View source for Monomorphism by annotation of type variables

[[Category:Proposals]]
Introduce the reserved word <code>monomo</code> for rendering type variables monomorphic:

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

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:

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


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

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

<i>For the moment, this proposal can be "emulated" using Haskell's type classes:</i>
<tt>
: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
</tt>
----
See also:

* [https://gitlab.haskell.org/ghc/ghc/-/issues/19800 GHC ticket 19800: Allow monomorphism annotations for type variables]


[[User:Atravers|Atravers]] 00:21, 7 January 2019 (UTC)