Move identity' from unstable/function' into `racket/function'.

collects/racket/function.rkt

@@ -1,6 +1,8 @@
 #lang scheme/base
 
-(provide const negate curry curryr)
+(provide identity const negate curry curryr)
+
+(define (identity x) x)
 
 (define (const c)
   (define (const . _) c)

collects/scribblings/reference/procedures.scrbl

@@ -482,6 +482,10 @@ applied.}
 @(define fun-eval (make-base-eval))
 @(interaction-eval #:eval fun-eval (require racket/function))
 
+@defproc[(identity [v any/c]) any/c]{
+Returns @scheme[v].
+}
+
 @defproc[(const [v any]) procedure?]{
 
 Returns a procedure that accepts any arguments and returns @scheme[v].

collects/tests/racket/function.rktl

@@ -43,6 +43,13 @@
   (test 'f object-name (rec  f (lambda (x) x)))
   (test (list 2) (rec (f . x) (if (= (car x) 3) (f 2) x)) 3))
 
+;; ---------- identity ----------
+(let ()
+  (test 'foo identity 'foo)
+  (test 1 identity 1)
+  (err/rt-test (identity 1 2))
+  (err/rt-test (identity)))
+
 ;; ---------- const ----------
 (let ()
   (test 'foo (const 'foo))

collects/tests/unstable/function.rkt

@@ -9,11 +9,6 @@
 
    (test-suite "Simple Functions"
 
-     (test-suite "identity"
-       (test-case "unique symbol"
-         (let* ([sym (gensym)])
-           (check-eq? (identity sym) sym))))
-
      (test-suite "thunk"
        (test-case "unique symbol"
          (let* ([count 0]

collects/unstable/function.rkt

@@ -91,8 +91,6 @@
 ;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(define (identity x) x)
-
 (define-syntax (thunk stx)
   (syntax-case stx ()
     [(thunk body ...)
@@ -365,7 +363,6 @@
 
 (provide
  ;; functions
- identity
  thunk
  conjoin disjoin
  curryn currynr papply papplyr call

collects/unstable/scribblings/function.scrbl

@@ -14,12 +14,6 @@ This module provides tools for higher-order programming and creating functions.
 
 @section{Simple Functions}
 
-@defproc[(identity [x any/c]) (one-of/c x)]{
-
-Returns @scheme[x].
-
-}
-
 @defform[(thunk body ...)]{
 
 Creates a function that ignores its inputs and evaluates the given body.  Useful