Improve documentation

src/resource.rs

@@ -1,14 +1,38 @@
 use std::any::{type_name, Any};
 
+use frunk::{prelude::HList, HCons, HNil};
+
+use crate::Aero;
+
 /// Bound on the types that can be used as an aerosol resource.
 pub trait Resource: Any + Send + Sync + Clone {}
 impl<T: Any + Send + Sync + Clone> Resource for T {}
 
+/// A compile-time list of resource types which are statically guaranteed to be present.
+pub trait ResourceList: HList + Any + Send + Sync + Clone {
+    /// Test at runtmie whether every resource in this list is present in the given Aero instance.
+    fn test<R: ResourceList>(aero: &Aero<R>) -> bool;
+}
+impl ResourceList for HNil {
+    fn test<R: ResourceList>(_aero: &Aero<R>) -> bool {
+        true
+    }
+}
+impl<H: Resource, T: ResourceList> ResourceList for HCons<H, T> {
+    fn test<R: ResourceList>(aero: &Aero<R>) -> bool {
+        aero.has::<H>() && T::test(aero)
+    }
+}
+
+pub(crate) fn missing_resource<T: Resource>() -> ! {
+    panic!("Resource `{}` does not exist", type_name::<T>())
+}
+
 pub(crate) fn unwrap_resource<T: Resource>(opt: Option<T>) -> T {
     if let Some(value) = opt {
         value
     } else {
-        panic!("Resource `{}` does not exist", type_name::<T>())
+        missing_resource::<T>()
     }
 }
 
@@ -19,6 +43,17 @@ pub(crate) fn unwrap_constructed<T: Resource, U>(res: Result<U, impl Into<anyhow
     }
 }
 
+pub(crate) fn unwrap_constructed_hlist<T, U>(res: Result<U, impl Into<anyhow::Error>>) -> U {
+    match res {
+        Ok(x) => x,
+        Err(e) => panic!(
+            "Failed to construct one of `{}`: {}",
+            type_name::<T>(),
+            e.into()
+        ),
+    }
+}
+
 pub(crate) fn duplicate_resource<T: Resource>() -> ! {
     panic!(
         "Duplicate resource: attempted to add a second `{}`",