lib/strings: add a `toInt` helper (close #11242)

While the function itself is pretty easy, it's not straitforward to find a way to convert string to int with nix.

lib/strings.nix

@@ -223,4 +223,12 @@ rec {
   # Check whether a value is a store path.
   isStorePath = x: builtins.substring 0 1 (toString x) == "/" && dirOf (builtins.toPath x) == builtins.storeDir;
 
+  # Convert string to int
+  # Obviously, it is a bit hacky to use fromJSON that way.
+  toInt = str:
+    let may_be_int = builtins.fromJSON str; in
+    if builtins.isInt may_be_int
+    then may_be_int
+    else throw "Could not convert ${str} to int.";
+
 }

lib/tests.nix

@@ -110,4 +110,14 @@ runTests {
     expected = [2 30 40 42];
   };
 
+  testToIntShouldConvertStringToInt = {
+    expr = toInt "27";
+    expected = 27;
+  };
+
+  testToIntShouldThrowErrorIfItCouldNotConvertToInt = {
+    expr = builtins.tryEval (toInt "\"foo\"");
+    expected = { success = false; value = false; };
+  };
+
 }