the DrScheme repl isn't the one in Emacs

Dear old Lisper,

You have found drscheme and it is almost like your old Lisp machine. It is easy to program in it, it has things that nobody else has, and we all love parentheses. But after some initial enthusiasm you are wondering why in the world, we decided not to provide commands for sending individual definitions and expressions from the Definitions window to the Interactions window, aka, REPL.

It wasn't an accident. It was by design after some difficult experiences. I am personally a product of the Emacs world that you are describing below, and my advisor Dan Friedman was called the "Lispman" on his door sign at Indiana.

When I first discovered the idea of sending individual expressions and definitions from a buffer to a repl, it was a near-religious revelation to me. I wanted everyone to know this trick and use it. When I started teaching the freshman course at Rice, I told our chairman so and he asked "why". I was shocked, awed, and I failed to explain to him how it mattered. He was a mathematician and I wrote it off. They don't know.

Then I started watching my sophomores and juniors at Rice in lab. Now that was a true disappointment. Few if any used this trick and when they did, they more often tripped up and got the repl into a state where they didn't know what was going on.

In the mid 90s, I wrote some more Little books with Dan, and boy, time and again, I watched him stumble across the state of the repl. I even watched him re-start the repl and load the whole buffer more often than not.

Why? In the presence of macros and higher-order functions and other beasts, it is difficult for masters of the universe with 30 years of experience to keep track of things. What do you think students with 10 or 20 days worth of experience will do? Is it really such a deep principle of computing to create the objects incrementally in the repl as opposed to thinking systematically through the design of a program?

I decided not and asked Robby to make DrScheme's repl transparent. That is, it re-starts the repl and re-loads the buffer every time. I consider this behavior a suitable compromise: have a repl but don't confuse yourself with send-defs and send-exprs. This is especially true in an age when sending an entire buffer takes as much time as sending an individual expression or definition. Soon we'll get "compilation behind your back" so that only the current buffer is re-interpreted. It'll start things even faster.

Even though I had used the incremental mode for more than a decade when I switched from Emacs to DrScheme in 1998, I have hardly ever looked back. I miss a few other things but the incremental repl is one of those rituals old Lispers acquired and never questioned ... but it isn't fundamental and critical to anything. (Note there is no qualifying clauses, no when no if no but. I really mean this last sentence the way I spelled it.)

Web Application Development in PLT Scheme

Many users often post to the plt-scheme mailing list asking for introductions to Web application development in PLT Scheme. They've heard of the continuation-based PLT Web Server and want a gentle introduction. Unfortunately, there has been a distinct lack of good documentation and tutorials for the server. Taking the cue from two users: Jens Axel Soegaard and David Reynolds, we've written a tutorial with Danny Yoo.

The tutorial is available at http://docs.plt-scheme.org/continue/index.html. It walks through the creation of a blog application, introducing features slowly and culminates in an SQL-backed database for the posts. Of particular interest, is the fast start servlet setup based on the Instaservlet package from Untyped.

Please take a look and pass along this as a pointer to those who may be interested in PLT Scheme.

Search in DrScheme

I've just overhauled search in DrScheme. Instead of popping up a dialog box and asking you for a search string, you get a new editor along the bottom of the DrScheme window where you can type a search string and DrScheme responds by circling (in purple) all of the occurrences of the search string in the file, both in the main window and in the contour window.

From there, you can use the new menu shortcuts to navigate forwards and backwards through the hits, optionally replacing occurrences of the search string with the replace strings (see the Edit menu).

With the default settings, typing a search string does not move around in the main window. Specifically (unlike Emacs, Safari, or Firefox), you don't move immediately to the first occurrence of the search string. You have to hit return or one of the search keys (in the Edit menu) for that to happen. But you can enable this in the General pane of the Editing tab in the preferences dialog. Click on "Search using anchors". Now, when you search for a string, DrScheme will whizz you right to it and when you edit the search string, DrScheme will shift the insertion point based on the search string. In order for you to keep track of where edits will go, you'll see a funny looking little red dongle in your text. That's where the insertion point was when you started your search, and that's where DrScheme starts all of these implicit searches from.

Try it out and let us know what you think! In particular, is the anchor-based search or the non-anchor-based search more intuitive for you?

The Tour in Video

Putting behind its stodgy textual past, the DrScheme tour is now in video! It's rather preliminary, and can use lots of improvement, but now we're really taking it to the kids.

Your security hole is my fun hack, or: computing factorial in DrScheme with a click-powered loop.

One of the many changes in v4.0 is to close a security hole in DrScheme. Specifically, DrScheme v371 lets the program in the definitions window get a hold of the editor containing said program and manipulate it programmatically. There are lots of bad things one might do with this fact, like circumventing DrScheme's protections and cause it to crash, or even spontaneously exit.

