Implement -XStaticValues.wip/static-pointers

Contains contributions from Alexander Vershilov and Mathieu Boespflug.

As proposed in [1], this extension introduces a new syntactic form
`static e`, where `e :: a` can be any closed expression. The static form
produces a value of type `StaticPtr a`, which works as a reference that
programs can "dereference" to get the value of `e` back. References are
like `Ptr`s, except that they are stable across invocations of a
program.

In essence the extension collects the arguments of the static form into
a global static pointer table. The expressions can be looked up by a
fingerprint computed from the package, the module and a fresh name
given to the expression. For more details we refer to the users guide
section contained in the patch.

The extension is a contribution to the Cloud Haskell ecosystem
(distributed-process and related), and thus has the potential to foster
Haskell as a programming language for distributed systems.

The immediate improvement brought by the extension is the elimination of
remote tables from Cloud Haskell applications. Such applications contain
table fragments spread throughout multiple modules and packages.
Eliminating these fragments saves the programmer the burden required to
construct and assemble the global remote table, a verbose and
error-prone process, even with the help of Template Haskell, that
moreover pollutes the export lists of all modules.

[1] Jeff Epstein, Andrew P. Black, and Simon Peyton-Jones. Towards
Haskell in the cloud. SIGPLAN Not., 46(12):118–129, September 2011. ISSN
0362-1340.

1 files changed, 33 insertions, 0 deletions

diff --git a/compiler/rename/RnExpr.lhs b/compiler/rename/RnExpr.lhs
index b24956c85e..be9d9116d8 100644
--- a/compiler/rename/RnExpr.lhs
+++ b/compiler/rename/RnExpr.lhs
@@ -310,6 +310,39 @@ rnExpr e@(ELazyPat {}) = patSynErr e
 
 %************************************************************************
 %*                                                                      *
+        Static values
+%*                                                                      *
+%************************************************************************
+
+For the static form we check that the free variables are all top-level
+value bindings. This is done by checking that the name is external or
+wired-in. See the Note about the NameSorts in Name.lhs.
+
+\begin{code}
+rnExpr e@(HsStatic expr) = do
+    (expr',fvExpr) <- rnLExpr expr
+    stage <- getStage
+    case stage of
+      Brack _ _ -> return () -- Don't check names if we are inside brackets.
+                             -- We don't want to reject cases like:
+                             -- \e -> [| static $(e) |]
+                             -- if $(e) turns out to produce a legal expression.
+      _ -> do
+       let isTopLevelName n = isExternalName n || isWiredInName n
+       case nameSetToList $ filterNameSet (not . isTopLevelName) fvExpr of
+         [] -> return ()
+         fvNonGlobal -> addErr $ cat
+             [ ptext $ sLit $ "Only identifiers of top-level bindings can "
+                           ++ "appear in the body of the static form:"
+             , nest 2 $ ppr e
+             , ptext $ sLit "but the following identifiers were found instead:"
+             , nest 2 $ vcat $ map ppr fvNonGlobal
+             ]
+    return (HsStatic expr', fvExpr)
+\end{code}
+
+%************************************************************************
+%*                                                                      *
         Arrow notation
 %*                                                                      *
 %************************************************************************