chore: prefer · == a over a == ·
#430
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joehendrix
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We recently discovered inconsistencies in Mathlib and Std over the ordering of the arguments for `==`. The most common usage puts the "more variable" term on the LHS, and the "more constant" term on the RHS, however there are plenty of exceptions, and they cause unnecessary pain when switching (particularly, sometimes requiring otherwise unneeded `LawfulBEq` hypotheses). This convention is consistent with the (obvious) preference for `x == 0` over `0 == x` when one term is a literal. We recently updated Std to use this convention leanprover-community/batteries#430 This PR changes the two major places in Lean that use the opposite convention, and adds a suggestion to the docstring for `BEq` about the preferred convention.
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Mathlib currently uses
· == aeverywhere, and nevera == ·. Std is a mix; this PR brings it into line with Mathlib.However,
List.elemin Lean usesa == ·.I will, at some point, investigate if it is possible to change that. If not, we probably should switch everything downstream. (And add doc-strings on
BEqexplaining which is the preferred convention.)