Monomorphism by annotation of type variables

Introduce the reserved word monomo for rendering type variables monomorphic:

         primitive newIORef :: monomo a . a -> IO (IORef a)

Its use in the type signatures for such primitives would prevent the definition of polymorphic references in all contexts - the (abbreviated) canonical example of abuse:

         let v = ... $ newIORef undefined in
         do writeIORef v ("0" :: [Char])
            n <- readIORef v
            return (n + 1 :: Int)

...would cause a type error - the type of undefined is also made monomorphic by the annotated type of newIORef, resulting in the type monomo a . IORef a for v, preventing its dual specialisation to IORef [Char] and IORef Int.

Definitions relying on the old behaviour would be modified to use, or replaced by new primitives.

Atravers 00:21, 7 January 2019 (UTC)