Enhance transcoder-related functionality of `(rnrs io ports)'

* module/rnrs/io/ports.scm (transcoder-eol-style)
  (transcoder-error-handling-mode): Export these.
  (textual-port?): Implement this procedure and export it.
* module/rnrs.scm: Export these here as well.

* module/rnrs/io/ports.scm (port-transcoder): Implement this procedure.
  (binary-port?): Treat only ports without an encoding as binary ports,
  add docstring.
  (standard-input-port, standard-output-port, standard-error-port):
  Ensure these are created without an encoding.
  (eol-style): Add `none' as enumeration member.
  (native-eol-style): Switch to `none' from `lf'.

* test-suite/tests/r6rs-ports.test (7.2.7 Input ports)
  (8.2.10 Output ports): Test binary-ness of `standard-input-port',
  `standard-output-port' and `standard-error-port'.
  (8.2.6 Input and output ports): Add test for `port-transcoder'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

module/rnrs.scm

@@ -162,12 +162,14 @@
 
 	  file-options buffer-mode buffer-mode?
 	  eol-style native-eol-style error-handling-mode
-	  make-transcoder transcoder-codec native-transcoder
+	  make-transcoder transcoder-codec transcoder-eol-style
+          transcoder-error-handling-mode native-transcoder
 	  latin-1-codec utf-8-codec utf-16-codec
 	  
 	  eof-object? port? input-port? output-port? eof-object port-eof?
 	  port-transcoder
-	  binary-port? transcoded-port port-position set-port-position!
+	  binary-port? textual-port? transcoded-port
+	  port-position set-port-position!
 	  port-has-port-position? port-has-set-port-position!?
           close-port call-with-port
 	  open-bytevector-input-port make-custom-binary-input-port get-u8 

module/rnrs/io/ports.scm

@@ -32,13 +32,14 @@
           ;; auxiliary types
           file-options buffer-mode buffer-mode?
           eol-style native-eol-style error-handling-mode
-          make-transcoder transcoder-codec native-transcoder
+          make-transcoder transcoder-codec transcoder-eol-style
+          transcoder-error-handling-mode native-transcoder
           latin-1-codec utf-8-codec utf-16-codec
            
           ;; input & output ports
           port? input-port? output-port?
           port-eof?
-          port-transcoder binary-port? transcoded-port
+          port-transcoder binary-port? textual-port? transcoded-port
           port-position set-port-position!
           port-has-port-position? port-has-set-port-position!?
           call-with-port close-port
@@ -129,11 +130,11 @@
   (enum-set-member? symbol (enum-set-universe (buffer-modes))))
 
 (define-enumeration eol-style
-  (lf cr crlf nel crnel ls)
+  (lf cr crlf nel crnel ls none)
   eol-styles)
 
 (define (native-eol-style)
-  (eol-style lf))
+  (eol-style none))
 
 (define-enumeration error-handling-mode
   (ignore raise replace)
@@ -190,10 +191,30 @@
 ;;;
 
 (define (port-transcoder port)
-  (error "port transcoders are not supported" port))
+  "Return the transcoder object associated with @var{port}, or @code{#f}
+if the port has no transcoder."
+  (cond ((port-encoding port)
+         => (lambda (encoding)
+              (make-transcoder
+               encoding
+               (native-eol-style)
+               (case (port-conversion-strategy port)
+                 ((error) 'raise)
+                 ((substitute) 'replace)
+                 (else
+                  (assertion-violation 'port-transcoder
+                                       "unsupported error handling mode"))))))
+        (else
+         #f)))
 
 (define (binary-port? port)
-  ;; So far, we don't support transcoders other than the binary transcoder.
+  "Returns @code{#t} if @var{port} does not have an associated encoding,
+@code{#f} otherwise."
+  (not (port-encoding port)))
+
+(define (textual-port? port)
+  "Always returns @var{#t}, as all ports can be used for textual I/O in
+Guile."
   #t)
 
 (define (port-eof? port)
@@ -408,13 +429,16 @@ the characters read."
 ;;;
 
 (define (standard-input-port)
-  (dup->inport 0))
+  (with-fluids ((%default-port-encoding #f))
+    (dup->inport 0)))
 
 (define (standard-output-port)
-  (dup->outport 1))
+  (with-fluids ((%default-port-encoding #f))
+    (dup->outport 1)))
 
 (define (standard-error-port)
-  (dup->outport 2))
+  (with-fluids ((%default-port-encoding #f))
+    (dup->outport 2)))
 
 )