Mike Schaeffer's Blog

In my Lisp programming, I find myself using Anaphoric Macros quite a bit. My first exposure to this type of macro (and deliberate variable capture) was in Paul Graham's On Lisp. Since I haven't been able to find Emacs Lisp implementations of these macos, I wrote my own.

The first of the two macros is an anaphoric version of the standard if special form:

(defmacro aif (test if-expr &optional else-expr)
  "An anaphoric variant of (if ...). The value of the test
expression is locally bound to 'it' during execution of the
consequent clauses. The binding is present in both consequent
branches."
  (declare (indent 1))
  `(let ((it ,test))
     (if it ,if-expr ,else-expr)))

The second macro is an anaphoric version of while:

(defmacro awhile (test &rest body)
  "An anaphoric varient of (while ...). The value of the test
expression is locally bound to 'it' during execution of the body
of the loop."
  (declare (indent 1))
  (let ((escape (gensym "awhile-escape-")))
    `(catch ',escape
       (while t
         (let ((it ,test))
           (if it
               (progn ,@body)
             (throw ',escape ())))))))

What both of these macros have in common is that they emulate an existing conditional special form, while adding a local binding that makes it possible to access the result of the condition. This is particularly useful in scenarios where a predicate function returns a true value that contains useful information beyond t or nil.

In my career, I've done a bit of switching back and forth between Emacs and various IDE's. One of the IDE features I've come to depend on is quick access to the compiler. Typically, IDE's make it possible to compile your project with a keystroke, and then navigate from error to error at the press of a key. It's easy to recreate this in Emacs. The following two expressions make Emacs work a lot like Visual Studio in this regard.

(global-set-key [(shift f5)] 'compile)
(global-set-key [f12] 'next-error)

After these forms are evaluated, pressing Shift-F5 invokes the compile command, which asks for a command to be run in an inferior shell, typically make, ant, or some other build utility. The catch is that it runs the command in the directory of the current buffer, which implies that the build script can be found in the same directory as the current source file. For a Java project with a per-package directory hierarchy, this is often not true. There are probably a bunch of ways to fix this, but I've solved it with a Windows NT batch file, ant-up.bat, that repeatedly searches up the directory hierarchy for build.xml. I just compile with ant-up, rather than a direct invocation of ant. This is not the most elegant solution, I'm sure, but it took about five minutes to implement and works well.

@echo off

setlocal

:retry

set last_path=%CD%

echo Searching %CD% ...

if exist build.xml goto compile-now

cd ..

if "%last_path%"=="%CD%" goto abort

goto retry

:compile-now

call ant -k %1 %2 %3 %4 %5

if errorlevel 1 goto failure

goto success

:abort

echo build.xml not found... compile failed

:failure

exit /b 1

:success

exit /b 0

I just started reading this blog, but it already looks useful: m-x all-things-emacs.

Lately, I've been finding myself spending lots of time toggling between two Excel spreadsheets to make edits. This little macro makes it easy in Excel 2000 to toggle between two spreadsheet windows. I reccomend you bind it to a keystroke.

 Option Explicit
Dim lastWindow As Variant
Sub HereAndThere()     If IsEmpty(lastWindow) Then         Set lastWindow = ActiveWindow     Else         Dim currentWindow As Window         Set currentWindow = ActiveWindow         
    lastWindow.Activate
    
    Set lastWindow = currentWindow
End If
End Sub 

Here's how you use it:

• Run the macro once to save your current location.
• Switch to your other spreadsheet
• Now, running the macro will switch back and forth betweeen the two worksheets.

The "saving excursions" in the title is a reference to the save-excursion special form in Emacs Lisp. This macro isn't quite the same (and not nearly as powerful), but it reminded me of the Emacs feature. If it turns out to be useful, I might generalize my little macro to include some of the capabilities of Emacs' save-excursion.