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

[Alexander Kjeldaas]

Immediately, the alternative of introducing bound variables in the
environment that is available to rewrite rules comes to mind as a more
general way of doing this.

So this example from the GHC docs:
{-# RULES
  "map/map"    forall f g xs.  map f (map g xs) = map (f.g) xs
  "map/append" forall f xs ys. map f (xs ++ ys) = map f xs ++ map f ys
 #-}

For some source:
  map f (map g xs)

it is translated into:

   let location = "somefile.hs:234"
   in map (f.g) xs

So for error:

{-# RULES
  "error/location"    error s = errorLoc location s
 #-}

is translated into:
   let location = "somefile.hs:345"
   in errorLoc location s


Alexander
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