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 = 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.

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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

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See also:

• GHC ticket 19800: Allow monomorphism annotations for type variables

Atravers 00:21, 7 January 2019 (UTC)