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| author | Kaz Kylheku <kaz@kylheku.com> | 2021-01-20 22:59:02 -0800 |
|---|---|---|
| committer | Kaz Kylheku <kaz@kylheku.com> | 2021-01-20 22:59:02 -0800 |
| commit | 947b0979d05b190fee65698864cd1e14b5797f66 (patch) | |
| tree | 1e5e93f7e4ba5c69d0bb3815c2eeb30904a74fd7 /txr.1 | |
| parent | d30af5639c1a3a27452a7cd63c6f91201eac687b (diff) | |
| download | txr-947b0979d05b190fee65698864cd1e14b5797f66.tar.gz txr-947b0979d05b190fee65698864cd1e14b5797f66.tar.bz2 txr-947b0979d05b190fee65698864cd1e14b5797f66.zip | |
matcher: allow variables to back-reference.
Multiple occurrences of variables unify using equal.
* share/txr/stdlib/match.tl (var-list): New struct type.
Used for tracking what variables have been defined.
(compile-struct-match, compile-vec-match, compile-atom-match,
compile-op-match, compile-cons-structure,
compile-require-match, compile-let-match, compile-loop-match,
compile-parallel-match, compile-not-match): Take var-match
argument and pass it down.
(compile-parallel-match): Take var-match argument and pass
copies of it down to the compile jobs of the branches, since
they do not unify.
(compile-var-match, comiple-let-match, compile-op-match):
Handle variables carefully: check for variable already
being defined and generate a backreference instead of
a new binding match.
(compile-match): Take optional var-list argument,
instantiating the object if it is missing, pass down to all
recursive compile unctions.
* txr.1: Documented.
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 23 |
1 files changed, 23 insertions, 0 deletions
@@ -39646,6 +39646,29 @@ notation. The reason is that a struct literal produces an object which loses information about how it was specified in the literal syntax, but those details are critically important in pattern matching. +A pattern can contain multiple occurrences of the same variable. +Except in the case when these variables occur in different branches +of an +.code @(or) +or +.code @(and) +pattern operator, those repeated variables denote one variable. +The left-most, or in the case of +.code @(let) +nesting, outermost, occurrence of the variable binds to the corresponding +element of the object. The remaining occurrences of the variable must +correspond to objects which are +.code equal +to that object, or else there is no match. +For instance, the pattern +.code "(@a @a)" +matches a list like +.code "(1 1)" +and binds a to the leftmost +.codn 1 , +and fails to match a list like +.codn "(1 2)" . + The pattern-matching notation is documented in the following sections; sections describing the pattern matching macros follow. |