#2800: deprecated OPTIONS flag warnings generated during dep chasing?
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Reporter: duncan | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.3 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown/Multiple | Os: Unknown/Multiple
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It would appear that warnings about deprecated flags used in `OPTIONS`
pragmas get spat out during the early pre-processing and module chasing
phase when they would not be generated after pre-processing.
`ghc --make` has to scan each module to look for options and language
extensions to know if it has to run cpp. At this stage it cannot of course
run cpp so it does not necessarily get the full set of options and
extensions since some may be conditional. So it should not at this stage
generate any warnings since they may be suppressed by later cpp-
conditional flags.
Another possible theory is that it generates the warnings before having
read all the `OPTIONS` flags so the later warning suppression is
ineffective. Unfortunately we have to put them in the order we do for
compatibility with older versions of ghc.
Here's an example out of Cabal:
{{{
{-# OPTIONS -cpp -fffi #-}
-- OPTIONS required for ghc-6.4.x, and must appear first
{-# LANGUAGE CPP, ForeignFunctionInterface #-}
{-# OPTIONS_GHC -cpp -fffi #-}
{-# OPTIONS_NHC98 -cpp #-}
{-# OPTIONS_JHC -fcpp -fffi #-}
#if __GLASGOW_HASKELL__ >= 610
{-# OPTIONS_GHC -fno-warn-deprecated-flags #-}
-- the ghc flag -fffi is deprecated in ghc-6.10. We're
-- supposed to use the LANGUAGE pragmas instead. However
-- we have to maintain compatibility with older ghc versions
-- so we (try to) suppress this warning.
#endif
}}}
What we're trying to do here is make it work with ghc-6.4 + and not
generate any warnings with the latest ghc. This seems quite tricky to do.
To support ghc-6.4 we have to put the `OPTIONS` pragma first, since it
apparently only looks at the first couple lines. We then use the compiler-
specific `OPTIONS` pragmas. In particular these are needed for ghc-6.6.
For ghc-6.8 and later we can use the `LANGUAGE` pragmas.
In ghc-6.10 the `-fffi` option is deprecated. However we cannot
conditionally compile the `OPTIONS -cpp -fffi` since ghc needs to see the
`-cpp` to know that it has to cpp the module. Making the `OPTIONS -cpp`
unconditional and the `OPTIONS -cpp -fffi` conditional does not work with
ghc-6.4.
So instead we try suppressing the warning. We cannot do that
unconditionally since the flag is only present in newer ghc versions.
However if we do suppress conditionally then ghc still warns anyway,
presumably because it is warning on the first pass rather than in the pass
after running cpp.
All in all it's a bit tricky to do right. Indeed I cannot currently see a
way to make it work at all without generating warnings. The sledgehammer
would be to put a flag in the .cabal file and have it apply to all
modules, however that does not prevent the warning happening during Cabal
bootstrapping which uses plain `ghc --make`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2800>
GHC <http://www.haskell.org/ghc/>
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