diff --git a/fnarg/fnarg-test.rkt b/fnarg/fnarg-test.rkt
new file mode 100644
index 0000000..20765b8
--- /dev/null
+++ b/fnarg/fnarg-test.rkt
@@ -0,0 +1,58 @@
+#lang racket/base
+(module+ test
+  (require rackunit syntax/macro-testing
+           (for-syntax racket/base syntax/parse syntax/transformer syntax-parse-example/fnarg/fnarg))
+
+  ;; macro for defining functions that cannot mutate their parameters and do
+  ;;  not accept keyword arguments
+  (define-syntax (define/immutable-parameter stx)
+    (syntax-parse stx
+      [(_ (name:id (~var arg (fnarg #:keyword? #f)) ...) body:expr ...+)
+       ;; The `~@` form _splices_ the keyword at (A), if it exists, into the
+       ;;  function header.
+       ;; The `~?` form at (B) chooses the first form if both the arg.fresh-var
+       ;;  and arg.default attributes have values. Otherwise, `~?` chooses the
+       ;;  second form.
+       #'(define (name (~@ (~? arg.keyword)                ;; (A)
+                           (~? [arg.fresh-var arg.default] ;; (B)
+                               arg.fresh-var))
+                       ...)
+           ;; Disable set! for arg
+           (define-syntax arg.name
+             (make-variable-like-transformer #'arg.fresh-var #f))
+           ...
+           body ...)]))
+
+  (check-exn #rx"set!: cannot mutate identifier"
+    (lambda ()
+      (convert-compile-time-error
+        (let ()
+          (define/immutable-parameter (bad-fn x)
+            (set! x 'oops)
+            (void))
+          (void)))))
+
+  (check-exn #rx"keyword argument is not allowed"
+    (lambda ()
+      (convert-compile-time-error
+        (define/immutable-parameter (has-kw a #:cache cache?)
+          (void)))))
+
+  (test-case "fib/cache"
+    (define fib-cache (make-hash))
+    (define/immutable-parameter (fib n [cache #f])
+      (cond
+        [(<= n 1) n]
+        [(and cache (hash-has-key? cache n))
+         (hash-ref cache n)]
+        [else
+         (define result
+           (+ (fib (- n 1) cache)
+              (fib (- n 2) cache)))
+         (when cache
+           (hash-set! cache n result))
+         result]))
+
+    (check-equal? (fib 8) 21)
+    (check-equal? (fib 40 fib-cache) 102334155))
+)
diff --git a/fnarg/fnarg.rkt b/fnarg/fnarg.rkt
new file mode 100644
index 0000000..9b73b3e
--- /dev/null
+++ b/fnarg/fnarg.rkt
@@ -0,0 +1,28 @@
+#lang racket/base
+
+(provide fnarg)
+
+(require racket/syntax syntax/parse)
+
+;; Use _splicing_ to parse a sequence of elements for keyword args.
+(define-splicing-syntax-class (fnarg [allow-optional? #t]
+                                     #:keyword? [allow-keyword? #t])
+  #:attributes (name fresh-var keyword default)
+  #:commit
+  (pattern name:id
+           #:attr keyword #f
+           #:attr default #f
+           #:with fresh-var (generate-temporary #'name))
+  (pattern [name:id default:expr]
+           #:fail-unless allow-optional? "optional argument is not allowed"
+           #:attr keyword #f
+           #:with fresh-var (generate-temporary #'name))
+  ;; The ~seq pattern describes a _sequence_ of elements
+  (pattern (~seq keyword:keyword name:id)
+           #:fail-unless allow-keyword? "keyword argument is not allowed"
+           #:attr default #f
+           #:with fresh-var (generate-temporary #'name))
+  (pattern (~seq keyword:keyword [name:id default:expr])
+           #:fail-unless allow-optional? "optional argument is not allowed"
+           #:fail-unless allow-keyword? "keyword argument is not allowed"
+           #:with fresh-var (generate-temporary #'name)))
diff --git a/fnarg/fnarg.scrbl b/fnarg/fnarg.scrbl
new file mode 100644
index 0000000..eadff8b
--- /dev/null
+++ b/fnarg/fnarg.scrbl
@@ -0,0 +1,31 @@
+#lang syntax-parse-example
+@require[
+  (for-label racket/base syntax/parse syntax-parse-example/fnarg/fnarg)]
+
+@title{Function Parameter Syntax Class}
+@stxbee2021["shhyou" 23]
+
+@; =============================================================================
+
+@defmodule[syntax-parse-example/fnarg/fnarg]{}
+
+Syntax classes offer a powerful mechanism for abstracting over classes of
+conceptually related patterns. Moreover, syntax classes can be parameterized,
+with @racket[expr/c] being one prominent example.
+
+In this example, we define a syntax class that roughly captures the grammar of
+formal parameters in function headers. It parses the name of the formal
+parameter, the default expressions of optional arguments and the keywords of
+keyworded arguments.
+
+@defidform[fnarg]{
+  The @racket[fnarg] syntax class matches the basic argument patterns that
+  can appear in function headers: mandatory, optional, and keyword arguments.
+  The class optionally accepts two parameters to toggle whether optional and/or
+  keyword arguments are allowed.
+
+  Refer to the source file @tt{fnarg-test.rkt} for an example use.
+
+  @racketfile{fnarg.rkt}
+
+}
diff --git a/index.scrbl b/index.scrbl
index 30350b9..0219dcb 100644
--- a/index.scrbl
+++ b/index.scrbl
@@ -43,3 +43,4 @@
 @include-example{flaggable-app}
 @include-example{js-dict}
 @include-example{define-freevar}
+@include-example{fnarg}