Andreas Klebinger
My core questions are:
* Should variables representing the same thing always have the same unique? * If not how can one assert they actually represent the same thing?
Working on the pattern matching code I came across this assertion:
ASSERT(tvs1 `equalLength` ex_tvs ) http://git.haskell.org/ghc.git/blob/HEAD:/compiler/deSugar/MatchCon.hs#l125
tvs1 and ex_tvs are both the existentially quantified type variables of a pattern. One gained by taking apart the pattern itself and one by taking apart the ConLike in the pattern.
Could you elaborate a bit more here? This sounds like a bug but I'm not sure I completely follow how you came about these tyvars.
Follow up question: The whole assert is essentially a unit test as ex_tvs isn't used outside of the assert. Is there a solution to check these invariants in tests instead of the source code?
If I understand the question correctly: no. GHC contains hundreds of assertions like this which are only enabled when the compiler is compiled with -DDEBUG. The advantage of including them in the source instead of an external test is that it keeps the invariant check right next to the implementation, where it belonds. Cheers, - Ben