Extraction: forbid Prop-polymorphism of inductives when extracting to Ocaml

 A particular case in sort-polymorphism of inductive types allows
 an informative type (such as prod) to have instances in Prop:

   (I,I) : True * True : Prop

 This is due to the fact that prod is a singleton type: indeed (I,I)
 has no informative content. But this invalidates an important invariant
 for the correctness of the extraction: inductive constructors stop
 having always the same sort as their inductive type. Consider for instance:

  Definition f (X:Type)(x:X*X)(g:X->nat) := g (fst x).
  Definition test := f _ (I,I) (fun _ => 0).

 Then the inductive element (I,I) is extracted as a logical part __,
 but during a strict evaluation (i.e. in Ocaml, not Haskell), this __
 will be given to fst, and hence to a match, leading to a nasty result
 (potentially segfault). Haskell is not affected, since fst is never
 evaluated.

 This patch adds a check for this situation during any Ocaml extraction,
 leading for the moment to a fatal error. Some functions in inductive.ml
 and retyping.ml now have an extra optional argument ?(polyprop=true)
 that should stay untouched in regular Coq usage, while type-checking
 done during extraction will disable this prop-polymorphism.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@14256 85f007b7-540e-0410-9357-904b9bb8a0f7

1 files changed, 9 insertions, 0 deletions

diff --git a/plugins/extraction/table.ml b/plugins/extraction/table.ml
index ad5424deb..aa312321b 100644
--- a/plugins/extraction/table.ml
+++ b/plugins/extraction/table.ml
@@ -334,6 +334,15 @@ let error_no_module_expr mp =
        ++ str "some Declare Module outside any Module Type.\n"
        ++ str "This situation is currently unsupported by the extraction.")
 
+let error_singleton_become_prop id =
+  err (str "The informative inductive type " ++ pr_id id ++
+       str " has a Prop instance.\n" ++
+       str "This happens when a sort-polymorphic singleton inductive type\n" ++
+       str "has logical parameters, such as (I,I) : (True * True) : Prop.\n" ++
+       str "The Ocaml extraction cannot handle this situation yet.\n" ++
+       str "Instead, use a sort-monomorphic type such as (True /\\ True)\n" ++
+       str "or extract to Haskell.")
+
 let error_unknown_module m =
   err (str "Module" ++ spc () ++ pr_qualid m ++ spc () ++ str "not found.")