expand-hook-combine: bugfix.

Here we fix bugs in expand-hook-combine, imrprove the tests
and make different recommendations in the manual about hook
order.

* eval.c (expand_hook_combine_fun): Fix incorrect tests
which cause the next function to be ignored.

* tests/011/exphook.tl: (pico-style-expand-hook): Needs tweak
to evaluate constantp using standard expansion (without
pico-style), so that pico-style can nest with ifx in either
order.
(pico-style): Now when we call expand-hook-combine
we give the new hook first, and the existing one next.
This behavior makes more sense as a default go-to strategy
because it gives priority to the innermost hook-based macro,
closest to the code.
(infix-expand-hook, ifx): Add test cases which test nesting of
hook-based macros.

* txr.1: Opposite recommendation made about chaining of
expand hooks: new first, fall back on old.
Example adjusted.

3 files changed, 45 insertions, 15 deletions

diff --git a/eval.c b/eval.c
index 63026443..9cb982b3 100644
--- a/eval.c
+++ b/eval.c
@@ -5564,13 +5564,13 @@ static val expand_hook_combine_fun(val env, val form, val menv, val type)
 
   if (first) {
     val nform = funcall3(first, form, menv, type);
-    if (nform)
+    if (nform != form)
       return nform;
   }
 
   if (next) {
     val nform = funcall3(next, form, menv, type);
-    if (nform)
+    if (nform != form)
       return nform;
   }
 
diff --git a/tests/011/exphook.tl b/tests/011/exphook.tl
index 8439ac56..7f745da9 100644
--- a/tests/011/exphook.tl
+++ b/tests/011/exphook.tl
@@ -2,16 +2,45 @@
 
 (defun-match pico-style-expand-hook
   ((@(as form (@obj . @rest)) @nil nil)
-   (if (constantp obj)
-     ^(quote ,form)
-      form))
+   (let ((*expand-hook* nil))
+     (if (constantp obj)
+       ^(quote ,form)
+        form)))
   ((@form @nil @nil) form))
 
 (defmacro pico-style (:env env . body)
-  (let ((*expand-hook* [expand-hook-combine *expand-hook*
-                                            pico-style-expand-hook]))
+  (let ((*expand-hook* [expand-hook-combine pico-style-expand-hook
+                                            *expand-hook*]))
     (expand ^(progn ,*body) env)))
 
 (mtest
   (pico-style (1 2 3)) (1 2 3)
   (pico-style (let ((a 0)) (cons a (1 2 3)))) (0 1 2 3))
+
+(defun-match infix-expand-hook
+  ((@(as form (@x @y . @rest)) @nil nil)
+   (if (fboundp y)
+     ^(,y ,x ,*rest)
+      form))
+  ((@form @nil @nil) form))
+
+(defmacro ifx (:env env . body)
+  (let ((*expand-hook* [expand-hook-combine infix-expand-hook *expand-hook*]))
+    (expand ^(progn ,*body) env)))
+
+(test
+   (ifx (let ((a 1) (b 2) (c 3))
+           (- ((a + b) * 3))))
+   -9)
+
+(test
+  (ifx
+    (pico-style
+      ((1) cons (2))))
+  ((1) . (2)))
+
+(test
+  (pico-style
+    (ifx
+      ((1) cons (2))))
+  ((1) . (2)))
diff --git a/txr.1 b/txr.1
index daace25b..b9a68529 100644
--- a/txr.1
+++ b/txr.1
@@ -44087,16 +44087,17 @@ rather than assign a value to it. Assigning an incorrect value to
 to it in a Listener session may be difficult to recover from, since
 it may render the Listener incapable of evaluating forms.
 
-Note: it is recommended to capture the previous value of
+Note: it is recommended for a hook to have access to the previous value of
 .code *expand-hook*
-and call that function before performing any transformation of
+and call that function in cases where
+.meta form
+the hook would decline to perform a transformation by returning
 .metn form .
-If that function performs a transformation, indicated by
-returning a different form, then return that form, only
-performing local processing when that function has declined.
 The function
 .code expand-hook-combine
-is provided which encapsulates this logic.
+is provided which encapsulates this logic. It allows a hook to easily
+insert itself before the other hooks, as recommended above, or
+after the other hooks.
 
 .TP* Example
 
@@ -44116,8 +44117,8 @@ is provided which encapsulates this logic.
   ;; hooks are tried first.
 
   (defmacro pico-style (:env env . body)
-    (let ((*expand-hook* [expand-hook-combine *expand-hook*
-                                              pico-style-expand-hook]))
+    (let ((*expand-hook* [expand-hook-combine pico-style-expand-hook
+                                              *expand-hook*]))
       (expand ^(progn ,*body) env)))
 
   ;; Under pico-style, (1 2 3) converts to '(1 2 3)