I'm talking about the transformer binding. Specifically the result of extract-struct-info that both struct-copy and match use in their expansion, which produces a six-element list that satisfies struct-info?. This six-element list has optional identifier bound to type descriptor optional identifier bound to type constructor optional identifier bound to type predicate list of field accessor identifiers (optional last value of #f) list of optional field mutator identifiers optional super type identifier
I want a 7th element that is the list of field identifiers themselves as given to the struct form or the define-signature form (which I believe produces the expected struct-info transformer binding, so I don't have to actually change define-signature). The expectation is that the field names and the field accessor names correspond 1-to-1. 7 is not 6, thus backwards-incompatible. I know many programs use positional accessors rather than match on the whole structure of the list, so I imagine adding a 7th element is not very intrusive. -Ian ----- Original Message ----- From: "Sam Tobin-Hochstadt" <sa...@cs.indiana.edu> To: "J. Ian Johnson" <i...@ccs.neu.edu> Cc: "users" <users@racket-lang.org> Sent: Wednesday, May 14, 2014 12:42:56 PM GMT -05:00 US/Canada Eastern Subject: Re: [racket] Struct fields in struct-info + match enhancements On Wed, May 14, 2014 at 12:35 PM, J. Ian Johnson <i...@ccs.neu.edu> wrote: > tl;dr if you use struct-info in your programs, I might break them. Please > continue reading. > > I had a PR a while ago suggesting a change to struct-copy due to its > unhygienic nature with fields. It did not go through since there wasn't > enough information in the struct-info to separate the struct-name and the > field-name. Because struct-info does not have a procedural-only interface, > changing it to instead or also hold the individual field identifiers would be > backwards incompatible. However, I also expect that struct-info manipulation > outside of core Racket is rare. > > Is there anyone out there that would be affected by a this change that would > be unwilling to make slight modifications to support the new struct-info? > I ask not because of struct-copy itself, but for an additional enhancement to > racket/match: named field selection from structs instead of positional only. > I'm getting bitten by pervasive refactoring woes whenever I add fields to > structs. All of my match patterns must change to have an extra _ somewhere. I don't understand why this would require a backwards-incompatible change to struct-info. Also, this discussion is confusing because it's not clear whether you mean the dynamic value produced by the `struct-info` procedure, or the structure type transformer binding. I think you mean the latter, in which case I expect you could do what you want by implementing `prop:struct-info` appropriately with an extended structure, and handling existing values (such as six-element lists) appropriately with defaults. Sam ____________________ Racket Users list: http://lists.racket-lang.org/users
