Re: [Haskell-cafe] RFC: rewrite-with-location proposal

On Mon, Feb 25, 2013 at 4:42 PM, Simon Peyton-Jones 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.****

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Do you mean that the proposal itself won't work, or specifically implementing this features in terms of existing rewrite rules won't work?

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See http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack; and please edit it.****

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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.

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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)****

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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****

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myErr :: ?loc:Location => Int -> a****

myErr n = errLoc (show n)****

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foo :: Int -> int****

foo n | n<0 = myErr n****

| otherwise = ...whatever...****

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When typechecking ‘foo’ we need ?loc:Location, and so the magic is that we use the location of the call of myErr in foo.****

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Simon****

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*From:* haskell-cafe-bounces@haskell.org [mailto: haskell-cafe-bounces@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****

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On Mon, Feb 25, 2013 at 12:46 PM, Simon Hengel wrote:*** *

On Mon, Feb 25, 2013 at 10:40:29AM +0100, Twan van Laarhoven wrote:

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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.****

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I could imagine that source locations being useful when debugging rewrite rules for example.****

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I think your argument makes sense, but why not fix that specifically?****

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{-# REWRITE ALWAYS "errorLoc" error = errorLoc currentLocation #-}****

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Alexander****

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