeFreezeByteArray mv1; return () } prefetch.hs:42:12: No instance for (PrimMonad m0) arising from a use of ‛newByteArray’ The type variable ‛m0’ is ambiguous Relevant bindings include main :: m0 () (bound at prefetch.hs:41:1) Note: there are several potential instances: instance PrimMonad IO -- Defined in ‛Control.Monad.Primitive’ instance PrimMonad (ST s) -- Defined in ‛Control.Monad.Primitive’ In a stmt of a 'do' block: mv1 <- newByteArray 17 In the expression: do { mv1 <- newByteArray 17; v1 <- unsafeFreezeByteArray mv1; return () } In an equation for ‛main’: main = do { mv1 <- newByteArray 17; v1 <- unsafeFreezeByt }}} this is because main seems to not be assumed to have type IO () If i add that type ascription to main, or I comment out the name, type inference succeeds. I understand and know we have special treatment for whats the main module, but this is an example of that special treatment changing the type inference! Is that a known issue? (its obvious to me now, but is it reasonable?) this is pretty much a nonissue, but i think its still worth noting. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8334 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler