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feat: nonempty list type #1607

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feat: nonempty list type #1607

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@fgdorais fgdorais commented Jan 7, 2026

Experimental WIP.

@fgdorais fgdorais added the WIP work in progress label Jan 7, 2026
leanprover-community-mathlib4-bot added a commit to leanprover-community/mathlib4-nightly-testing that referenced this pull request Jan 7, 2026
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Update Batteries branch for testing leanprover-community/batteries#1607
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leanprover-community-bot commented Jan 7, 2026

Mathlib CI status (docs):

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414owen commented Jan 11, 2026

Hi @fgdorais
There's probably a reason why this isn't so, but wouldn't a length-indexed list obviate the need for a nonempty list, and be more general?
NonEmpty a could be modelled as IList (succ n) a, for some n.
You'd equally be able to express "a list with at least two elements", etc.

I'm also wondering whether we're going to see Semigroup, Functor, Applicative, and Monad instances for this?

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@fgdorais @leanprover-community-bot @414owen