Re: A better way to access records.

[Bengt Richter <bokr@bokr.com>]

Hi Brendan,

On +2020-10-30 21:28:38 +1100, Brendan Tildesley wrote:
> From the little bit of SICP that I've done, I recall watching the lectures
> where
> they put a mage hat on and talk about the power of names. One could perhaps
> say
> the most powerful tool in a programming language is the ability to give
> something a name and then refer to those names.
> 
> In guix/guile, record types are  list of names given to some data.
> For example:
> 
> (define foo
>   (package
>    (name "bar")
>    (version "1.0")
>    ...)
> 
> Here we the names foo, name, version, that refer to things of interest. We
> can
> call foo easily enough to get the record, but we cannot refer to name or
> version so easily.  We instead have to use accessors like (package-name
> foo),
> which requires us to write foo each time explicitly and have repeat package-
> for each accessor.
> In the guix codebase, on many occasions there appear things like this:
> 
> (match-lambda
>     (($ <agetty-configuration> agetty tty term baud-rate auto-login
>         login-program login-pause? eight-bits? no-reset? remote?
> flow-control?
>         host no-issue? init-string no-clear? local-line extract-baud?
>         skip-login? no-newline? login-options chroot hangup? keep-baud?
> timeout
>         detect-case? wait-cr? no-hints? no-hostname? long-hostname?
>         erase-characters kill-characters chdir delay nice extra-options)
>      (list
>       ....
> 
> Here we have given some names to things, abandoned those names, and once
> again
> gone to the trouble of naming them again, in order, just for one local
> environment. We'd have to do it again to make use of it elsewhere, and I
> assume
> they would have to change if the record type it self needed to be updated.
> 
> Wouldn't be nice if we could just step inside a record type whenever we
> pleased?
> The above would be like this perhaps:
> 
> (let-from-record-type <agetty-configuration>
>  (list ...))
> 
> "let-from-record-type" i just made up since i dont know what it should be
> called.  Anyhow, it seems like we're stepping back a few centuries in
> computer
> science by needing to jump through these hoops.
> 
> The list of symbols can be retreived with (record-type-fields
> <agetty-configuration>), but I can't think of how one would write the above
> syntax.
> 
> Opinions?
> 
> 
>

    info guile record

may be useful :)

-- 
Regards,
Bengt Richter