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feat(style): avoid nonrec #689

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feat(style): avoid nonrec #689

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7 changes: 7 additions & 0 deletions templates/contribute/style.md
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Expand Up @@ -638,3 +638,10 @@ We allow, but discourage, contributors from simultaneously renaming declarations
In this case, no deprecation attribute is required for X, but it is for W.

Named instances do not require deprecations. Deprecated declarations can be deleted after 6 months.

### Avoid `nonrec`

The `nonrec` keyword tells Lean to assume that apparently recursive calls in the declaration body
are not actually recursive, and instead look for declarations in other namespaces with the same name.
It is nearly always clearer to disambiguate the call by using an explicit namespace,
and this should be preferred where possible. (There are currently many places in Mathlib that break this rule.)
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@jcommelin jcommelin Oct 28, 2025

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Suggested change
It is nearly always clearer to disambiguate the call by using an explicit namespace,
and this should be preferred where possible. (There are currently many places in Mathlib that break this rule.)
Avoid `nonrec` when the recursive call conflicts with another declaration *in a namespace*, because then adding the namespace to that declaration is more informative (to both Lean and the user). If it conflicts with a declaration in the root namespace, then both `nonrec` and `_root_.[...]` are acceptable.
(There are currently many places in Mathlib that break this rule.)