diff --git a/lib/std/core/maybe.kk b/lib/std/core/maybe.kk
index f96c329ae..801e553c6 100644
--- a/lib/std/core/maybe.kk
+++ b/lib/std/core/maybe.kk
@@ -26,10 +26,16 @@ pub fun default( m : maybe<a>, nothing : a ) : a
     Just(x) -> x
 
 // Get the value of the `Just` constructor or raise an exception
-pub fun unjust( m : maybe<a> ) : exn a
+pub fun unjust( m : maybe<a>, ?kk-file-line: string) : exn a
   match m
     Just(x) -> x
-    Nothing -> throw("unexpected Nothing in std/core/maybe/unjust")
+    Nothing -> throw("unexpected Nothing in " ++ ?kk-file-line)
+
+// Get the value of the `Just` constructor or raise an exception with `error-msg`
+pub fun expect( m : maybe<a>, error-msg: string) : exn a
+  match m
+    Just(x) -> x
+    Nothing -> throw(error-msg)
 
 pub fun map( m : maybe<a>, f : a -> e b ) : e maybe<b>
   match m
diff --git a/lib/std/core/maybe2.kk b/lib/std/core/maybe2.kk
index 10c7ff0a5..f7278a23b 100644
--- a/lib/std/core/maybe2.kk
+++ b/lib/std/core/maybe2.kk
@@ -26,10 +26,16 @@ pub fun default( m : maybe2<a,b>, nothing : (a,b) ) : (a,b)
     Just2(x,y) -> (x,y)
 
 // Get the value of the `Just` constructor or raise an exception
-pub fun unjust2( m : maybe2<a,b> ) : exn (a,b)
+pub fun unjust( m : maybe2<a,b>, ?kk-file-line: string) : exn (a,b)
   match m
     Just2(x,y) -> (x,y)
-    Nothing2   -> throw("unexpected Nothing2 in std/core/maybe/unjust2")
+    Nothing2   -> throw("unexpected Nothing2 in " ++ ?kk-file-line)
+
+// Get the value of the `Just` constructor or raise an exception
+pub fun expect( m : maybe2<a,b>, error-msg: string) : exn (a,b)
+  match m
+    Just2(x,y) -> (x,y)
+    Nothing2   -> throw(error-msg)
 
 pub fun map( m : maybe2<a,b>, f : (a,b) -> e (c,d) ) : e maybe2<c,d>
   match m