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.

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

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-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

On Mon, Feb 25, 2013 at 12:46 PM, Simon Hengel <sol@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