infix: phony infix requires 3 or more elements.

It is undesirable to translate (1 fun) into (fun 1).
Only cases similar to these patterns, using list
as an example:

  (1 list 2)     ->  (list 1 2)
  (1 list 2 3)   ->  (list 1 2 3)
  (1 list 2 + 3) ->  (list 1 (+ 2 3))
  (list 2 3)     ->  (list 2 3)
  (list 2 + 3)   ->  (list (+ 2 3))

* stdlib/infix.tl (infix-expand-hook): Restrict the phony
infix case to three or more elements.

* txr.1: Update phony infix case 1 as requiring three
or more elements. Also add (1 list) example emphasizing
that it's not recognized.

2 files changed, 8 insertions, 1 deletions

diff --git a/stdlib/infix.tl b/stdlib/infix.tl
index 54d24ca9..7eee5ad7 100644
--- a/stdlib/infix.tl
+++ b/stdlib/infix.tl
@@ -305,6 +305,7 @@
                       (boundp tok)))
          tok)
        (@(require (@x @y . @rest)
+                  (consp rest)
                   (and (not (funp env x)) (funp env y)))
         (let ((rexp (infix-expand-hook rest env nil)))
           (if (eq rexp rest)
diff --git a/txr.1 b/txr.1
index 6c44b77d..d81a0b3b 100644
--- a/txr.1
+++ b/txr.1
@@ -54420,7 +54420,7 @@ A form not falling into any of the above rules is recognized as "phony infix"
 if it conforms to one of two situations:
 .RS
 .IP 1.
-It consists of at least two elements, where the first element is not
+It consists of at least three elements, where the first element is not
 a function, and the second element is a function. Both the lexical and
 global function namespaces are considered.
 When a form is thus recognized, it is transformed by exchanging the first and
@@ -54480,6 +54480,12 @@ same manner as for the expression
 which is transformed to
 .codn "(list 1 (+ 2 2))" .
 
+The expression
+.code "(1 list)"
+is not recognized as phony infix and not thus not transformed into
+.code "(list 1)"
+because it doesn't have three or more elements.
+
 Only compound expressions are recognized as infix. A sequence
 of atoms like
 .code "a * b"