add app special form for convenience

should probably add some sort of macro system

src/main.rs

@@ -100,6 +100,7 @@ impl Sexp {
         }
     }
 
+    #[must_use]
     pub fn rw(&self, rule: &Rule) -> Sexp {
         rw(rule, self)
     }
@@ -177,6 +178,14 @@ impl Sexp {
             },
         }
     }
+
+    #[must_use]
+    pub fn is_this_atom(&self, atom: Symbol) -> bool {
+        match self {
+            Sexp::Atom(at) => *at == atom,
+            Sexp::List(_) => false,
+        }
+    }
 }
 
 impl fmt::Display for Sexp {
@@ -597,10 +606,12 @@ pub fn rw(rule: &Rule, sexp: &Sexp) -> Sexp {
     }
 }
 
+#[must_use]
 pub fn simp(rules: &[Rule], mut expr: Sexp, step_limit: usize, complexity_limit: usize) -> Sexp {
     for _ in 0..step_limit {
         expr = expr
             .apply_special_form::<Genslop>()
+            .apply_special_form::<App>()
             .apply_special_form::<Log>()
             .apply_special_form::<Let>()
             .apply_special_form::<Eq>();
@@ -743,6 +754,44 @@ impl<'src> SpecialForm<'src> for Log<'src> {
     }
 }
 
+static APP: Lazy<Symbol> = Lazy::new(|| TABLE.lock().unwrap().intern("app").unwrap());
+
+#[derive(Debug, Clone)]
+struct App<'src> {
+    f: &'src Sexp,
+    args: &'src [Sexp],
+}
+
+impl<'src> SpecialForm<'src> for App<'src> {
+    fn from_sexp(sexp: &'src Sexp) -> Option<Self> {
+        match sexp {
+            Sexp::Atom(_) => None,
+            Sexp::List(xs) => {
+                if xs.len() >= 2 && xs[0].is_this_atom(*APP) {
+                    Some(Self {
+                        f: xs.get(1).unwrap(),
+                        args: &xs[2..],
+                    })
+                } else {
+                    None
+                }
+            }
+        }
+    }
+
+    fn eval(self) -> Sexp {
+        let Self { f, args } = self;
+
+        let mut out = f.clone();
+
+        for arg in args {
+            out = Sexp::List(vec![out, arg.clone()]);
+        }
+
+        out
+    }
+}
+
 pub fn make_rule(sexp: &Sexp) -> Option<Rule> {
     match sexp {
         Sexp::List(xs) => match &xs[..] {