Asserting vars with different Uniques represent the same Object

Ben Gamari ben at
Sat Nov 11 22:25:02 UTC 2017

Andreas Klebinger <klebinger.andreas at> writes:

> 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 )
> 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.


- Ben
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