[plt-scheme] keywords (a backward-incompatible change)

From: Hans Oesterholt (hdnews at gawab.com)
Date: Sat Oct 1 15:52:31 EDT 2005

My first impression:

Add them. Named arguments are the best amoung things I've encountered
in languages. I'll quickly adapt ROOS and mzgtk2 to make use of
keywords instead of the way I've implemented them currently.

Best whishes,

Hans Oesterholt.

Matthew Flatt schreef:

>With version 299.403, we plan to add keywords to MzScheme. Keywords are
>self-quoting, symbol-like values that can be used for keyword-based
>optional arguments.
>For now, we've settled on colon-prefixed keywords --- which means that
>a symbol or identifier can't start with an unquoted colon:
> (keyword? :a)   ; => #t
> (eq? :a ':a)    ; => #t
> (keyword? ':a)  ; => #t
> (keyword? 'a)   ; => #f
> (keyword? ':|a b|) ; => #t
> (keyword? '|:a b|) ; => #f
> (keyword? :)    ; => #t
> (symbol? :a)    ; => #f
> (symbol? :)     ; => #f
> (lambda (:x) 10) ; => syntax error: :x is not an identifier
> * Self-quoting keywords support keyword-based optional arguments with
>   a relatively standard syntax.
> * Syntactic forms can also use keywords, and pattern matching "just
>   works". Using keywords can help clarify when a syntactic form
>   expects to match literals (e.g., the colon in a `compound-unit/sig'
>   form) versus bindings (e.g., `quasiquote' in a `match' pattern).
> * Some people like the idea of keywords, and they'll put them to good
>   use.
> * R5RS allows a colon at the beginning of a symbol/identifier, and
>   some of our code uses identifiers that start with a colon. Adding
>   keywords of this form is a significant backward-incompatible change.
> * Some people dislike the idea of keywords, and they'll be forced to
>   use libraries that depend on keywords.
>To help port code, `#k-' can be used in front of an S-expression to
>disable keywords, just like `#ci' selects case-insensitive parsing. For
>example, if a module uses colon-prefixed identifiers/symbols and the
>module does not need keywords, then it can be prefixed with `#k-'.
>Naturally, `#k+' would enable keyword parsing, so `#k+:apple' is always
>the keyword `:apple'.
>Assuming that we add keywords to MzScheme, a MzLib library will quickly
>follow for defining procedures with keyword-based arguments. MzScheme's
>built-in `lambda' and `case-lambda' will not change.
>Among the people that I've polled so far, this plan has been relatively
>uncontroversial; discussion quickly turns to the best uses keywords,
>instead of whether they should exist or how they should be written. So
>I'm broadening the poll here...
>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme

Posted on the users mailing list.