diff --git a/packages/core/src/utils/isWhat.ts b/packages/core/src/utils/isWhat.ts
new file mode 100644
index 0000000..25a0094
--- /dev/null
+++ b/packages/core/src/utils/isWhat.ts
@@ -0,0 +1,21 @@
+export const notMatched: unique symbol = Symbol('notMatched')
+
+/**
+ * By using the built-in type guarding strategy, you can indirectly generate an `is` function without explicitly declaring
+ * the return type of the `is` function. This allows you to focus less on type declarations.
+ *
+ * @example
+ * ```ts
+ * const onlyStrings = ['a', 'b', 1, 2]
+ *   .filter((x): x is string => typeof x === 'string')
+ * // Code like the one above can be handled using `isWhat` without explicitly declaring the return type
+ * const onlyStrings = ['a', 'b', 1, 2]
+ *   .filter(isWhat(x => typeof x === 'number' ? x : notMatched))
+ * // The general pattern can be written as
+ * const onlyWhatType = ['a', 'b', 1, 2]
+ *   .filter(isWhat(x => (${write type guard conditional expression}) ? x : notMatched))
+ * ```
+ */
+export function isWhat<T>(match: (x: unknown) => T | typeof notMatched): (x: unknown) => x is T {
+  return (x): x is T => match(x) !== notMatched
+}