Finish up ep11

docs/videos.org

@@ -598,7 +598,8 @@ CLOSED: [2022-09-11 Sun 11:08]
 - https://lxsameer.com
 - [[https://twitter.com/lxsameer][@lxsameer]] on Twitter
 
-* Episode 10 - More on Macros
+* DONE Episode 10 - More on Macros
+CLOSED: [2022-11-04 Fri 15:25]
 ** Quasiquote aka backquote (`)
 - Backquote constructs allow us to quote an expression, but selectively evaluate sub-expressions
 - Similar to a template engine for lisp expressions
@@ -628,3 +629,78 @@ CLOSED: [2022-09-11 Sun 11:08]
 Let's discuss ~defflag~ and ~when-flag~ macros in [[file:../core/fg42/flags.el]]
 
 * Episode 11 - Common pitfalls of Macros
+** Compiletime vs runtime
+#+BEGIN_SRC emacs-lisp
+  ;; `do-something-with-side-effect' evaluates on compile time
+  ;; in the macro's context
+  (defmacro wrong (x)
+    (when x
+      (do-something-with-side-effect x)))
+
+  ;; `do-something-with-side-effect' evaluates on run time
+  ;; in the caller's context
+  (defmacro correct (x)
+    (when x
+      `(do-something-with-side-effect ,x)))
+#+END_SRC
+** Variable Capture
+#+BEGIN_SRC emacs-lisp
+  ;; `result' will hide any binding with the same name in the
+  ;; parent scope
+  (defmacro wrong (f &rest body)
+    `(let ((result ,f))
+       (if result
+           ,@body
+         (message "Failed: %s" result))))
+
+  (defvar result 1)
+  (wrong 200 (message "> %s" result))
+
+  ;; Instead we need to generate a local binding
+  (defmacro correct (f &rest body)
+    (let ((var (gensym)))
+      `(let ((,var ,f))
+         (if ,var
+             ,@body
+           (message "Failed: %s" ,var)))))
+
+  (defvar result 1)
+  (correct 200 (message "%s" result))
+#+END_SRC
+
+More on *uninterned* symbols: https://www.gnu.org/software/emacs/manual/html_node/elisp/Creating-Symbols.html
+
+** Many evaluation
+#+BEGIN_SRC emacs-lisp
+  ;; The argument `x' will be evaluated 2 times in this example.
+  ;; what if it contains side effects?
+  (defmacro wrong (x)
+    `(if ,x
+         (do-something-with ,x)
+       (do-somthing-else ,x)))
+
+  ;; It's better to use a local binding for it and evaluate it
+  ;; just once
+  (defmacro correct (x)
+    (let ((var (gensym)))
+    `(let ((,var ,x))
+       (if ,var
+           (do-something-with ,var)
+         (do-somthing-else ,var)))))
+#+END_SRC
+** Evaluating arguments of a macro
+Never evaluate the arguments of a macro manually. E.g. with =eval=
+
+#+BEGIN_SRC emacs-lisp
+  ;; `eval' here will be evaluated before the caller context
+  ;; is even populated
+  (defmacro wrong (x)
+    (list 'setq (eval x) 'blah))
+
+  ;; The correct way is to mark `x' for evaluation instead
+  (defmacro correct (x)
+    `(setq ,x 'blah))
+#+END_SRC
+
+More info: https://www.gnu.org/software/emacs/manual/html_node/elisp/Eval-During-Expansion.html
+** What's next?