But, we can do something even more fun. Put the following program into a DrScheme window (in v371) and set the language to the mzscheme/textual language. Change "input" to whatever number you wish to compute the factorial of and then hit the Run button until your program transforms itself into the final result.

(define input 10)
(require (lib "mred.ss" "mred") (lib "class.ss"))
(let* ([ed (let-syntax ([m (λ (stx) (with-syntax ([x (syntax-source stx)]) #'x))])
             (m))]
       [mth (regexp-match 
             #rx"^; ([0-9]+) ([0-9]+)" 
             (send ed get-text 0 
                   (send ed paragraph-end-position 0)))]
       [lckd (send ed is-locked?)])
  (send ed begin-edit-sequence)
  (send ed lock #f)
  (if mth
      (let ([n (string->number (list-ref mth 1))]
            [acc (string->number (list-ref mth 2))])
        (send ed delete 0 (send ed paragraph-end-position 0))
        (if (= n 1)
            (begin (send ed delete 0 (send ed paragraph-end-position 0))
                   (send ed insert (format "~a\n#|" acc) 0)
                   (send ed insert "\n|#" (send ed last-position)))
            (begin (send ed delete 0 (send ed paragraph-end-position 0))
                   (send ed insert (format "; ~a ~a" (- n 1) (* n acc)) 0 0))))
      (send ed insert (format "; ~a 1\n" input) 0))
  (send ed lock lckd)
  (send ed end-edit-sequence))

Completions in DrScheme (finally)

DrScheme now supports a language- sensitive (but not lexical- scope sensitive) completion feature. Type <menukey>-/ and see what names are available to finish off the word you're typing.

Thanks to Jacob (and do follow that link; we all need a little more love in our lives) and Mike for taking the initiative to actually implement what is probably the most requested feature in DrScheme at the moment.

Birthday Easter Eggs in DrScheme

DrScheme has five birthday easter eggs in it, one for each of the main contributers to the PLT Scheme infrastructure (Matthias, Matthew, Eli, Shriram, and me). I put four of them in there, and mostly concentrated on making them fun. Matthew added mine and the best part of that one is figuring out on earth it shows up (it is quite tricky to find the code that actually makes that one appear).

I don't want to ruin the fun of searching for the Easter Eggs yourself, but just to get you started, do have a look at plt/collects/framework/private/bday.ss for Matthias, Matthew, Shriram, and Eli's birthdays. Mine is July 2nd.

Happy Hunting!

New Debugger Features

As Eli mentioned, v371 introduces support for debugging several files at a time, as well as new buttons for stepping Over and Out of expressions in the debugger.

Debugging across multiple files is easy. Start by opening the "main" file that you want to debug and all of the files it requires (directly or indirectly) that you want to debug along with it. Then click Debug in the main file's frame. For example, if I wanted to see what the FrTime dataflow engine (in frp-core.ss) does when a particular program (say demo-module.ss) runs, I would open these two files and click Debug in the frame for demo-module.ss.

As each required file loads, DrScheme offers the option of debugging it. If you choose "yes", then the file is included in the debugging session, so you can set breakpoints and step into it. (Note that this will make the code in the file run more slowly, and single-stepping at calls to its functions will bring you into it.) A file can only participate in one debugging session at a time, so if you're already debugging it with some other program, DrScheme will tell you so (instead of asking whether to debug it). For best results, all of the files you debug should be modules. Once a file is included in the debugging session, you can set breakpoints and step into it as if you were debugging it by itself.

As soon as you can debug programs that span several files, it's particularly valuable to be able to do more than set breakpoints and single-step. This is the motivation for the new Over and Out buttons, which are also quite simple. If the execution marker is at the start of an expression that's not in tail position, then you can step over the entire expression, which is equivalent to setting a one-shot breakpoint at the end of the expression and continuing. (If you've set breakpoints inside the expression, or inside any functions it calls, then execution may suspend before reaching the end.) Likewise, if execution is suspended and the current expression is evaluating within a debugging-enabled context, then you can step out to the innermost such context. This would be difficult to simulate by hand, since you'd need to keep track of recent callers.

At any given point, either or both of the Over and Out buttons may be disabled, but over the course of a session they can eliminate a lot of tedium.

The screenshot above shows a session debugging frp-core.ss as used by demo-module.ss. Execution is suspended on a right paren, so stepping Over is disabled, but we see the expression's value at the upper left, we've moused over b to see its value at the upper right, and it's possible to step Out.

Adjusting DrScheme's Keybindings

Check out Kyle Smith's blog post on how to change DrScheme's keybindings:

Keybinding 101 in DrScheme