[Git][ghc/ghc][wip/andreask/cast_any] Improve documentation of @Any@ type.

[Andreas Klebinger (@AndreasK)]

Andreas Klebinger pushed to branch wip/andreask/cast_any at Glasgow Haskell Compiler / GHC


Commits:
ff10db0f by Andreas Klebinger at 2024-05-22T15:04:08+02:00
Improve documentation of @Any@ type.

In particular mention possible uses for non-lifted types.

Fixes #23100.

- - - - -


2 changed files:

- compiler/GHC/Builtin/Types.hs
- libraries/ghc-prim/GHC/Types.hs


Changes:

=====================================
compiler/GHC/Builtin/Types.hs
=====================================
@@ -447,6 +447,11 @@ It has these properties:
 
   * You can safely coerce any /lifted/ type to Any, and back with unsafeCoerce.
 
+  * You can safely coerce any type to Any, and back with unsafeCoerce#, as long as
+    the kinds match up.
+    While this allows coercion of lifted types, one should prefer unsafeCoerce
+    over unsafeCoerce# whenever possible.
+
   * It does not claim to be a *data* type, and that's important for
     the code generator, because the code gen may *enter* a data value
     but never enters a function value.


=====================================
libraries/ghc-prim/GHC/Types.hs
=====================================
@@ -283,11 +283,44 @@ data Symbol
 *                                                                      *
 ********************************************************************* -}
 
--- | The type constructor 'Any' is type to which you can unsafely coerce any
--- lifted type, and back. More concretely, for a lifted type @t@ and
--- value @x :: t@, @unsafeCoerce (unsafeCoerce x :: Any) :: t@ is equivalent
--- to @x at .
+-- | The type constructor @Any :: forall k. k@ is a type to which you can unsafely coerce any type, and back.
 --
+-- For @unsafeCoerce@ this means for all lifted types @t@ that
+-- @unsafeCoerce (unsafeCoerce x :: Any) :: t@ is equivalent to @x@ and safe.
+--
+-- The same is true for *all* types when using unsafeCoerce# like this:
+-- @unsafeCoerce# (unsafeCoerce# x :: (Any :: TYPE r)) :: (t :: TYPE r)@.
+-- But users should always prefer unsafeCoerce over unsafeCoerce# when possible.
+--
+-- For example, if @x :: t@ and @t :: Type@, you can do this:
+--
+-- @
+--    a :: Any @Type
+--    a = unsafeCoerce x
+--    x2 :: t
+--    x2 = unsafeCoerce a
+-- @
+--
+-- But you must obey the following rule: do not change the *kind* of the type you are coercing.  For example:
+-- @
+--    a1 :: Any @(TYPE 'IntRep)
+--    a1 = unsafeCoerce# True
+--
+--    a2 :: Any @(TYPE ('BoxedRep 'UnliftedRep))
+--    a2 = unsafeCoerce# True
+-- @
+-- Here @a1@ is bad because we started with @True :: (Bool :: Type)@, represented by a boxed heap pointer,
+-- and coerced it to @a1 :: Any @(TYPE 'IntRep)@, whose representation is a non-pointer integer.
+-- That's why we had to use `unsafeCoerce#`; it is really unsafe because it can change representations.
+-- Similarly @a2@ is bad because although both @True@ and @a2@ are represented by a heap pointer,
+-- @True@ is lifted but @a2@ is not; bugs here may be rather subtle.
+--
+-- To cast a boxed unlifted type to `Any`, type annotations are recommended
+-- to make sure that @Any@ has the correct kind. (Coercion between lifted and unlifted types is unsound.)
+-- For example, for some @b :: ByteArray#@:
+-- @
+--    unsafeCoerce# b :: (Any :: TYPE ('BoxedRep 'Unlifted))
+-- @
 type family Any :: k where { }
 -- See Note [Any types] in GHC.Builtin.Types. Also, for a bit of history on Any see
 -- #10886. Note that this must be a *closed* type family: we need to ensure



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ff10db0fb77ed869a078089ff6ea1ff952620970

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ff10db0fb77ed869a078089ff6ea1ff952620970
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