Add subshell semantics

* geesh/shell.scm (subshell): New public function.
* tests/shell.scm: Test it.
* .dir-locals.el: Indent it nicely.

.dir-locals.el

@@ -9,4 +9,5 @@
    (eval . (put '<sh-with-redirects> 'scheme-indent-function 1))
    (eval . (put 'call-with-backquoted-input-port 'scheme-indent-function 1))
    (eval . (put 'make-script 'scheme-indent-function 1))
+   (eval . (put 'sh:subshell 'scheme-indent-function 1))
    (eval . (put 'sh:with-redirects 'scheme-indent-function 2)))))

geesh/shell.scm

@@ -5,6 +5,7 @@
   #:use-module (srfi srfi-26)
   #:export (sh:exec-let
             sh:exec
+            sh:subshell
             sh:with-redirects))
 
 ;;; Commentary:
@@ -149,3 +150,14 @@ filename used for the here-document contents."
       (lambda ()
         (flush-all-ports)
         (for-each restore-saved-fdes! (reverse saved-fds))))))
+
+
+;;; Subshells.
+
+(define (sh:subshell env thunk)
+  "Run @var{thunk} in a subshell environment."
+  (match (primitive-fork)
+    (0 (thunk)
+       (primitive-exit))
+    (pid (match-let (((pid . status) (waitpid pid)))
+           (set-var! env "?" (number->string (status:exit-val status)))))))

tests/shell.scm

@@ -311,4 +311,18 @@
 
 ;; TODO: Read-write tests, closing tests, clobbering tests.
 
+
+;;; Subshells.
+
+(test-equal "Subshells cannot change variables"
+  "foo"
+  (let ((env (make-environment '(("x" . "foo")))))
+    (sh:subshell env
+      (lambda ()
+        (set-var! env "x" "bar")))
+    (var-ref env "x")))
+
+;; TODO: Test other means of manipulating the environment and exit
+;; statuses.
+
 (test-end)