Allow plugins to define built-in rules
Conal Elliott has an application in which he wants a plugin to add some new
BuiltinRules to the
mg_rules of the
ModGuts of the module being compiled. He needs a built-in rule (which runs some custom code) rather than a vanilla
CoreRule (which pattern-matches the argument).
But then he got a GHC panic
get a GHC panic: “No match in record selector ru_args”.
Turns out it comes from these lines in
imp_rule_edges = foldr (plusVarEnv_C unionVarSet) emptyVarEnv [ mapVarEnv (const maps_to) (exprFreeIds arg `delVarSetList` ru_bndrs imp_rule) | imp_rule <- imp_rules , let maps_to = exprFreeIds (ru_rhs imp_rule) `delVarSetList` ru_bndrs imp_rule , arg <- ru_args imp_rule ]
This code is just working out what locally-defined variables should be kept alive by the
mg_rules for imported Ids. But
BuiltinRules don't have any
Easy fix: just don't traverse builtin rules in this scan. In Conal's case, the rules that his plugin adds certainly don't mention locally-defined Ids.
I'll do this shortly.