Changing the default-directory of an open compilation buffer

Question

I have two different compile commands, and they need to run in different directories.

I've gone through the trouble of making sure that if at all possible, I re-use the open compile buffer, rather than open a new view to it over and over again. The problem is that the default-directory of the buffer is set after I run one command, and subsequent calls don't seem to be able to override the default directory.

So:

(defun build-engine ()
  "Build the exe."
  (interactive)
  (let ((default-directory "f:/Projects/build/"))
    (compile "build.pl -target=release_fastbuild -platform=win64 %*")))

(defun build-test (name)
  "Build a test from the current tool executable with the specified name"
  (interactive "sMap Name: ")
  (let ((default-directory "f:/Projects/bin/"))
    (compile (concat "tool_win64.exe /genfastload:PC /name:" name))))

Those are the two compile commands.

This is how I always switch to the open compile buffer, so I don't keep opening new ones. (It's also how I keep the buffer from vertically splitting.)

(defadvice compile (around split-vertically activate)
  (if (and (get-buffer "*compilation*")
           (not (eq (current-buffer) (get-buffer "*compilation*"))))
      (switch-to-buffer-other-frame (get-buffer "*compilation*"))
    (let ((split-width-threshold 9999)
          (split-height-threshold 0))))
    ad-do-it)

If I run the first build command, it works. Then if I build the second, it fails because the default-directory is still set to f:/Projects/build/. If I kill the first compile buffer, my command works.

I could write some infrastructure so that the build-engine command only uses build buffers and the build-test command only uses test buffers, but it seems like that let should override the previous one, shouldn't it?

Answer 1

Based on Kyle's observation, I've made a few changes.

(defun build-engine ()
  "Build the exe."
  (interactive)
  (setq global-default-compile-directory "f:/Projects/build")
  (let (compile "build.pl -target=release_fastbuild -platform=win64 %*")))

(defun build-test (name)
  "Build a test from the current tool executable with the specified name"
  (interactive "sMap Name: ")
  (setq global-default-compile-directory "f:/Projects/bin")
  (let (compile (concat "tool_win64.exe /genfastload:PC /name:" name))))

(defadvice compile (around split-vertically activate)
    (let ( ((default-directory global-default-compile-directory))
     (split-width-threshold 9999)
     (split-height-threshold 0)))
    (switch-to-appropriate-compile-buffer)
    ad-do-it)

This works perfectly well.