add unquote special form

src/main.rs

@@ -611,6 +611,7 @@ pub fn simp(rules: &[Rule], mut expr: Sexp, step_limit: usize, complexity_limit:
     for _ in 0..step_limit {
         expr = expr
             .apply_special_form::<Quote>()
+            .apply_special_form::<Unquote>()
             .apply_special_form::<Genslop>()
             .apply_special_form::<LambdaLike>()
             .apply_special_form::<App>()
@@ -909,7 +910,50 @@ fn consify(xs: &[Sexp]) -> Sexp {
         None => Sexp::nil(),
     }
 }
-//        Sexp::List(vec![Sexp::Atom(*CONS), consify(&xs[1..])])
+
+static UNQUOTE: Lazy<Symbol> = Lazy::new(|| TABLE.lock().unwrap().intern("unquote").unwrap());
+
+// sbcl has quote take one arg....
+#[derive(Debug, Clone)]
+struct Unquote<'src>(&'src Sexp);
+
+impl<'src> SpecialForm<'src> for Unquote<'src> {
+    fn from_sexp(sexp: &'src Sexp) -> Option<Self> {
+        match sexp {
+            Sexp::Atom(_) => None,
+            Sexp::List(xs) => match &xs[..] {
+                [Sexp::Atom(unquote), expr] if *unquote == *UNQUOTE => Some(Self(expr)),
+                _ => None,
+            },
+        }
+    }
+
+    fn eval(self) -> Sexp {
+        let Self(expr) = self;
+
+        deconsify(expr)
+    }
+}
+
+fn deconsify(sexp: &Sexp) -> Sexp {
+    match sexp {
+        Sexp::Atom(at) => Sexp::Atom(*at),
+        Sexp::List(xs) => match &xs[..] {
+            [Sexp::Atom(cons), car, cdr] if *cons == *CONS => {
+                let mut out = vec![deconsify(car)];
+
+                let cdr = deconsify(cdr);
+                match cdr {
+                    Sexp::Atom(_) => out.push(cdr),
+                    Sexp::List(xs) => out.extend(xs),
+                }
+
+                Sexp::List(out)
+            }
+            _ => sexp.clone(),
+        },
+    }
+}
 
 pub fn make_rule(sexp: &Sexp) -> Option<Rule> {
     match sexp {