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Slow compile times for Happy-generated source

This came up while investigating the compile times of Happy grammars: https://github.com/simonmar/happy/issues/109. GHC 8 and later take a lot longer to compile ~~grammars with types that rely on PartialTypeSignatures and have lots of wildcards~~.

There is an example repo (https://github.com/wiz/too-happy) in that thread containing a grammar whose generated code has lots of wildcards.

On my laptop, the project takes about 25 seconds to compile on 7.10.3. With 8.0.2, 8.2.2, and HEAD it takes on the order of 4 minutes.

Edited by Alec Theriault
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