Redesign withDict (formerly magicDict)

This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition, this replaces the constant-folding rule for `magicDict` in
`GHC.Core.Opt.ConstantFold` with a special case in the desugarer in
`GHC.HsToCore.Expr.dsHsWrapped`. I have also renamed `magicDict` to `withDict`
in light of the discussion in
https://mail.haskell.org/pipermail/ghc-devs/2021-April/019833.html.
All of this has the following benefits:

* `withDict` is now more type safe than before. Moreover, if a user applies
  `withDict` at an incorrect type, the special-casing in `dsHsWrapped` will
  now throw an error message indicating what the user did incorrectly.
* `withDict` can now work with classes that have multiple type arguments, such
  as `Typeable @k a`. This means that `Data.Typeable.Internal.withTypeable` can
  now be implemented in terms of `withDict`.
* Since the special-casing for `withDict` no longer needs to match on the
  structure of the expression passed as an argument to `withDict`, it no
  longer cares about the presence or absence of `Tick`s. In effect, this
  obsoletes the fix for #19667.

The new `T16646` test case demonstrates the new version of `withDict` in
action, both in terms of `base` functions defined in terms of `withDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries. The `T16646Fail` test case demonstrates the error message that GHC
throws when `withDict` is applied incorrectly.

This fixes #16646. By adding more tests for `withDict`, this also
fixes #19673 as a side effect.

1 files changed, 24 insertions, 2 deletions

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 ~, ~~