Dead end with macros

src/utils/for_loop.rs

@@ -54,10 +54,10 @@ macro_rules! xloop {
   (for $p:pat in $it:expr; $body:stmt) => {
     xloop!(for $p in $it; $body; ())
   };
-  (for $p:pat in $it:expr; $body:stmt; $exit:stmt) => {
+  (for $p:pat in $iit:expr; $body:stmt; $exit:stmt) => {
     {
-      let mut __xloop__ = $it.into_iter();
-      xloop!(let Some($p) = __xloop__.next(); $body; $exit)
+      let mut i = $iit.into_iter();
+      xloop!(let Some($p) = i.next(); $body; $exit)
     }
   };
   (let $p:pat = $e:expr; $body:stmt) => {

src/utils/mod.rs

@@ -1,5 +1,8 @@
 mod cache;
 pub mod translate;
+mod replace_first;
+// mod visitor;
+pub use replace_first::replace_first;
 pub use cache::Cache;
 mod substack;
 pub use substack::Stackframe;

src/utils/replace_first.rs

@@ -0,0 +1,14 @@
+use std::iter;
+
+pub fn replace_first<'a, T, F>(slice: &'a [T], mut f: F) -> Option<impl Iterator<Item = T> + 'a>
+where T: Clone, F: FnMut(&T) -> Option<T> {
+  for i in 0..slice.len() {
+    if let Some(new) = f(&slice[i]) {
+      let subbed_iter = slice[0..i].iter().cloned()
+        .chain(iter::once(new))
+        .chain(slice[i+1..].iter().cloned());
+      return Some(subbed_iter)
+    }
+  }
+  None
+}

src/utils/visitor.rs

@@ -0,0 +1,18 @@
+pub trait Visit<T> {
+  type Return;
+  fn visit(&self, target: T) -> Return; 
+}
+
+pub trait ImpureVisit<T> {
+  type Shard;
+  type Return;
+  fn impure_visit(&self, target: T) -> (Shard, Return);
+  fn merge(&mut self, s: Shard);
+}
+
+pub struct OverlayVisitor<VBase, VOver>(VBase, VOver);
+
+impl<VBase, VOver, T, R> Visitor<T> for OverlayVisitor<VBase, VOver>
+where VBase: Visitor<T, Return = Option<R>>, VOver: Visitor<T, Return = Option<R>> {
+  
+}