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Opened by Divam Narula@dfordivamDeveloper

Semigroup instance of UniqDFM is wrong (not associative)

Summary

This is a very minor issue in code which possibly only a GHC dev can hit.

plusUDFM is incorrect, as the behavior of replacing the values of duplicate keys is dependent on the size of UniqDFMs being merged.

instance Semi.Semigroup (UniqDFM key a) where (<>) = plusUDFM

plusUDFM :: UniqDFM key elt -> UniqDFM key elt -> UniqDFM key elt
plusUDFM udfml@(UDFM _ i) udfmr@(UDFM _ j)
  -- we will use the upper bound on the tag as a proxy for the set size,
  -- to insert the smaller one into the bigger one
  | i > j = insertUDFMIntoLeft udfml udfmr
  | otherwise = insertUDFMIntoLeft udfmr udfml

A possible solution could be to use insertUDFMIntoLeft, but not sure if it would work correctly. Just documenting this as I hit it myself.

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Expected behavior

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Edited by Divam Narula