1 files changed, 51 insertions, 0 deletions
diff --git a/starlarktest/assert.star b/starlarktest/assert.star
new file mode 100644
index 0000000..c6e480f
--- /dev/null
+++ b/starlarktest/assert.star
@@ -0,0 +1,51 @@
+# Predeclared built-ins for this module:
+#
+# error(msg): report an error in Go's test framework without halting execution.
+#  This is distinct from the built-in fail function, which halts execution.
+# catch(f): evaluate f() and returns its evaluation error message, if any
+# matches(str, pattern): report whether str matches regular expression pattern.
+# module(**kwargs): a constructor for a module.
+# _freeze(x): freeze the value x and everything reachable from it.
+#
+# Clients may use these functions to define their own testing abstractions.
+
+def _eq(x, y):
+    if x != y:
+        error("%r != %r" % (x, y))
+
+def _ne(x, y):
+    if x == y:
+        error("%r == %r" % (x, y))
+
+def _true(cond, msg = "assertion failed"):
+    if not cond:
+        error(msg)
+
+def _lt(x, y):
+    if not (x < y):
+        error("%s is not less than %s" % (x, y))
+
+def _contains(x, y):
+    if y not in x:
+        error("%s does not contain %s" % (x, y))
+
+def _fails(f, pattern):
+    "assert_fails asserts that evaluation of f() fails with the specified error."
+    msg = catch(f)
+    if msg == None:
+        error("evaluation succeeded unexpectedly (want error matching %r)" % pattern)
+    elif not matches(pattern, msg):
+        error("regular expression (%s) did not match error (%s)" % (pattern, msg))
+
+freeze = _freeze  # an exported global whose value is the built-in freeze function
+
+assert = module(
+    "assert",
+    fail = error,
+    eq = _eq,
+    ne = _ne,
+    true = _true,
+    lt = _lt,
+    contains = _contains,
+    fails = _fails,
+)