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Diffstat (limited to 'NEWS')
| -rw-r--r-- | NEWS | 31 |
1 files changed, 31 insertions, 0 deletions
@@ -26,6 +26,37 @@ If you don't care whether the URI is a relative-ref or not, use In the future `uri?' will return a true value only for URIs that specify a scheme. +** Tail arrays deprecated + +Guile's structures used to have a facility whereby each instance of a +vtable can contain a variable-length tail array of values. The length +of the tail array was stored in the structure. This facility was +originally intended to allow C code to expose raw C structures with +word-sized tail arrays to Scheme. + +However, the tail array facility was confusing and doesn't work very +well. It was very rarely used, but it insinuates itself into all +invocations of `make-struct'. For this reason the clumsily-named +`make-struct/no-tail' procedure can actually be more elegant in actual +use, because it doesn't have a random `0' argument stuck in the middle. + +Tail arrays also inhibit optimization by allowing instances to affect +their shapes. In the absence of tail arrays, all instances of a given +vtable have the same number and kinds of fields. This uniformity can be +exploited by the runtime and the optimizer. The presence of tail arrays +make some of these optimizations more difficult. + +Finally, the tail array facility is ad-hoc and does not compose with the +rest of Guile. If a Guile user wants an array with user-specified +length, it's best to use a vector. It is more clear in the code, and +the standard optimization techniques will do a good job with it. + +For all of these reasons, tail arrays are deprecated in Guile 2.2 and +will be removed from Guile 3.0. Likewise, `make-struct' / +`scm_make_struct' is deprecated in favor of `make-struct/no-tail' / +`scm_make_struct_no_tail'. Perhaps one day we will be able to reclaim +the `make-struct' name! + * Bug fixes ** Enable GNU Readline 7.0's support for "bracketed paste". |