handle free variables correctly #58
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* Introduce `Context` to define free variables for parsing and display * Add `parse_with_context` to to resolve free variables based on a `Context` * Add `Term::with_context` for displaying terms using a given `Context`
ljedrz
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Sep 11, 2025
ljedrz
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Sep 11, 2025
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Yeah, I think this feature is long overdue in this repo; would you like to fix the clippy warnings and consider one suggestion I've left?
Thanks!
Accepting a slice is more flexible and idiomatic than a vector reference (@ljedrz) Co-authored-by: ljedrz <3750347+ljedrz@users.noreply.github.com>
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@ljedrz While I was at it, I also fixed a couple of other pre-existing lints in unrelated code (b8a8145, 1f59337), following the "Boy Scout Rule". These additional cleanups are not essential to this PR, so if you'd prefer a more focused commit history, I am perfectly happy to revert them. Just let me know! |
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Our current implementation of Term doesn't have free variable names as it is only a De Bruijn index expression, which is not correct. Free variable names are critical for open terms.
The issue is demonstrated by the following test, where free variable names were lost completely:
lambda_calculus/tests/reduction.rs
Line 62 in c961a86
This PR introduces a naming Context to give names to free variables, which is the same approach as the TAPL book.
Notes
A key inspiration for this change is the original TAPL implementation, which uses a top-level syntax to define variables (see https://github.com/mspertus/TAPL/blob/main/untyped/test.f ). In that implementation, you first have to define a variable like
x(usingx/;), which pushesxinto the naming context. Then, you can usexas a free variable.Since our implementation does not have such a top-level syntax, this PR introduces a Context type instead, which must be prepared manually by the user.