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charset="us-ascii"; protected-headers="v1" From: Philip McGrath <philip@philipmcgrath.com> Subject: Re: Equality and hashing for new datatypes Date: Thu, 22 Jun 2023 18:45:54 -0400 Message-ID: <2803364.tdWV9SEqCh@bastet> In-Reply-To: <01fa3142-02d8-9bb0-9dfd-5b233aa57bbf@gmail.com> MIME-Version: 1.0 On Thursday, June 22, 2023 7:33:06 AM EDT Taylan Kammer wrote: > On 22.06.2023 00:25, Philip McGrath wrote: > > Hi, > > > > What is the recommended way for a library to customize `equal?` and > > `equal?` -based hashing for new datatypes it defines? > > > > I know from [1] that `equal?` can be customized using GOOPS; however, some > > projects do not use GOOPS (e.g. [2]), so it would seem more friendly for a > > generic library to avoid it. Also, it is not clear if the same mechanism > > is > > even an option for customizing `equal-hash` to match `equal?`. > > > > I'm looking for something like Racket's `gen:equal+hash` [3] or Chez > > Scheme's `record-equal-procedure` and `record-hash-procedure` [4]. > > > > This came up while I was working on a portable version of the immutable > > hash-table implementations from Racket-on-Chez. > > > > -Philip > > > > [1]: > > https://www.gnu.org/software/guile/manual/html_node/GOOPS-Object-Miscella > > ny.html [2]: > > https://lists.gnu.org/archive/html/info-gnu/2023-05/msg00005.html [3]: > > https://docs.racket-lang.org/reference/Equality.html#%28part._.Implementi > > ng_.Equality_for_.Custom_.Types%29 [4]: > > https://cisco.github.io/ChezScheme/csug9.5/objects.html#./objects:h14 > Hi Philip, > > You can use SRFI 69 hash tables in Guile. This library offers a way to > specify the equality and hash procedures that should be used by a hash > table, e.g.: > > [...] > > Note that this doesn't affect the behavior of the standard 'equal?' > procedure, it only affects the SRFI 69 hash table operations on the created > hash table. > > I currently don't remember whether Guile's own non-portable hash table > library supports the same feature. It probably does in some way. > Thanks, but this is the opposite of what I want to do. The library I'm porting already supports creating tables with custom equality and hashing functions. What I'm trying to do is make the standard `equal?` procedure recognize when two of my table values are semantically the same even if internal implementation details differ. (Note also that SRFI 69 and other libraries provide *mutable* hash tables. The Racket-on-Chez library I'm porting provides immutable hash tables, more properly Hash Array Mapped Tries, as made famous by Clojure.) -Philip --nextPart3095218.irdbgypaU6 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part. 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