"concerns" chapter

.vscode/extensions.json

@@ -1,5 +1,6 @@
 {
     "recommendations": [
-        "rust-lang.rust-analyzer"
+        "rust-lang.rust-analyzer",
+        "yzhang.markdown-all-in-one"
     ]
 }

book/src/SUMMARY.md

@@ -5,3 +5,4 @@
 - [Background](./ch01/s00-background.md)
 - [Implementation](./ch02/s00-implementation.md)
 - [Usage]()
+- [Current Implementation Concerns](./ch04/s00-concerns.md)

book/src/ch04/s00-concerns.md

@@ -0,0 +1,32 @@
+# Concerns (questions) with the current implementaion
+
+## Can `WeakFunctor` be an associated type of a `Functor` instead of its supertype?
+
+## `Clone`-`FnMut` category functors
+
+## `Copy`-`Fn` category functors
+
+## `WeakFunctor::F<'a, A>` is not (yet) covariant over the lifetime `'a`.
+
+### Specific case: `Stackless<'a>` isn't covariant.
+Current hypothesis is that this comes from `EvalTree<'a>` being invariant over `'a`
+due to `FnOnce` being invariant over its output,
+which in turn comes from present typesysten limitations.
+
+## It might be better to have a per-lifetime trait for `Functor`s.
+
+Current:
+```rust
+pub trait WeakFunctor {
+    type F<'a, A: 'a>: 'a
+    where
+        Self: 'a;
+}
+```
+
+Proposed:
+```rust
+pub trait WeakFunctor<'a>: 'a {
+    type F<A: 'a>: 'a;
+}
+```