Add Path.relative for lexical relative-path computation#9
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`relative` computes the path to a target relative to a base, purely lexically (no filesystem access): it normalizes both inputs, finds their common leading component prefix, then walks up with `..` for each remaining base component before descending into the target's remainder. It returns a `(Maybe String)`: `Nothing` when the paths cannot be related lexically (one absolute and one relative, or different Windows drive roots) and `Just "."` for equal paths. Named `relative` rather than `relative-to` because the core `defndynamic relative-to` (the load-path helper in Dynamic.carp) shadows that name for unqualified calls, which would break the module's `use`-based convention. It pairs naturally with the existing `absolute`.
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Path.relative, the lexical companion toabsolute: it computes the path to atargetrelative to abasewithout touching the filesystem — the staple operation behind Python'sos.path.relpath, Rust'spathdiff, and Java'sPath.relativize.Behaviour
(relative target base)returns a(Maybe String):(relative "/a/b/c" "/a/b")Just "c"(relative "/a/b" "/a/b/c")Just ".."(relative "/a/x" "/a/b")Just "../x"(relative "/a/b" "/a/b")Just "."(relative "/a/b/c" "/x/y")Just "../../a/b/c"(relative "/a/b" "a/b")Nothing(absolute vs relative)It composes the existing primitives: both inputs run through
normalize(so./../redundant separators are resolved first), are split into components, stripped of the empty-string artifactssplityields at roots, then reduced to a common leading prefix plus one..per leftover base component followed by the target's remainder.Nothingis returned when the paths cannot be related lexically — one absolute and one relative, or (on Windows) different drive roots.Naming
The natural name would be
relative-to, but core'sDynamic.carpalready defines adefndynamic relative-to(the load-path helper behindadd-c/add-cflags). That dynamic binding shadows the name for unqualified calls, so(relative-to …)after(use Path)expands the macro instead of calling the function and fails to type-check. Since the module is built for unqualified use (use-all Paththroughout the tests), I named itrelative, which also pairs cleanly with the existingabsolute. Happy to switch to a qualified-onlyrelative-to, or another name, if you'd rather.Tests / CI
test/path.carpgains nineassert-equalcases covering every example above plus the relative→relative and reverse-mismatch cases. Local run: 56 passed, 0 failed.carp-fmt --checkandanglerare clean; committed docs regenerated viagendocs.carp.Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.