add syntax/for-transform

- provides expand-clause (as expand-for-clause) from racket/private/for.rkt

pkgs/racket-pkgs/racket-doc/syntax/scribblings/for-transform.scrbl

@@ -0,0 +1,33 @@
+#lang scribble/manual
+@(require (for-label racket/base
+                     racket/sequence))
+
+@title{@racket[for] Clause Transforms}
+
+@defmodule[syntax/for-transform]{The @racketmodname[syntax/for-transform] module
+provides a helper function that gives access to the sequence transformers 
+defined by @racket[define-sequence-syntax]. This is what the @racket[for] forms
+use and enables faster 
+@tech[#:doc '(lib "scribblings/reference/reference.scrbl")]{sequence} traversal
+than what the sequence interface provides.}
+
+@defproc[(expand-for-clause [orig-stx syntax?] [clause syntax?]) syntax?]{
+
+Expands a @racket[for] clause of the form @racket[[(x ...) seq-expr]], where
+@racket[x] are identifiers, to:
+
+@racketblock[
+(([(outer-id ...) outer-expr] ...)
+ outer-check
+ ([loop-id loop-expr] ...)
+ pos-guard
+ ([(inner-id ...) inner-expr] ...)
+ pre-guard
+ post-guard
+ (loop-arg ...))]
+
+which can then be spliced into the appropriate iterations. See @racket[:do-in]
+for more information.
+
+The first argument @racket[orig-stx] is used only for reporting syntax errors.
+}

pkgs/racket-pkgs/racket-doc/syntax/scribblings/syntax.scrbl

@@ -19,6 +19,8 @@
 
 @include-section["for-body.scrbl"]
 
+@include-section["for-transform.scrbl"]
+
 @include-section["srcloc.scrbl"]
 
 @include-section["quote.scrbl"]

racket/collects/racket/HISTORY.txt

@@ -2,6 +2,7 @@ Version 5.90.0.9
 Allow hash table chaperones and impersonators to Support efficient
  hash-clear and hash-clear!
 setup/collection-name: added
+syntax/for-transform: added
 
 Version 5.90.0.6
 Added path<?, symbol<?

racket/collects/racket/private/for.rkt

@@ -32,6 +32,7 @@
 
              for/fold/derived for*/fold/derived
              (for-syntax split-for-body)
+             (for-syntax (rename expand-clause expand-for-clause))
 
              (rename *in-range in-range)
              (rename *in-naturals in-naturals)

racket/collects/syntax/for-transform.rkt

@@ -0,0 +1,4 @@
+#lang racket/base
+
+(require (for-template (only-in racket/private/for expand-for-clause)))
+(provide expand-for-clause)