add lambda-like 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::<Genslop>()
+            .apply_special_form::<LambdaLike>()
             .apply_special_form::<App>()
             .apply_special_form::<Log>()
             .apply_special_form::<Let>()
@@ -792,6 +793,80 @@ impl<'src> SpecialForm<'src> for App<'src> {
     }
 }
 
+static LAMBDA_LIKE: Lazy<Symbol> =
+    Lazy::new(|| TABLE.lock().unwrap().intern("lambda-like").unwrap());
+
+#[derive(Debug, Clone)]
+struct LambdaLike<'src> {
+    lambda: &'src Sexp,
+    bindings: &'src [Sexp],
+    body: Option<&'src Sexp>,
+}
+
+impl<'src> SpecialForm<'src> for LambdaLike<'src> {
+    fn from_sexp(sexp: &'src Sexp) -> Option<Self> {
+        match sexp {
+            Sexp::Atom(_) => None,
+            Sexp::List(xs) => match &xs[..] {
+                [Sexp::Atom(lambda_like), lambda, Sexp::List(bindings), body]
+                    if *lambda_like == *LAMBDA_LIKE =>
+                {
+                    Some(Self {
+                        lambda,
+                        bindings,
+                        body: Some(body),
+                    })
+                }
+                [Sexp::Atom(lambda_like), lambda, Sexp::List(bindings)]
+                    if *lambda_like == *LAMBDA_LIKE =>
+                {
+                    Some(Self {
+                        lambda,
+                        bindings,
+                        body: None,
+                    })
+                }
+                _ => None,
+            },
+        }
+    }
+
+    // (lambda () body) -> (lambda () body)
+    // it's sort of incorrect (do zero arg functions exist? actual question), but i think i'll actually do:
+    // (lambda () body) -> body
+    // (lambda (a) body) -> (lambda a body)
+    // (lambda (a) body) -> (lambda a body)
+    // (lambda (a b) body) -> (lambda a (lambda b body))
+    fn eval(self) -> Sexp {
+        let Self {
+            lambda,
+            bindings,
+            body,
+        } = self;
+
+        if let Some(body) = body {
+            let mut out = body.clone();
+
+            for binding in bindings.iter().rev() {
+                out = Sexp::List(vec![lambda.clone(), binding.clone(), out]);
+            }
+
+            out
+        } else if let Some(body) = bindings.last() {
+            let mut out = body.clone();
+
+            for binding in bindings.iter().rev().skip(1) {
+                out = Sexp::List(vec![lambda.clone(), binding.clone(), out]);
+            }
+
+            out
+        } else {
+            // HACK: bad?
+            Sexp::nil()
+        }
+    }
+}
+
 pub fn make_rule(sexp: &Sexp) -> Option<Rule> {
     match sexp {
         Sexp::List(xs) => match &xs[..] {