Import all instances in cases like C (a b)
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.
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
\begin{code}
loadImportedInsts :: Class -> [Type] -> TcM PackageInstEnv
loadImportedInsts cls tys
-- Suck in the instances
; let { (inst_pool', iface_insts)
-- 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
-- Empty => finish up rapidly, without writing to eps
; if null iface_insts then
(insts', iface_insts)
= case lookupNameEnv insts cls of {
Nothing -> (insts, []) ;
(insts', iface_insts)
= case lookupNameEnv insts cls of {
Nothing -> (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') }}
(_, []) -> (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
-- 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}
| any (`elem` tycons) gates = (gis, decl:decls)
| otherwise = ((gates,decl) : gis, decls)
\end{code}