On Sat, Nov 14, 2015 at 10:52 AM, Kim-Ee Yeoh
What we're seeing is special handling for stage 3: reporting errors in a do-block. Sometimes it helps, sometimes it hinders. For instance, the derivation of this signature is confusing indeed:
IO System.IO.Handle -> (System.IO.Handle -> IO Data.ByteString.ByteString) -> Data.ByteString.ByteString
To elaborate, the error message comes from the compiler choking on the monadic bind (>>=) that reveals itself in the desugared code. The (>>=) results from desugaring file <- openBinaryFile filename ReadMode into openBinaryFile filename ReadMode >>= \file -> ... The compiler knows the type signature of (>>=), which wants to see IO Handle -> (Handle -> IO ByteString) -> IO ByteString But the signature of the top-level function insists on IO Handle -> (Handle -> IO ByteString) -> ByteString This is what's the compiler's trying to say. -- Kim-Ee