current-input-port et al are srfi-39 parameters

* libguile/ports.c (scm_init_ports): Export the port fluids to Scheme,
  temporarily.

* module/ice-9/boot-9.scm (fluid->parameter): Turn `current-input-port'
  et al into srfi-39 parameters, backed by the exported fluids, then
  remove the fluids from the guile module.
  (%cond-expand-features): Add srfi-39.

* module/srfi/srfi-39.scm: Re-export features from boot-9.

* test-suite/tests/parameters.test: Add tests.

2 files changed, 40 insertions, 94 deletions

diff --git a/module/ice-9/boot-9.scm b/module/ice-9/boot-9.scm
index d5ba67a6d..03dad9b0d 100644
--- a/module/ice-9/boot-9.scm
+++ b/module/ice-9/boot-9.scm
@@ -2915,6 +2915,36 @@ module '(ice-9 q) '(make-q q-length))}."
 
 
 ;;;
+;;; Current ports as parameters.
+;;;
+
+(let ((fluid->parameter
+       (lambda (fluid conv)
+         (make-struct <parameter> 0
+                      (case-lambda
+                        (() (fluid-ref fluid))
+                        ((x) (let ((prev (fluid-ref fluid)))
+                               (fluid-set! fluid (conv x))
+                               prev)))
+                      fluid conv))))
+  (define-syntax-rule (port-parameterize! binding fluid predicate msg)
+    (begin
+      (set! binding (fluid->parameter (module-ref (current-module) 'fluid)
+                                      (lambda (x)
+                                        (if (predicate x) x
+                                            (error msg x)))))
+      (module-remove! (current-module) 'fluid)))
+  
+  (port-parameterize! current-input-port %current-input-port-fluid
+                      input-port? "expected an input port")
+  (port-parameterize! current-output-port %current-output-port-fluid
+                      output-port? "expected an output port")
+  (port-parameterize! current-error-port %current-error-port-fluid
+                      output-port? "expected an output port"))
+
+
+
+;;;
 ;;; Warnings.
 ;;;
 
@@ -3657,8 +3687,9 @@ module '(ice-9 q) '(make-q q-length))}."
     srfi-4   ;; homogenous numeric vectors
     srfi-6   ;; open-input-string etc, in the guile core
     srfi-13  ;; string library
-    srfi-23  ;; `error` procedure
     srfi-14  ;; character sets
+    srfi-23  ;; `error` procedure
+    srfi-39  ;; parameterize
     srfi-55  ;; require-extension
     srfi-61  ;; general cond clause
     ))
diff --git a/module/srfi/srfi-39.scm b/module/srfi/srfi-39.scm
index d1c46d028..0d540633d 100644
--- a/module/srfi/srfi-39.scm
+++ b/module/srfi/srfi-39.scm
@@ -35,104 +35,19 @@
 ;;; Code:
 
 (define-module (srfi srfi-39)
-  #:use-module (srfi srfi-16)
-
-  #:export (make-parameter)
-  #:export-syntax (parameterize)
-
   ;; helper procedure not in srfi-39.
   #:export (with-parameters*)
-  #:replace (current-input-port current-output-port current-error-port))
-
-;; Make 'srfi-39 available as a feature identifiere to `cond-expand'.
-;;
-(cond-expand-provide (current-module) '(srfi-39))
-
-(define make-parameter
-  (case-lambda
-    ((val) (make-parameter/helper val (lambda (x) x)))
-    ((val conv) (make-parameter/helper val conv))))
-
-(define get-fluid-tag (lambda () 'get-fluid)) ;; arbitrary unique (as per eq?) value
-(define get-conv-tag (lambda () 'get-conv)) ;; arbitrary unique (as per eq?) value
-
-(define (make-parameter/helper val conv)
-  (let ((fluid (make-fluid (conv val))))
-    (case-lambda
-      (()
-       (fluid-ref fluid))
-      ((new-value)
-       (cond
-        ((eq? new-value get-fluid-tag) fluid)
-        ((eq? new-value get-conv-tag) conv)
-        (else (fluid-set! fluid (conv new-value))))))))
-
-(define-syntax-rule (parameterize ((?param ?value) ...) ?body ...)
-  (with-parameters* (list ?param ...)
-                    (list ?value ...)
-                    (lambda () ?body ...)))
+  #:re-export (make-parameter
+               parameterize
+               current-input-port current-output-port current-error-port))
 
-(define current-input-port
-  (case-lambda
-    (()
-     ((@ (guile) current-input-port)))
-    ((new-value)
-     (set-current-input-port new-value))))
-
-(define current-output-port
-  (case-lambda
-    (()
-     ((@ (guile) current-output-port)))
-    ((new-value)
-     (set-current-output-port new-value))))
-
-(define current-error-port
-  (case-lambda
-    (()
-     ((@ (guile) current-error-port)))
-    ((new-value)
-     (set-current-error-port new-value))))
-
-(define port-list
-  (list current-input-port current-output-port current-error-port))
-
-;; There are no fluids behind current-input-port etc, so those parameter
-;; objects are picked out of the list and handled separately with a
-;; dynamic-wind to swap their values to and from a location (the "value"
-;; variable in the swapper procedure "let").
-;;
-;; current-input-port etc are already per-dynamic-root, so this arrangement
-;; works the same as a fluid.  Perhaps they could become fluids for ease of
-;; implementation here.
-;;
-;; Notice the use of a param local variable for the swapper procedure.  It
-;; ensures any application changes to the PARAMS list won't affect the
-;; winding.
-;;
 (define (with-parameters* params values thunk)
   (let more ((params params)
 	     (values values)
 	     (fluids '())     ;; fluids from each of PARAMS
-	     (convs  '())     ;; VALUES with conversion proc applied
-	     (swapper noop))  ;; wind/unwind procedure for ports handling
+	     (convs  '()))    ;; VALUES with conversion proc applied
     (if (null? params)
-	(if (eq? noop swapper)
-	    (with-fluids* fluids convs thunk)
-	    (dynamic-wind
-		swapper
-		(lambda ()
-		  (with-fluids* fluids convs thunk))
-		swapper))
-	(if (memq (car params) port-list)
-	    (more (cdr params) (cdr values)
-		  fluids convs
-		  (let ((param (car params))
-			(value (car values))
-			(prev-swapper swapper))
-		    (lambda ()
-		      (set! value (param value))
-		      (prev-swapper))))
-	    (more (cdr params) (cdr values)
-		  (cons ((car params) get-fluid-tag) fluids)
-		  (cons (((car params) get-conv-tag) (car values)) convs)
-		  swapper)))))
+	(with-fluids* fluids convs thunk)
+        (more (cdr params) (cdr values)
+              (cons (parameter-fluid (car params)) fluids)
+              (cons ((parameter-converter (car params)) (car values)) convs)))))