add lang-reader-module-paths to factor out copy-pasted code (#1347)

* add lang-reader-module-paths to syntax/module-reader

to be used as the third argument to make-meta-reader in lang-extensions
like at-exp

* document lang-reader-module-paths

* use lang-reader-module-paths in at-exp

pkgs/at-exp-lib/at-exp/lang/reader.rkt

@@ -16,15 +16,7 @@
     (make-meta-reader
      'at-exp
      "language path"
-     (lambda (bstr)
-       (let* ([str (bytes->string/latin-1 bstr)]
-              [sym (string->symbol str)])
-         (and (module-path? sym)
-              (vector
-               ;; try submod first:
-               `(submod ,sym reader)
-               ;; fall back to /lang/reader:
-               (string->symbol (string-append str "/lang/reader"))))))
+     lang-reader-module-paths
      wrap-reader
      (lambda (orig-read-syntax)
        (define read-syntax (wrap-reader orig-read-syntax))

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

@@ -409,7 +409,8 @@ a vector of module paths, they are tried in order using
 @racket[#f], a reader exception is raised in the same way as when
 @racket[read-spec] produces a @racket[#f]. The @racketmodname[planet]
 languages supply a @racket[module-path-parser] that converts a byte
-string to a module path.
+string to a module path. Lang-extensions like @racketmodname[at-exp]
+use @racket[lang-reader-module-paths] as this argument.
 
 If loading the module produced by @racket[module-path-parser]
 succeeds, then the loaded module's @racketidfont{read},
@@ -426,6 +427,14 @@ The procedures generated by @racket[make-meta-reader] are not meant
 for use with the @racketmodname[syntax/module-reader] language; they
 are meant to be exported directly.}
 
+@defproc[(lang-reader-module-paths [bstr bytes?]) (or/c #f (vectorof module-path?))]{
+To be used as the third argument to @racket[make-meta-reader] in
+lang-extensions like @racketmodname[at-exp]. On success, it returns a
+vector of module paths, one of which should point to the reader module
+for the @hash-lang[] @racket[bstr] language. These paths are
+@racket[(submod base-path reader)] and @racket[base-path/lang/reader].
+}
+
 
 @defproc[(wrap-read-all [mod-path module-path?]
                         [in input-port?]

racket/collects/syntax/module-reader.rkt

@@ -5,6 +5,7 @@
   (provide (rename-out [provide-module-reader #%module-begin]
                        [wrap wrap-read-all])
            make-meta-reader
+           lang-reader-module-paths
            (except-out (all-from-out racket/private/base) #%module-begin))
 
   (define ar? procedure-arity-includes?)
@@ -296,4 +297,20 @@
       (read-fn inp 'read-syntax (list src) src mod line col pos
                convert-read-syntax))
 
-    (values -read -read-syntax -get-info)))
+    (values -read -read-syntax -get-info))
+
+  ;; lang-reader-module-paths : Byte-String -> (U False (Vectorof Module-Path))
+  ;; To be used as the third argument to make-meta-reader in lang-extensions
+  ;; like at-exp. On success, returns a vector of module paths, one of which
+  ;; should point to the reader module for the #lang bstr language.
+  (define (lang-reader-module-paths bstr)
+    (let* ([str (bytes->string/latin-1 bstr)]
+           [sym (string->symbol str)])
+      (and (module-path? sym)
+           (vector
+            ;; try submod first:
+            `(submod ,sym reader)
+            ;; fall back to /lang/reader:
+            (string->symbol (string-append str "/lang/reader"))))))
+
+  )