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Simplified lookup of property values #940

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Simplified lookup of property values #940

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e76d89a
add property-value procedure
mnieper Apr 28, 2025
3cd352d
use continuation marks instead of parameterize
mnieper Apr 29, 2025
909beeb
add stubs for continuation marks for bootstrapping the expander
mnieper Apr 29, 2025
138b3f3
enable the use of property-value outside of calls to macro transformers
mnieper Apr 30, 2025
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51 changes: 37 additions & 14 deletions csug/syntax.stex
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Original file line number Diff line number Diff line change
Expand Up @@ -574,6 +574,9 @@ With two arguments, \var{id} and \var{key}, \var{lookup} returns the
value of \var{id}'s \var{key} property, or \scheme{#f} if \var{id}
has no \var{key} property.

However, in the case of properties, there is a second, usually easier
way, to obtain property values, namely by calling the
\scheme{property-value} procedure described below.
Comment on lines +577 to +579
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For compile-time properties, the \scheme{property-value} procedure can be used instead of the lookup procedure in macro transformers.

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The best wording probably depends on what the "recommended" way to retrieve property values should be. With property-value being available, I see no reason to use the older way for new code.

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I'd like to avoid making a recommendation and simply present the options.


%----------------------------------------------------------------------------
\entryheader
Expand Down Expand Up @@ -709,6 +712,29 @@ Attaching a new property with the same name as an property already
attached to a binding shadows the existing property with the new
property.

%----------------------------------------------------------------------------
\entryheader
\formdef{property-value}{\categoryprocedure}{(property-value \var{identifier} \var{key-identifier})}
\formdef{property-value}{\categoryprocedure}{(property-value \var{identifier} \var{key-identifier} \var{default})}
\returns see below
\listlibraries
\endentryheader

The \scheme{property-value} procedure returns the value of the
\var{key-identifier} property of \var{identifier}.
Comment on lines +723 to +724
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The \scheme{property-value} procedure is used in macro transformers to look up the value of the
\var{key-identifier} compile-time property of \var{identifier}.
Compile-time properties are not usually available after macro expansion.

If \var{identifier} has no \var{key-identifier} property, \var{default}
is returned.
If omitted, \var{default} defaults to \scheme{#f}.
As the right-hand side of a \scheme{define-property} form is evaluated
at expand-time, note that invoking \scheme{property-value} at relative
run-time within the same library or top-level program won't see the
property.
This is the reason why the \scheme{get-property} macro wrapper around
\scheme{property-value} is used below.
Comment on lines +728 to +733
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Delete this section. I tried to include the essence of the content in the revision above.


If \var{identifier} or \var{key-identifier} have no visible binding,
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have => has

an exception with condition type \scheme{&undefined} is raised.

The following example defines a macro, \scheme{get-info}, that retrieves
the \scheme{info} property of a binding, defines the variable \scheme{x},
attaches an \scheme{info} property to the binding of \scheme{x}, retrieves
Expand All @@ -721,11 +747,10 @@ properties are shadowed as well as the outer binding of \scheme{x}.
(define info)
(define-syntax get-info
(lambda (x)
(lambda (lookup)
(syntax-case x ()
[(_ q)
(let ([info-value (lookup #'q #'info)])
#`'#,(datum->syntax #'* info-value))]))))
(syntax-case x ()
[(_ q)
(let ([info-value (property-value #'q #'info)])
#`'#,(datum->syntax #'* info-value))])))
(define x "x-value")
(define-property x info "x-info")
(get-info x) ;=> "x-info"
Expand All @@ -741,10 +766,9 @@ just that.
\schemedisplay
(define-syntax get-property
(lambda (x)
(lambda (r)
(syntax-case x ()
[(_ id key)
#`'#,(datum->syntax #'* (r #'id #'key))]))))
(syntax-case x ()
[(_ id key)
#`'#,(datum->syntax #'* (property-value #'id #'key))])))
Comment on lines +769 to +771
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Please keep the original code, because it demonstrates how to use the lookup procedure. Add a note:

This macro can also be defined without the lookup procedure using \scheme{property-value} as in the \scheme{get-info} example above.

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This is related to my comment above; if we agree that new code probably wants to use property-value, the primary example should be formulated with it. I could add the old version as a secondary example.

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It's fine with me to have get-info use the lookup procedure and get-property use property-value. I want to have an example of each way.

(get-property x info) ;=> "x-info"
\endschemedisplay

Expand Down Expand Up @@ -810,11 +834,10 @@ cannot be accessed or forged.
(do-drt #'rname #'(fname ...) #f)]
[(_ rname pname (fname ...))
(for-all identifier? #'(rname pname fname ...))
(lambda (lookup)
(let ([prtd (lookup #'pname #'drt-key)])
(unless prtd
(syntax-error #'pname "unrecognized parent record type"))
(do-drt #'rname #'(fname ...) prtd)))]))))
(let ([prtd (property-value #'pname #'drt-key)])
(unless prtd
(syntax-error #'pname "unrecognized parent record type"))
(do-drt #'rname #'(fname ...) prtd)))])))
\endschemedisplay

\schemedisplay
Expand Down
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