quasiquote: support @,expr hack.

For better or worse, TXR Lisp has a dichotomy of
representation that @<atom> produces sys:var syntax, whereas
@<compound> produces sys:expr. This can cause an issue in
backquoting. Suppose you want to use backquote to generate
sytax like (a @b) where the b comes from a variable.
The problem is that (let ((x 'b)) ^(a @,x)) doesn't do
what you might expect: it produces (sys:expr b) rather
than (sys:var b).

This patch adds a hack into the quasiquote expander which
causes it to generate code to do what you expect.

Old behavior:

  1> (expand '^(a @,x))
  (list 'a (list 'sys:expr x))

New behavior:

  1> (expand '^(a @,x))
  (list 'a (let ((#:g0012 x))
             (if (atom #:g0012)
               (list 'sys:var #:g0012)
               (list 'sys:expr #:g0012))))

In other words, x will be evaluted, and the based on the
type of the object which emerges, either sys:var or
sys:expr syntax is generated.

* eval.c (expand_qquote_rec): Implement the above hack.
We are careful to only do this when this exact shape occurs
in the syntax: (sys:expr (sys:unquote item)).

* tests/010/qquote.tl: New file.

* txr.1: Documented.

3 files changed, 73 insertions, 0 deletions

diff --git a/eval.c b/eval.c
index 81c7cdca..41777807 100644
--- a/eval.c
+++ b/eval.c
@@ -3661,6 +3661,7 @@ static val expand_qquote_rec(val qquoted_form, val qq, val unq, val spl)
     return cons(quote_s, cons(qquoted_form, nil));
   } else {
     val sym = car(qquoted_form);
+    val args, uqform;
 
     if (sym == spl) {
       val error_msg = if3(spl == sys_splice_s,
@@ -3691,6 +3692,18 @@ static val expand_qquote_rec(val qquoted_form, val qq, val unq, val spl)
       val opts = expand_qquote(second(qquoted_form), qq, unq, spl);
       val keys = expand_qquote(rest(rest(qquoted_form)), qq, unq, spl);
       return rlcp(list(tree_construct_s, opts, keys, nao), qquoted_form);
+    } else if (sym == expr_s && consp((args = cdr(qquoted_form)))
+               && !cdr(args) && consp((uqform = car(args))) &&
+               car(uqform) == unq && consp(cdr(uqform)) && !cddr(uqform))
+    {
+      val gs = gensym(nil);
+      val ret = list(let_s, cons(list(gs, cadr(uqform), nao), nil),
+                     list(if_s, list(atom_s, gs, nao),
+                          list(list_s, list(quote_s, var_s, nao),
+                               gs, nao),
+                          list(list_s, list(quote_s, expr_s, nao),
+                               gs, nao), nao), nao);
+      return rlcp_tree(ret, qquoted_form);
     } else {
       val f = sym;
       val r = cdr(qquoted_form);
diff --git a/tests/010/qquote.tl b/tests/010/qquote.tl
new file mode 100644
index 00000000..e6daad3a
--- /dev/null
+++ b/tests/010/qquote.tl
@@ -0,0 +1,30 @@
+(let ((nullsym nil)
+      (sym 's)
+      (atom "abc")
+      (cons '(x y z))
+      (dwim '[]))
+  (tree-bind (x y (op arg)) ^(a b @,nullsym)
+    (assert (eq op 'sys:var))
+    (assert (eq arg nullsym)))
+  (tree-bind (x y (op arg)) ^(a b @,sym)
+    (assert (eq op 'sys:var))
+    (assert (eq arg sym)))
+  (tree-bind (x y (op arg)) ^(a b @,atom)
+    (assert (eq op 'sys:var))
+    (assert (eq arg atom)))
+  (tree-bind (x y (op arg)) ^(a b @,cons)
+    (assert (eq op 'sys:expr))
+    (assert (eq arg cons)))
+  (tree-bind (x y (op arg)) ^(a b @,dwim)
+    (assert (eq op 'sys:expr))
+    (assert (eq arg dwim)))
+  (tree-bind (x y (op arg . tail)) ^(a b (sys:expr ,sym . foo))
+    (assert (eq op 'sys:expr))
+    (assert (eq arg sym))
+    (assert (eq tail 'foo)))
+  (tree-bind (x y (op arg0 arg1)) ^(a b (sys:expr ,sym foo))
+    (assert (eq op 'sys:expr))
+    (assert (eq arg0 sym))
+    (assert (eq arg1 'foo)))
+  (tree-bind (x y (op)) ^(a b (sys:expr))
+    (assert (eq op 'sys:expr))))
diff --git a/txr.1 b/txr.1
index 4ee8ba01..3439afbc 100644
--- a/txr.1
+++ b/txr.1
@@ -12175,6 +12175,36 @@ The expression
 is evaluated to produce the list
 .codn "(6 8)" ,
 and this list is spliced into the quoted template.
+
+.meIP >> @,* expr
+This syntax is not a distinct quasiquoting operator, but rather the combination of
+an unquote occurring as a meta-expression, denoting the structure
+.codn "(sys:expr ,expr)" .
+This structure is treated specially by the quasiquote expander. Code is generated
+for it such that if
+.meta expr
+evaluates to an value
+.meta val
+which is an
+.codn atom ,
+then the result will be the
+.mono
+.meti (sys:var << val )
+.onom
+structure. If
+.meta val
+is a
+.code cons
+rather than an
+.codn atom ,
+then the result is the
+.mono
+.meti (sys:expr << val )
+.onom
+structure. In other words, when quasiquoting is used to insert a value under the
+.code @
+meta prefix, the expander generates code to analyze the type of the value, and
+produce to the form which is most likely intended.
 .RE
 
 .TP* "Dialect Notes:"