ListPair.map2, based on code by escalier@riseup.net

1 files changed, 11 insertions, 0 deletions

diff --git a/lib/ur/listPair.ur b/lib/ur/listPair.ur
index 0182af19..d69993f9 100644
--- a/lib/ur/listPair.ur
+++ b/lib/ur/listPair.ur
@@ -33,3 +33,14 @@ fun all [a] [b] f =
     in
         all'
     end
+
+fun map2 [a] [b] [c] (f : a -> b -> c) =
+    let
+        fun map2' ls1 ls2 =
+            case (ls1, ls2) of
+                ([], []) => []
+              | (x1 :: ls1, x2 :: ls2) => f x1 x2 :: map2' ls1 ls2
+              | _ => error <xml>ListPair.map2: Unequal list lengths</xml>
+    in
+        map2'
+    end