On Mon, Feb 25, 2013 at 4:42 PM, Simon Peyton-Jones <simo...@microsoft.com>wrote:
> I’m afraid the rewrite-rule idea won’t work. RULES are applied during > optimisation, when tons of inlining has happened and the program has been > shaken around a lot. No reliable source location information is available > there.**** > > ** > Do you mean that the proposal itself won't work, or specifically implementing this features in terms of existing rewrite rules won't work? > ** > > See http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack; and > please edit it.**** > > ** > One thing I'd disagree with on that page is point (3). While it's certainly nice to have a full stack trace, implementing just shallow call information is incredibly useful. For logging and test framework usages, it in fact completely covers the use case. And even for debugging, I think it would be a massive step in the right direction. I'll admit to ignorance on the internals of GHC, but it seems like doing the shallow source location approach would be far simpler than a full trace. I'd hate to lose a very valuable feature because we can't implement the perfect feature. > ** > > One idea I had, which that page does not yet describe, is to have an > implicit parameter, > something like ?loc::Location, with**** > > errLoc :: ?loc:Location => String -> a**** > > errLoc s = error (“At “ ++ ?loc ++ “\n” ++ s)**** > > ** ** > > This behave exactly like an ordinary implicit parameter, EXCEPT that if > there is no binding for ?loc::Location, then the current location is used. > Thus**** > > ** ** > > myErr :: ?loc:Location => Int -> a**** > > myErr n = errLoc (show n)**** > > ** ** > > foo :: Int -> int**** > > foo n | n<0 = myErr n**** > > | otherwise = ...whatever...**** > > ** ** > > When typechecking ‘foo’ we need ?loc:Location, and so the magic is that we > use the location of the call of myErr in foo.**** > > ** ** > > Simon**** > > ** ** > > ** ** > > ** ** > > *From:* haskell-cafe-boun...@haskell.org [mailto: > haskell-cafe-boun...@haskell.org] *On Behalf Of *Alexander Kjeldaas > *Sent:* 25 February 2013 12:16 > *To:* Simon Hengel > *Cc:* Haskell Cafe > *Subject:* Re: [Haskell-cafe] RFC: rewrite-with-location proposal**** > > ** ** > > On Mon, Feb 25, 2013 at 12:46 PM, Simon Hengel <s...@typeful.net> wrote:*** > * > > On Mon, Feb 25, 2013 at 10:40:29AM +0100, Twan van Laarhoven wrote: > > I think there is no need to have a separate REWRITE_WITH_LOCATION > > rule. What if the compiler instead rewrites 'currentLocation' to the > > current location? Then you'd just define the rule: > > > > {-# REWRITE "errorLoc" error = errorLoc currentLocation #-}**** > > REWRITE rules are only enabled with -O. Source locations are also > useful during development (when you care more about compilation time > than efficient code and hence use -O0). So I'm not sure whether it's a > good idea to lump those two things together.**** > > ** ** > > I could imagine that source locations being useful when debugging rewrite > rules for example.**** > > ** ** > > I think your argument makes sense, but why not fix that specifically?**** > > ** ** > > {-# REWRITE ALWAYS "errorLoc" error = errorLoc currentLocation #-}**** > > ** ** > > Alexander**** > > ** ** > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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