TcPlugin: access to irreducible givens + fix passed ev_binds_var

1 files changed, 9 insertions, 4 deletions

diff --git a/docs/users_guide/extending_ghc.rst b/docs/users_guide/extending_ghc.rst
index 1cec248364..998ddeeec0 100644
--- a/docs/users_guide/extending_ghc.rst
+++ b/docs/users_guide/extending_ghc.rst
@@ -577,12 +577,12 @@ is defined thus:
 
     data TcPlugin = forall s . TcPlugin
       { tcPluginInit    :: TcPluginM s
-      , tcPluginSolve   :: s -> EvBindsVar -> TcPluginSolver
+      , tcPluginSolve   :: s -> TcPluginSolver
       , tcPluginRewrite :: s -> UniqFM TyCon TcPluginRewriter
       , tcPluginStop    :: s -> TcPluginM ()
       }
 
-    type TcPluginSolver = [Ct] -> [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
+    type TcPluginSolver = EvBindsVar -> [Ct] -> [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
 
     type TcPluginRewriter = RewriteEnv -> [Ct] -> [Type] -> TcPluginM TcPluginRewriteResult
 
@@ -652,8 +652,8 @@ The key component of a typechecker plugin is a function of type
 
 ::
 
-    solve :: [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
-    solve givens wanteds = ...
+    solve :: EvBindsVar -> [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
+    solve binds givens wanteds = ...
 
 This function will be invoked in two different ways:
 
@@ -701,6 +701,11 @@ the plugin to create equality axioms for use in evidence terms, but GHC
 does not check their consistency, and inconsistent axiom sets may lead
 to segfaults or other runtime misbehaviour.
 
+Evidence is required also when creating new Given constraints, which are
+usually implied by old ones. It is not uncommon that the evidence of a new
+Given constraint contains a removed constraint: the new one has replaced the
+removed one.
+
 .. _type-family-rewriting-with-plugins:
 
 Type family rewriting with plugins