Pattern checker status

David Feuer david at
Wed Jan 11 04:04:59 UTC 2017

Could you possibly give us an update about the current status of your 
pattern checker work? I know the number of tickets may seem a bit 
overwhelming; please reach out to the ghc-devs list, or individual 
developers, to get whatever help you need. Knocking out some of 
tickets is a priority for 8.2, and the freeze is coming up fast. In my 
the top priorities for GHC 8.2 should probably be #10746 and #12949. 
We really want to get those squashed. I think #11195 is also a fairly 
priority (if it's still an issue!).

#10746 is a serious correctness issue, and Simon's suggested fix 
straightforward. Have you run into trouble?

#12949 is also a serious correctness issue. You're right that desugaring 
doesn't happen till after type checking, but I believe the type checker 
should have already worked out what it needs to be able to desugar 
overloading. It would be worth asking how you might access that 

#11195 looks like a practically important and serious performance 
problem. I know you spent some time investigating it months ago; do 
you have any more recent progress to report? Do you know if the 
problem is still there?

David Feuer

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