leanprover-community / leanprover-community/lean
`expr.fold` uneccesarily(?) is not defined for `pexpr`s
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- C++
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Description
The statement of expr.fold is
meta constant expr.fold {α : Type} : expr → α → (expr → nat → α → α) → α
which prohibits folding over pexprs. Is there any good reason for this (the only thing I can think of is a possible VM representation difference?).
If not, let's change it.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start at the expr.fold declaration quoted in the issue and compare its expr parameter with pexpr, including the VM representation concern raised there. Determine whether folding pexprs is supported; done means the requested definition change is made if it is compatible, with relevant behavior checked.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100