4 files changed, 32 insertions, 3 deletions
diff --git a/tests/lang/eval-fail-substring.nix b/tests/lang/eval-fail-substring.nix
new file mode 100644
index 000000000000..f37c2bc0a160
--- /dev/null
+++ b/tests/lang/eval-fail-substring.nix
@@ -0,0 +1 @@
+builtins.substring (builtins.sub 0 1) 1 "x"
diff --git a/tests/lang/eval-okay-substring.exp b/tests/lang/eval-okay-substring.exp
new file mode 100644
index 000000000000..d6098476b820
--- /dev/null
+++ b/tests/lang/eval-okay-substring.exp
@@ -0,0 +1 @@
+Str("ooxfoobarybarzobaabb",[])
diff --git a/tests/lang/eval-okay-substring.nix b/tests/lang/eval-okay-substring.nix
new file mode 100644
index 000000000000..184d72580c51
--- /dev/null
+++ b/tests/lang/eval-okay-substring.nix
@@ -0,0 +1,19 @@
+with builtins;
+
+let
+
+  s = "foobar";
+
+in
+
+substring 1 2 s
++ "x"
++ substring 0 (stringLength s) s
++ "y"
++ substring 3 100 s
++ "z"
++ substring 2 (sub (stringLength s) 3) s
++ "a"
++ substring 3 0 s
++ "b"
++ substring 3 1 s
diff --git a/tests/lang/lib.nix b/tests/lang/lib.nix
index f888453ffbea..ec705299a7a9 100644
--- a/tests/lang/lib.nix
+++ b/tests/lang/lib.nix
@@ -1,18 +1,26 @@
+with builtins;
+
 rec {
 
   fold = op: nul: list:
     if list == []
     then nul
-    else op (builtins.head list) (fold op nul (builtins.tail list));
+    else op (head list) (fold op nul (tail list));
 
   concat =
     fold (x: y: x + y) "";
 
   flatten = x:
-    if builtins.isList x
+    if isList x
     then fold (x: y: (flatten x) ++ y) [] x
     else [x];
 
-  sum = fold (x: y: builtins.add x y) 0;
+  sum = fold (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let lenFileName = stringLength fileName;
+        lenExt = stringLength ext;
+    in !(lessThan lenFileName lenExt) &&
+       substring (sub lenFileName lenExt) lenFileName fileName == ext;
 
 }