[Git][ghc/ghc][wip/jakobbruenker/T24909] docs: Update mention of ($) type in user guide

[Jakob Brünker (@JakobBruenker)]

Jakob Brünker pushed to branch wip/jakobbruenker/T24909 at Glasgow Haskell Compiler / GHC


Commits:
5d55f178 by Jakob Bruenker at 2024-06-06T05:14:44+02:00
docs: Update mention of ($) type in user guide

Fixes #24909

- - - - -


1 changed file:

- docs/users_guide/exts/representation_polymorphism.rst


Changes:

=====================================
docs/users_guide/exts/representation_polymorphism.rst
=====================================
@@ -79,14 +79,26 @@ representation-polymorphic type.
 
 However, not all is lost. We can still do this: ::
 
-  ($) :: forall r (a :: Type) (b :: TYPE r).
+  good :: forall r (a :: Type) (b :: TYPE r).
          (a -> b) -> a -> b
-  f $ x = f x
+  good f x = f x
 
 Here, only ``b`` is representation-polymorphic. There are no variables
 with a representation-polymorphic type. And the code generator has no
-trouble with this. Indeed, this is the true type of GHC's ``$`` operator,
-slightly more general than the Haskell 98 version.
+trouble with this. Nonetheless, there is a way to write a definition with
+``bad``'s type: ::
+
+
+  ($) :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
+                (a :: TYPE r1) (b :: TYPE r2).
+         (a -> b) -> a -> b
+  ($) f = f
+
+By eta-reducing, we got rid of ``x``, and thus have no variable with a
+representation-polymorphic type.  Indeed, this is the true type of GHC's ``$``
+operator, slightly more general than the Haskell 98 version. However, its
+strictness properties are different: ``(good undefined) `seq` ()`` is equivalent
+to ``()``, whereas ``(($) undefined) `seq` ()`` diverges.
 
 Because the code generator must store and move arguments as well
 as variables, the logic above applies equally well to function arguments,



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/5d55f178e9e35ac78307e6be8a0021f221f253f3

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/5d55f178e9e35ac78307e6be8a0021f221f253f3
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