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

[Twan van Laarhoven]

On 25/02/13 13:41, Michael Snoyman wrote:
>
> At that point, we've now made two changes to REWRITE rules:
>
> 1. They can takes a new ALWAYS parameters.
> 2. There's a new, special identifier currentLocation available.
>
> What would be the advantage is of that approach versus introducing a single new
> REWRITE_WITH_LOCATION pragma?


You are probably right. Ghc already has some logic in place for doing this with 
'assert':

-- Return an expression for (assertError "Foo.hs:27")
mkAssertErrorExpr = ..

finishHsVar name
  = do { ignore_asserts <- goptM Opt_IgnoreAsserts
       ; if ignore_asserts || not (name `hasKey` assertIdKey)
	then return (HsVar name, unitFV name)
	else do { e <- mkAssertErrorExpr
		; return (e, unitFV name) } }

So the check is name `hasKey` assertIdKey. I.e. it is a literal check whether 
the name is assert. Maybe that could be extended to check whether the name is 
declared as assert-like.

Of course the real solution is to have proper stack traces.


Twan