Heterogeneous lists.

theories/hlist.v

+From stdpp Require Import base.
+
+(* Not using [list Type] in order to avoid universe inconsistencies *)
+Inductive tlist := tnil : tlist | tcons : Type → tlist → tlist.
+
+Inductive hlist : tlist → Type :=
+  | hnil : hlist tnil
+  | hcons {A As} : A → hlist As → hlist (tcons A As).
+
+Fixpoint himpl (As : tlist) (B : Type) : Type :=
+  match As with tnil => B | tcons A As => A → himpl As B end.
+
+Definition happly {As B} (f : himpl As B) (xs : hlist As) : B :=
+  (fix go As xs :=
+    match xs in hlist As return himpl As B → B with
+    | hnil => λ f, f
+    | hcons A As x xs => λ f, go As xs (f x)
+    end) _ xs f.