Sunday, January 06, 2008

Thank heavens for PLT v4

I've just finally installed my first pre-release of PLT v4 (actually v3.99.0.9 at present), and there are so many little conveniences that are already qualitatively improving my life. Here are a few of the ways v4 is making me happy:

automatic opt-lambda: As I understand it, the underlying low-level #%plain-lambda is like the original lambda, but in the full scheme language, the automatically available, high-level lambda has support for the functionality of opt-lambda from the old (lib "") library. More often than not, that was the only reason I needed to require that library. It's so nice just to be able to reach for opt-args right away, without having to jump through any extra hoops.

keyword integration: Moreover, the high-level lambda also has automatic support for keyword arguments! This means there's a very smooth evolution path for functions: 1) positional when you just need a few arguments; 2) optional when one or two of them have reasonable defaults; and 3) keywords when there are enough arguments with enough defaults that it's too hard for the user to remember their order.

built-in pattern matching: No need to require (lib "") -- it's already in the full scheme language by default. I stumbled on that by accident!

generalized define-struct: There used to be a huge gap between what you could do with define-struct and what you could do with the very general but hard-to-use make-struct-type. But thanks to the expressivity of keywords, the new define-struct form is much more powerful. Plus it's so much easier to understand what
(define-struct posn (x y) #:transparent)
means, as opposed to
(define-struct posn (x y) #f)

built-in list libraries: Many more commonplace list operations are just automatically there when you use the full scheme language, such as foldl, foldr, last, andmap, ormap, build-list, etc. It used to be confusing trying to remember what was in (lib "") and what was in (lib "" "srfi" "1"). Now, if I'm not mistaken, I just don't ever need (lib "") anymore.

my first two columns are mine again: Last but not least, the new #lang module declaration form means I no longer have to indent the entire body of a module.

It looks like the PLT crew have really worked hard on fixing lots of these little inconveniences (some of which I only noticed consciously now that I no longer have to deal with them!).

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