[project @ 2003-11-06 09:42:45 by simonpj]

Import all instances in cases like C (a b)

diff --git a/ghc/compiler/iface/TcIface.lhs b/ghc/compiler/iface/TcIface.lhs
index c8c27e9..071948b 100644 (file)
--- a/ghc/compiler/iface/TcIface.lhs
+++ b/ghc/compiler/iface/TcIface.lhs
@@ -446,6 +446,13 @@ Then, if we are trying to resolve (C Int x), we need (a)
 if we are trying to resolve (C x [y]), we need *both* (b) and (c),
 even though T is not involved yet, so that we spot the overlap.
 
+
+NOTE: if you use an instance decl with NO type constructors
+       instance C a where ...
+and look up an Inst that only has type variables such as (C (n o))
+then GHC won't necessarily suck in the instances that overlap with this.
+
+
 \begin{code}
 loadImportedInsts :: Class -> [Type] -> TcM PackageInstEnv
 loadImportedInsts cls tys
@@ -467,7 +474,9 @@ loadImportedInsts cls tys
 
        -- Suck in the instances
        ; let { (inst_pool', iface_insts) 
-                   = selectInsts (eps_insts eps) cls_gate tc_gates }
+                   = WARN( null tc_gates, ptext SLIT("Interesting! No tycons in Inst:") 
+                                               <+> pprClassPred cls tys )
+                     selectInsts (eps_insts eps) cls_gate tc_gates }
 
        -- Empty => finish up rapidly, without writing to eps
        ; if null iface_insts then
@@ -504,16 +513,22 @@ selectInsts pool@(Pool insts n_in n_out) cls tycons
     (insts', iface_insts) 
        = case lookupNameEnv insts cls of {
                Nothing -> (insts, []) ;
-               Just gated_insts -> 
+               Just gated_insts ->
        
-         case foldl choose ([],[]) gated_insts of {
+         case choose1 gated_insts  of {
            (_, []) -> (insts, []) ;    -- None picked
            (gated_insts', iface_insts') -> 
 
          (extendNameEnv insts cls gated_insts', iface_insts') }}
 
+    choose1 gated_insts
+       | null tycons                   -- Bizarre special case of C (a b); then there are no tycons
+       = ([], map snd gated_insts)     -- Just grab all the instances, no real alternative
+       | otherwise                     -- Normal case
+       = foldl choose2 ([],[]) gated_insts
+
        -- Reverses the gated decls, but that doesn't matter
-    choose (gis, decls) (gates, decl)
+    choose2 (gis, decls) (gates, decl)
        | any (`elem` tycons) gates = (gis,                decl:decls)
        | otherwise                 = ((gates,decl) : gis, decls)
 \end{code}