From 597661fa4e90475843318d052859aedabbcff9f7 Mon Sep 17 00:00:00 2001
From: Alexis King <lexi.lambda@gmail.com>
Date: Tue, 3 Jan 2017 12:01:39 -0800
Subject: [PATCH] Make range from racket/list act like in-range when used with
 for

---
 .../scribblings/reference/pairs.scrbl         |  9 ++++++-
 racket/collects/racket/list.rkt               | 25 +++++++++++++++----
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/pkgs/racket-doc/scribblings/reference/pairs.scrbl b/pkgs/racket-doc/scribblings/reference/pairs.scrbl
index c3a46df4ba..ab115961a1 100644
--- a/pkgs/racket-doc/scribblings/reference/pairs.scrbl
+++ b/pkgs/racket-doc/scribblings/reference/pairs.scrbl
@@ -1264,12 +1264,19 @@ predecessor until @racket[end] (excluded) is reached.  If no starting
 point is provided, @racket[0] is used. If no @racket[step] argument is
 provided, @racket[1] is used.
 
+Like @racket[in-range], a @racket[range] application can provide better
+performance when it appears directly in a @racket[for] clause.
+
 @mz-examples[#:eval list-eval
   (range 10)
   (range 10 20)
   (range 20 40 2)
   (range 20 10 -1)
-  (range 10 15 1.5)]}
+  (range 10 15 1.5)]
+
+@history[#:changed "6.7.0.4"
+         @elem{Adjusted to cooperate with @racket[for] in the same
+               way that @racket[in-range] does.}]}
 
 
 @defproc[(append-map [proc procedure?] [lst list?] ...+)
diff --git a/racket/collects/racket/list.rkt b/racket/collects/racket/list.rkt
index e23c526c91..eefd263433 100644
--- a/racket/collects/racket/list.rkt
+++ b/racket/collects/racket/list.rkt
@@ -61,6 +61,8 @@
          remf
          remf*)
 
+(require (for-syntax racket/base))
+
 (define (first x)
   (if (and (pair? x) (list? x))
     (car x)
@@ -561,11 +563,24 @@
         (if (pred x) (loop l (cons x i) o) (loop l i (cons x o)))))))
 
 ;; similar to in-range, but returns a list
-(define range
-  (case-lambda
-    [(end)            (for/list ([i (in-range end)])            i)]
-    [(start end)      (for/list ([i (in-range start end)])      i)]
-    [(start end step) (for/list ([i (in-range start end step)]) i)]))
+(define range-proc
+  (let ()
+    ; make sure range has the right runtime name
+    (define range
+      (case-lambda
+        [(end)            (for/list ([i (in-range end)])            i)]
+        [(start end)      (for/list ([i (in-range start end)])      i)]
+        [(start end step) (for/list ([i (in-range start end step)]) i)]))
+    range))
+
+(define-sequence-syntax range
+  (λ () #'range-proc)
+  (λ (stx)
+    (syntax-case stx ()
+      [[(n) (_ end)]            #'[(n) (in-range end)]]
+      [[(n) (_ start end)]      #'[(n) (in-range start end)]]
+      [[(n) (_ start end step)] #'[(n) (in-range start end step)]]
+      [[ids range-expr]         #'[ids (#%expression range-expr)]])))
 
 (define append-map
   (case-lambda [(f l)      (apply append (map f l))]