Make folders abstract

2 files changed, 9 insertions, 5 deletions

diff --git a/lib/ur/top.ur b/lib/ur/top.ur
index 5ce6ea9a..bfee2dfb 100644
--- a/lib/ur/top.ur
+++ b/lib/ur/top.ur
@@ -7,6 +7,8 @@ con folder = K ==> fn r :: {K} =>
                       -> tf [] -> tf r
 
 structure Folder = struct
+    fun fold K (r ::: {K}) (fl : folder r) = fl
+
     fun nil K (tf :: {K} -> Type)
             (f : nm :: Name -> v :: K -> r :: {K} -> tf r
                  -> [[nm] ~ r] => tf ([nm = v] ++ r))
diff --git a/lib/ur/top.urs b/lib/ur/top.urs
index 29b34b10..a9e5b6b3 100644
--- a/lib/ur/top.urs
+++ b/lib/ur/top.urs
@@ -1,12 +1,14 @@
 (** Row folding *)
 
-con folder = K ==> fn r :: {K} =>
-                      tf :: ({K} -> Type)
-                      -> (nm :: Name -> v :: K -> r :: {K} -> tf r
-                          -> [[nm] ~ r] => tf ([nm = v] ++ r))
-                      -> tf [] -> tf r
+con folder :: K --> {K} -> Type
 
 structure Folder : sig
+    val fold : K --> r ::: {K} -> folder r
+               -> tf :: ({K} -> Type)
+               -> (nm :: Name -> v :: K -> r :: {K} -> tf r
+                   -> [[nm] ~ r] => tf ([nm = v] ++ r))
+               -> tf [] -> tf r
+
     val nil : K --> folder (([]) :: {K})
     val cons : K --> r ::: {K} -> nm :: Name -> v :: K
                -> [[nm] ~ r] => folder r -> folder ([nm = v] ++ r)