| | -type FamInstEnv = UniqFM FamilyInstEnv -- Maps a family to its | instances | +type FamInstEnv = UniqDFM FamilyInstEnv -- Maps a family to its | +instances | -- See Note [FamInstEnv] Bartosz, could you add your comment from the commit (or variant thereof) as a comment to this type definition, so we know WHY you are using UniqDFM here? Thanks Simon We turn FamInstEnvs into lists in some places which don't directly affect the ABI. That happens in family consistency checks and when producing output for `:info`. Unfortunately that nondeterminism is nonlocal and it's hard to tell locally what it affects. Furthermore the envs should be relatively small, so it should be free to use deterministic maps here. Testing with nofib and ./validate detected no difference between UniqFM and UniqDFM. GHC Trac: #4012