Update AST for command substitutions

The AST generated by the parser did not match the syntax document.
However, neither were right.  This commit updates the syntax document
to describe a more consistent form, and fixes the code to follow it.

* doc/syntax.txt (word): Make the children of '<sh-cmd-sub>' a
possibly empty list of 'list's.
* geesh/lexer.scm (get-bracketed-command): Splice in children for
'<sh-cmd-sub>'.
(get-backquoted-command): Ditto.
* geesh/parser.scm (read-sh/bracketed): Make sure that result is
always a list.
(read-sh/backquoted): Ditto.
* tests/lexer.scm: Update tests accordingly.
* tests/parser.scm: Ditto.

doc/syntax.txt

@@ -40,7 +40,7 @@ redir ::= ('> fdes word)
 word  ::= string
         | (word ...)
         | ('<sh-quote> word)
-        | ('<sh-cmd-sub> sync ...)
+        | ('<sh-cmd-sub> [list] ...)
         | ('<sh-ref> var)
         | ('<sh-ref-or> var [word])
         | ('<sh-ref-or*> var [word])

geesh/lexer.scm

@@ -322,7 +322,7 @@ leading '$')."
     (#\(
      (let ((result ((read-bracketed-command) port)))
        (match (get-char port)
-         (#\) `(<sh-cmd-sub> ,result)))))))
+         (#\) `(<sh-cmd-sub> ,@result)))))))
 
 (define (get-backquoted-command port)
   "Get a backquoted command ('`...`') from @var{port}."
@@ -330,7 +330,7 @@ leading '$')."
     (#\`
      (let ((result ((read-backquoted-command) port)))
        (match (get-char port)
-         (#\` `(<sh-cmd-sub> ,result)))))))
+         (#\` `(<sh-cmd-sub> ,@result)))))))
 
 (define (get-expansion port)
   "Get an expansion ('$name', '${...}', '$(...)', or '`...`') from

geesh/parser.scm

@@ -766,7 +766,8 @@ bracket."
          (parse (make-parser #:open-bracket-hook incr-bracket-depth!
                              #:close-bracket-hook decr-bracket-depth!)))
     (match (parse lex syntax-error)
-      ((? eof-object?) #f)
+      ((? eof-object?) '())
+      (((? symbol? tag) . rest) `((,tag . ,rest)))
       (code code))))
 
 (define (read-sh/backquoted port)
@@ -776,7 +777,8 @@ bracket."
       (let ((lex (make-lexer port read-sh/bracketed read-sh/backquoted))
             (parse (make-parser)))
         (match (parse lex syntax-error)
-          ((? eof-object?) #f)
+          ((? eof-object?) '())
+          (((? symbol? tag) . rest) `((,tag . ,rest)))
           (code code))))))
 
 (define* (read-sh #:optional (port #f))

tests/lexer.scm

@@ -340,7 +340,7 @@
   (parameterize ((read-bracketed-command
                   (lambda (port)
                     (string-for-each (lambda _ (read-char port)) "foo")
-                    '(<sh-exec> "foo"))))
+                    '((<sh-exec> "foo")))))
     (tokenize "$(foo)")))
 
 ;;;
@@ -352,7 +352,7 @@
   (parameterize ((read-backquoted-command
                   (lambda (port)
                     (string-for-each (lambda _ (read-char port)) "foo")
-                    '(<sh-exec> "foo"))))
+                    '((<sh-exec> "foo")))))
     (tokenize "`foo`")))
 
 (test-end)

tests/parser.scm

@@ -314,12 +314,12 @@
   (parse "echo $(foo $(bar))"))
 
 (test-equal "Parses empty bracketed command substitions"
-  '(<sh-exec> "echo" (<sh-cmd-sub> #f))
+  '(<sh-exec> "echo" (<sh-cmd-sub>))
   (parse "echo $()"))
 
 (test-equal "Parses multiline bracketed command substitions"
-  '(<sh-exec> "echo" (<sh-cmd-sub> ((<sh-exec> "foo")
-                                    (<sh-exec> "bar"))))
+  '(<sh-exec> "echo" (<sh-cmd-sub> (<sh-exec> "foo")
+                                   (<sh-exec> "bar")))
   (parse "echo $(foo
                  bar)"))
 
@@ -336,12 +336,12 @@
   (parse "echo `foo \\`bar\\``"))
 
 (test-equal "Parses empty backquoted command substitions"
-  '(<sh-exec> "echo" (<sh-cmd-sub> #f))
+  '(<sh-exec> "echo" (<sh-cmd-sub>))
   (parse "echo ``"))
 
 (test-equal "Parses multiline backquoted command substitions"
-  '(<sh-exec> "echo" (<sh-cmd-sub> ((<sh-exec> "foo")
-                                    (<sh-exec> "bar"))))
+  '(<sh-exec> "echo" (<sh-cmd-sub> (<sh-exec> "foo")
+                                   (<sh-exec> "bar")))
   (parse "echo `foo
                 bar`"))