Use temporary assignments when calling functions.

* gash/environment.scm (save-variables-excursion): New procedure.
* gash/shell.scm (sh:exec-let): Use it to set up temporary assignments
during the extent of a function.
* tests/temporary-assignments.org: New file.
* Makefile.am (TESTS): Add it.

Makefile.am

@@ -144,6 +144,7 @@ FULL_TESTS =                                    \
   tests/pipes-and-booleans.org                  \
   tests/redirects.org                           \
   tests/signals.org                             \
+  tests/temporary-assignments.org               \
   tests/variable-and.org                        \
   tests/variable-or.org                         \
   tests/variable-patterns.org                   \

gash/environment.scm

@@ -33,6 +33,7 @@
             read-only?
             set-read-only!
             with-variables
+            save-variables-excursion
             get-environ
             with-environ
             getfun
@@ -202,6 +203,26 @@ extent of @var{thunk}."
         (set! inside-variables *variables*)
         (set! outside-variables *variables*)))))
 
+(define (save-variables-excursion names thunk)
+  "Save the variables listed in @var{names}, and call @var{thunk}.  When
+@var{thunk}'s dynamic extent is left, restore the variables to their
+saved state."
+  (let ((saved-variables #f))
+    (dynamic-wind
+      (lambda ()
+        (set! saved-variables
+          (map (lambda (name)
+                 (match (hash-ref *variables* name)
+                   ((? vector? vec) (cons name (vector-copy vec)))
+                   (x (cons name x))))
+               names)))
+      thunk
+      (lambda ()
+        (for-each (match-lambda
+                    ((name . #f) (hash-remove! *variables* name))
+                    ((name . value) (hash-set! *variables* name value)))
+                  saved-variables)))))
+
 (define* (get-environ #:optional (bindings '()))
   "Return a value that represents the set of current variables is
 suitable for passing to @code{environ}.  If @var{bindings} is set,

gash/shell.scm

@@ -137,11 +137,17 @@ environment variable bindings @var{bindings}."
                      (set-status! exit-val))))
           (and=> (getfun name)
                  (lambda (proc)
-                   (with-arguments (cons (car (program-arguments)) args)
+                   (save-variables-excursion (map car bindings)
                      (lambda ()
-                       (call-with-return
-                        (lambda ()
-                          (apply proc args)))))))
+                       (for-each (match-lambda
+                                   ((name . value)
+                                    (setvar! name value)))
+                                 bindings)
+                       (with-arguments (cons (car (program-arguments)) args)
+                         (lambda ()
+                           (call-with-return
+                            (lambda ()
+                              (apply proc args)))))))))
           (and=> (search-built-ins name)
                  (lambda (proc)
                    ;; TODO: Use 'bindings' here.

tests/temporary-assignments.org

@@ -0,0 +1,216 @@
+;;; Gash -- Guile As SHell
+;;; Copyright © 2019 Timothy Sample <samplet@ngyro.com>
+;;;
+;;; This file is part of Gash.
+;;;
+;;; Gash is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Gash is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Gash.  If not, see <http://www.gnu.org/licenses/>.
+
+
+;; Special built-ins
+
+* Assignments escape special built-ins
+:script:
+#+begin_src sh
+  eval 'X=foo'
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Temporary assignments escape special built-ins
+:script:
+#+begin_src sh
+  X=bar eval 'X=foo'
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Updates of temporary assignments escape special built-ins
+:script:
+#+begin_src sh
+  X=foo
+  X=bar eval 'X=baz'
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  baz
+#+end_example
+
+* Temporary assignments are not exported in special built-ins
+:script:
+#+begin_src sh
+  X=foo eval 'guile -c '\''(format #t "~a~%" (getenv "X"))'\'
+#+end_src
+:stdout:
+#+begin_example
+  #f
+#+end_example
+
+* Temporary exports do not escape special built-ins
+:script:
+#+begin_src sh
+  X=bar
+  X=foo :
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  #f
+#+end_example
+
+* Exports escape special built-ins
+:script:
+#+begin_src sh
+  X=foo
+  eval 'export X'
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Exports of temporary assignments escape special built-ins
+:script:
+#+begin_src sh
+  X=foo eval 'export X'
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+
+;; Functions
+
+* Assignments escape functions
+:script:
+#+begin_src sh
+  f() {
+      X=foo
+  }
+  f
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Temporary assignments do not escape functions
+:script:
+#+begin_src sh
+  f() {
+      X=foo
+  }
+  X=bar f
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  
+#+end_example
+
+* Updates of temporary assignments do not escape functions
+:script:
+#+begin_src sh
+  f() {
+      X=baz
+  }
+  X=foo
+  X=bar f
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Temporary assignments are not exported in functions
+:script:
+#+begin_src sh
+  f() {
+      guile -c '(format #t "~a~%" (getenv "X"))'
+  }
+  X=foo f
+#+end_src
+:stdout:
+#+begin_example
+  #f
+#+end_example
+
+* Temporary exports do not escape functions
+:script:
+#+begin_src sh
+  f() {
+      :
+  }
+  X=bar
+  X=foo f
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  #f
+#+end_example
+
+* Exports escape functions
+:script:
+#+begin_src sh
+  f() {
+      export X
+  }
+  X=foo
+  f
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  foo
+#+end_example
+
+* Exports of temporary assignments do not escape functions
+:script:
+#+begin_src sh
+  f() {
+      export X
+  }
+  X=foo f
+  guile -c '(format #t "~a~%" (getenv "X"))'
+#+end_src
+:stdout:
+#+begin_example
+  #f
+#+end_example
+
+* Read-only temporary assignments do not escape functions
+:script:
+#+begin_src sh
+  f() {
+      readonly X
+  }
+  X=foo f
+  echo $X
+#+end_src
+:stdout:
+#+begin_example
+  
+#+end_example