3 files changed, 69 insertions, 2 deletions
diff --git a/libraries/ghc-prim/GHC/Magic/Dict.hs b/libraries/ghc-prim/GHC/Magic/Dict.hs
new file mode 100644
index 0000000000..12861db568
--- /dev/null
+++ b/libraries/ghc-prim/GHC/Magic/Dict.hs
@@ -0,0 +1,44 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ImpredicativeTypes #-}
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE Unsafe #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.Magic.Dict
+-- Copyright   :  (c) The University of Glasgow 2009
+-- License     :  see libraries/ghc-prim/LICENSE
+--
+-- Maintainer  :  cvs-ghc@haskell.org
+-- Stability   :  internal
+-- Portability :  non-portable (GHC Extensions)
+--
+-- Defines the 'withDict' function. For more information, see
+-- @Note [withDict]@ in "GHC.HsToCore.Expr" in GHC.
+-- The definition of 'withDict' is located in a separate module from
+-- "GHC.Magic" because 'withDict' is @Unsafe@ (it threatens type class
+-- coherence) while "GHC.Magic" is @Trustworthy@.
+--
+-- Use "GHC.Exts" from the @base@ package instead of importing this
+-- module directly.
+--
+-----------------------------------------------------------------------------
+
+module GHC.Magic.Dict (withDict) where
+
+import GHC.Prim.Panic (panicError)
+import GHC.Types (RuntimeRep, TYPE)
+
+-- | @'withDict' d f@ provides a way to call a type-class–overloaded function
+-- @f@ by applying it to the supplied dictionary @d@.
+--
+-- 'withDict' can only be used if the type class has a single method with no
+-- superclasses. For more (important) details on how this works, see
+-- @Note [withDict]@ in "GHC.HsToCore.Expr" in GHC.
+withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r
+{-# NOINLINE withDict #-}
+withDict = panicError "Non-rewritten withDict"#
diff --git a/libraries/ghc-prim/changelog.md b/libraries/ghc-prim/changelog.md
index 6c7723068f..1ce61e2e61 100644
--- a/libraries/ghc-prim/changelog.md
+++ b/libraries/ghc-prim/changelog.md
@@ -1,3 +1,25 @@
+## next (edit as necessary)
+
+- `magicDict` has been renamed to `withDict` and is now defined in
+  `GHC.Magic.Dict` instead of `GHC.Prim`. `withDict` now has the type:
+
+  ```
+  withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r
+  ```
+
+  Unlike `magicDict`, `withDict` can be used without defining an
+  intermediate data type. For example, the `withTypeable` function from the
+  `Data.Typeable` module can now be defined as:
+
+  ```
+  withTypeable :: forall k (a :: k) rep (r :: TYPE rep). ()
+               => TypeRep a -> (Typeable a => r) -> r
+  withTypeable rep k = withDict @(TypeRep a) @(Typeable a) rep k
+  ```
+
+  Note that the explicit type applications are required, as the call to
+  `withDict` would be ambiguous otherwise.
+
 ## 0.8.0 (edit as necessary)
 
 - Change array access primops to use type with size maxing the element size:
@@ -23,7 +45,7 @@
 - Add known-key `cstringLength#` to `GHC.CString`. This is just the
   C function `strlen`, but a built-in rewrite rule allows GHC to
   compute the result at compile time when the argument is known.
-  
+
 - In order to support unicode better the following functions in `GHC.CString`
   gained UTF8 counterparts:
 
@@ -47,7 +69,7 @@
         atomicCasAddrAddr# :: Addr# -> Addr# -> Addr# -> State# s -> (# State# s, Addr# #)
         atomicCasWordAddr# :: Addr# -> Word# -> Word# -> State# s -> (# State# s, Word# #)
 
-- Add an explicit fixity for `(~)` and `(~~)`: 
+- Add an explicit fixity for `(~)` and `(~~)`:
 
         infix 4 ~, ~~
 
diff --git a/libraries/ghc-prim/ghc-prim.cabal b/libraries/ghc-prim/ghc-prim.cabal
index 05fd60f09a..61840021c1 100644
--- a/libraries/ghc-prim/ghc-prim.cabal
+++ b/libraries/ghc-prim/ghc-prim.cabal
@@ -45,6 +45,7 @@ Library
         GHC.Debug
         GHC.IntWord64
         GHC.Magic
+        GHC.Magic.Dict
         GHC.Prim.Ext
         GHC.Prim.Panic
         GHC.Prim.Exception