feat(wpcarro/emacs): Support default value for al-get r/4345

Similar to Elixir's `Map.get/3`.

Change-Id: I736f7e618aafc786a32eeba46ca635cb27db18b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6000
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

2 files changed, 18 insertions, 3 deletions

diff --git a/users/wpcarro/emacs/pkgs/al/al.el b/users/wpcarro/emacs/pkgs/al/al.el
index aa818941535f..4c37526c644a 100644
--- a/users/wpcarro/emacs/pkgs/al/al.el
+++ b/users/wpcarro/emacs/pkgs/al/al.el
@@ -111,10 +111,12 @@ Note that this doesn't append to the alist in the way that most alists handle
   (map-put! xs k v))
 
 ;; Read
-(defun al-get (k xs)
-  "Return the value at K in XS; otherwise, return nil.
+(defun al-get (k xs &optional default)
+  "Return the value at K in XS; otherwise, return nil or DEFAULT (if set).
 Returns the first occurrence of K in XS since alists support multiple entries."
-  (cdr (assoc k xs)))
+  (if (not (al-has-key? k xs))
+      default
+    (cdr (assoc k xs))))
 
 (defun al-get-entry (k xs)
   "Return the first key-value pair at K in XS."
diff --git a/users/wpcarro/emacs/pkgs/al/tests.el b/users/wpcarro/emacs/pkgs/al/tests.el
index 5146ee6b21a4..6556ddabc359 100644
--- a/users/wpcarro/emacs/pkgs/al/tests.el
+++ b/users/wpcarro/emacs/pkgs/al/tests.el
@@ -14,6 +14,19 @@
    (al-has-key? 'fname '((fname . "William")))
    (not (al-has-key? 'lname '((fname . "William"))))))
 
+(ert-deftest al-get ()
+  (let ((xs (->> (al-new)
+                 (al-set 'fname "John")
+                 (al-set 'employed? nil))))
+    (and
+     (string= "John" (al-get 'fname xs))
+     (string= "Cleese" (al-get 'lname xs "Cleese"))
+     ;; Test that the value of nil is returned even when a default is defined,
+     ;; which could be a subtle bug in the typical Elisp pattern of supporting
+     ;; defaults with: (or foo default).
+     (eq nil (al-get 'employed? xs))
+     (eq nil (al-get 'employed? xs "default")))))
+
 (ert-deftest al-has-value? ()
   (and
    (al-has-value? "William" '((fname . "William")))