New primitives:

* `sub' to subtract two numbers.
* `stringLength' to get the length of a string.
* `substring' to get a substring of a string.  These should be enough
  to allow most string operations to be expressed.

tests/filter-source.nix.in

@@ -4,6 +4,9 @@ derivation {
   builder = "@shell@";
   args = ["-e" "-x" (builtins.toFile "builder" "PATH=@testPath@; ln -s $input $out")];
   input =
-    let filter = path: type: type != "symlink" && baseNameOf (toString path) != "foo";
+    let filter = path: type:
+      type != "symlink"
+      && baseNameOf path != "foo"
+      && !((import ./lang/lib.nix).hasSuffix ".bak" (baseNameOf path));
     in builtins.filterSource filter ./test-tmp/filterin;
 }

tests/filter-source.sh

@@ -5,6 +5,9 @@ mkdir $TEST_ROOT/filterin
 mkdir $TEST_ROOT/filterin/foo
 touch $TEST_ROOT/filterin/foo/bar
 touch $TEST_ROOT/filterin/xyzzy
+touch $TEST_ROOT/filterin/b
+touch $TEST_ROOT/filterin/bak
+touch $TEST_ROOT/filterin/bla.c.bak
 ln -s xyzzy $TEST_ROOT/filterin/link
 
 $NIX_BIN_DIR/nix-build ./filter-source.nix -o $TEST_ROOT/filterout
@@ -12,4 +15,6 @@ $NIX_BIN_DIR/nix-build ./filter-source.nix -o $TEST_ROOT/filterout
 set -x
 test ! -e $TEST_ROOT/filterout/foo/bar
 test -e $TEST_ROOT/filterout/xyzzy
+test -e $TEST_ROOT/filterout/bak
+test ! -e $TEST_ROOT/filterout/bla.c.bak
 test ! -L $TEST_ROOT/filterout/link

tests/lang/eval-fail-substring.nix

@@ -0,0 +1 @@
+builtins.substring (builtins.sub 0 1) 1 "x"

tests/lang/eval-okay-substring.exp

@@ -0,0 +1 @@
+Str("ooxfoobarybarzobaabb",[])

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

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;
 
 }