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I've run into situations where I need deterministic `tyVarsOfType` and this implementation achieves that and also brings an algorithmic improvement. Union of two `VarSet`s takes linear time the size of the sets and in the worst case we can have `n` unions of sets of sizes `(n-1, 1), (n-2, 1)...` making it quadratic. One reason why we need deterministic `tyVarsOfType` is in `abstractVars` in `SetLevels`. When we abstract type variables when floating we want them to be abstracted in deterministic order. Test Plan: harbormaster Reviewers: simonpj, goldfire, austin, hvr, simonmar, bgamari Reviewed By: simonmar Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1468 GHC Trac Issues: #4012
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