2 files changed, 30 insertions, 0 deletions
diff --git a/nix/yants/default.nix b/nix/yants/default.nix
index 6da99fa3c8c4..3e9a4c00a29a 100644
--- a/nix/yants/default.nix
+++ b/nix/yants/default.nix
@@ -296,4 +296,27 @@ in lib.fix (self: {
     in sig: func: if length sig < 2
       then (throw "Signature must at least have two types (a -> b)")
       else defun' sig func;
+
+  # Restricting types
+  #
+  # `restrict` wraps a type `t`, and uses a predicate `pred` to further
+  # restrict the values, giving the restriction a descriptive `name`.
+  #
+  # First, the wrapped type definition is checked (e.g. int) and then the
+  # value is checked with the predicate, so the predicate can already
+  # depend on the value being of the wrapped type.
+  restrict = name: pred: t:
+    let restriction = "${t.name}[${name}]"; in typedef' {
+    name = restriction;
+    checkType = v:
+      let res = t.checkType v;
+      in
+        if !(t.checkToBool res)
+        then res
+        else {
+          ok = pred v;
+          err = "${prettyPrint v} does not conform to restriction '${restriction}'";
+        };
+  };
+
 })
diff --git a/nix/yants/tests/default.nix b/nix/yants/tests/default.nix
index da539ca3562b..8ad306df932b 100644
--- a/nix/yants/tests/default.nix
+++ b/nix/yants/tests/default.nix
@@ -92,4 +92,11 @@ deepSeq rec {
     (struct { a = int; b = option string; })
     (sum { a = int; b = option string; })
   ];
+
+  testRestrict = [
+    ((restrict "< 42" (i: i < 42) int) 25)
+    ((restrict "not too long" (l: builtins.length l < 3) (list int)) [ 1 2 ])
+    (list (restrict "eq 5" (v: v == 5) any) [ 5 5 5 ])
+  ];
+
 } (pkgs.writeText "yants-tests" "All tests passed!")