Monomorphism by annotation of type variables

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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 = unsafeLocalState $ 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.

For the moment, this proposal can be "emulated" using Haskell's type classes:

module Monomo where
import Prelude(Bool, Char, Double, Either, Float, Int, IO, Integer)
import Prelude(Maybe, String)
import Data.IORef(IORef)
import Data.STRef(STRef)
import Data.Time(UTCTime, NominalDiffTime, Day, TimeOfDay)
import Data.Time(LocalTime, TimeZone, ZonedTime)
import Data.Time(DiffTime)
import Data.Time(UniversalTime)
import Control.Monad.ST(ST)
import System.Directory(XdgDirectory, XdgDirectoryList, Permissions)
import System.IO(Handle, IOMode, BufferMode, SeekMode, HandlePosn)
import System.IO(TextEncoding, Newline, NewlineMode)
import GHC.Base(RealWorld)
class Monomo a
-- add suitable new instances as needed...
instance Monomo Bool
instance Monomo BufferMode
instance Monomo Char
instance Monomo Day
instance Monomo DiffTime
instance Monomo Double
instance (Monomo a, Monomo b) => Monomo (Either a b)
instance Monomo Float
instance Monomo Handle
instance Monomo HandlePosn
instance Monomo Int
instance Monomo Integer
instance Monomo (IO a)
instance Monomo IOMode
instance Monomo LocalTime
instance Monomo a => Monomo (IORef a)
instance Monomo a => Monomo [a]
instance Monomo a => Monomo (Maybe a)
instance Monomo Newline
instance Monomo NewlineMode
instance Monomo NominalDiffTime
instance Monomo Permissions
instance Monomo RealWorld
instance Monomo SeekMode
instance Monomo (ST s a)
instance Monomo a => Monomo (STRef s a)
instance Monomo TextEncoding
instance Monomo TimeOfDay
instance Monomo TimeZone
instance (Monomo a, Monomo b, Monomo c, Monomo d, Monomo e, Monomo f) => Monomo (a, b, c, d, e, f)
instance (Monomo a, Monomo b, Monomo c, Monomo d, Monomo e) => Monomo (a, b, c, d, e)
instance (Monomo a, Monomo b, Monomo c, Monomo d) => Monomo (a, b, c, d)
instance (Monomo a, Monomo b, Monomo c) => Monomo (a, b, c)
instance (Monomo a, Monomo b) => Monomo (a, b)
instance Monomo ()
instance Monomo UniversalTime
instance Monomo UTCTime
instance Monomo XdgDirectory
instance Monomo XdgDirectoryList
instance Monomo ZonedTime

Atravers 00:21, 7 January 2019 (UTC)