#2213: Confusing flags for rewrite rules
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Reporter: dons | Owner: dons
Type: bug | Status: assigned
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Changes (by simonpj):
* summary: -Wall incorrectly warns "Defined but not used" for functions
exported via RULES => Confusing flags for
rewrite rules
Comment:
I agree this is totally weird, and we must fix it for 6.10. The current
situation is:
* RULES pragmas are discarded by the lexer unless `explicitForallEnabled`
is on
* `explicitForAllEnabled` is set only by the flags Ian mentions above
* Those flags are set by `-fglasgow-exts`
There is an '''optimisation''' flag `-frewrite-rules`, which controls
whether rewrite rules are ''applied'' during optimisation. It's not meant
to be a language-extension flag.
We could have a '''language-extension''' flag `-XRewriteRules`, which
switches on RULES in source code. (Even without it, optimisation could
still use imported rules, as controlled by the optimisation flag
`-frewrite-rules`.) It could be switched on by `-fglasgow-exts`. It
would switch on lexing of `forall`.
Presumably RULES should be ignored without `-XRewriteRules`. (Would we
want a warning? And yet another warning-supression flag?) I'm a bit
concerned that this would break old programs that don't use `-fglasgow-
exts`, but do have `-XScopedTypeVariables` (say), by silently discarding
their rewrite rules. This seems like a strong argument for adding a
warning for discarded RULES. This warning would also have warned Don in
the program that started this ticket.
Should the optimisation flag still be called `-frewrite-rules`. I think we
should re-title it `-frun-rewrite-rules` (implied by -O of course).
Nothing very hard here, but better to get the design right first.
Comments?
Meanwhile I'll re-title the ticket.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2213#comment:3>
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