X-Git-Url: https://git.distorted.org.uk/~mdw/mLib/blobdiff_plain/55d45d31062ef774809ed151e9270ac1cbb6c12e..2edecb46edfcdc4b25c9d12bcfe23a5d66ec3f33:/utils/control.h?ds=sidebyside

diff --git a/utils/control.h b/utils/control.h
index fdc7125..77d985a 100644
--- a/utils/control.h
+++ b/utils/control.h
@@ -47,28 +47,47 @@
  */
 #define MCTRL__LABEL(tag) GLUE(_mctrl__##tag##__, __LINE__)
 
-/* @FIRSTBRANCH(tag_0) stmt_0@
- * [@MIDBRANCH(tag_i) stmt_i ...@]
- * @LASTBRANCH(tag_n) stmt_n@
- * @GOBRANCH(tag);@
- *
- * If control enters at the top, then only <stmt_0> is executed, followed by
- * the statement after.  Following @GOBRANCH(tag)@, control continues
- * from the correspondingly tagged statement, and continues with the
- * following statement again.
+/* @MC_ACT(stmt)@
+ * @MC_PASS@
+ *
+ * @MC_ACT@ is the main `trick' for constructing these flow-control
+ * operators.  It wraps up a statement as what we call an `action'.  Actions
+ * can be concatenated together to form a valid statement head, i.e., a
+ * sequence of actions can be followed by a statement, called the `body', to
+ * form a single syntactic statement.  The body can be simply `;', so an
+ * action can be treated as a simple statement.  However, if an action
+ * sequence is executed, only the first statement is executed.
+ *
+ * Actions can be labelled, e.g., using @MC_LABEL@, just like statements.  If
+ * control is passed to a label, e.g., by @MC_GOTO@, then the statement
+ * within the following action (only) is executed; the normal flow of control
+ * will then be to the statement following the containing action sequence and
+ * its body.
+ */
+#define MC_ACT(stmt)	if (1) stmt else
+#define MC_PASS		MC_ACT(;)
+
+
+/* @MC_LABEL(tag)@
+ * @MC_GOTO(tag)@
+ *
+ * @MC_LABEL@ just establishes a label which can be invoked (only) from the
+ * same top-level macro; and @MC_GOTO@ transfers control to it.
+ *
+ * The @MC_GOTO@ macro is special in that it can be used either as a plain
+ * statement, followed by a semicolon in the usual way, or as a prefix
+ * action in its own right, in place of @MC_ACT@.
  */
-#define FIRSTBRANCH(tag)  if (1) { goto MCTRL__LABEL(tag); MCTRL__LABEL(tag):
-#define MIDBRANCH(tag)    } else if (0) MCTRL__LABEL(tag): {
-#define LASTBRANCH(tag)   } else MCTRL__LABEL(tag):
-#define GOBRANCH(tag)     goto MCTRL__LABEL(tag)
+#define MC_LABEL(tag)	MCTRL__LABEL(tag):
+#define MC_GOTO(tag)	MC_ACT({ goto MCTRL__LABEL(tag); })
 
 /* @BEFORE(tag, stmt_0) stmt_1@
  *
  * Execute @stmt_0@ and then @stmt_1@.
  */
 #define BEFORE(tag, stmt)						\
-	if (1) { stmt goto MCTRL__LABEL(tag##__before_body); }		\
-	else MCTRL__LABEL(tag##__before_body):
+			MC_ACT({ stmt MC_GOTO(tag##__body); })		\
+  MC_LABEL(tag##__body)
 
 /* @AFTER(tag, stmt_0) stmt_1@
  *
@@ -77,11 +96,11 @@
  * either invokes @continue@, then control returns to @stmt_0@.
  */
 #define AFTER(tag, stmt)						\
-	if (1) goto MCTRL__LABEL(tag##__after_body);			\
-	else if (1) { MCTRL__LABEL(tag##__after_end): stmt }		\
-	else for (;;)							\
-	  if (1) goto MCTRL__LABEL(tag##__after_end);			\
-	  else MCTRL__LABEL(tag##__after_body):
+			MC_GOTO(tag##__body)				\
+  MC_LABEL(tag##__end)	MC_ACT(stmt)					\
+			for (;;)					\
+			  MC_GOTO(tag##__end)				\
+  MC_LABEL(tag##__body)
 
 /* @WRAP(tag, before, onend, onbreak) stmt@
  *
@@ -92,44 +111,26 @@
  * statements behave as one would expect from their context.
  */
 #define WRAP(tag, before, onend, onbreak)				\
-	if (1) { before goto MCTRL__LABEL(tag##__wrap_body); }		\
-	else if (1) { MCTRL__LABEL(tag##__wrap_end): onend }		\
-	else if (1) { MCTRL__LABEL(tag##__wrap_break): onbreak }	\
-	else for (;;)							\
-	  if (1) goto MCTRL__LABEL(tag##__wrap_break);			\
-	  else for (;;)							\
-	    if (1) goto MCTRL__LABEL(tag##__wrap_end);			\
-	    else MCTRL__LABEL(tag##__wrap_body):
-
-/* @ALLOWELSE(tag, before, onend, onbreak) stmt_0 [else stmt_1]@
+			MC_ACT({ before MC_GOTO(tag##__body); })	\
+  MC_LABEL(tag##__end)	MC_ACT(onend)					\
+  MC_LABEL(tag##__brk)	MC_ACT(onbreak)					\
+			for (;;)					\
+			  MC_GOTO(tag##__brk)				\
+			  for (;;)					\
+			    MC_GOTO(tag##__end)				\
+  MC_LABEL(tag##__body)
+
+/* @ALLOWELSE(tag) stmt_0 [else stmt_1]@
  * @GOELSE(tag);@
  *
- * Execute the @before@ statement, followed by @stmt_0@.  If @stmt_0@
- * completes, or invokes @break@ or @continue@, then control continues with
- * the next statement.  If @GOELSE(tag)@ is invoked anywhere in the
- * function, then @before@ is executed, followed by @stmt_1@ (if present).
- * If @stmt_1@ invokes @break@ then control passes to @onbreak@; if @stmt_1@
- * ends normally then control passes to @onend@.  Any @break@ and @continue@
- * in the @before@, @onend@, and @onbreak@ statements behave as one would
- * expect from their context.
+ * Executing @ALLOWELSE@ executes @stmt_0@, but not @stmt_1@.  If
+ * @GOELSE(tag)@ is executed, then control continues from @stmt_1@.
  */
-#define ALLOWELSE(tag, before, onend, onbreak)				\
-	if (1) goto MCTRL__LABEL(tag##__allowelse_body);		\
-	else if (1) MCTRL__LABEL(tag##__allowelse_body_end): ;		\
-	else if (1) { MCTRL__LABEL(tag##__allowelse_else_end): onend }	\
-	else if (1) { MCTRL__LABEL(tag##__allowelse_else_break): onbreak } \
-	else if (1) {							\
-	MCTRL__LABEL(tag##__allowelse_before_else):			\
-	  before goto MCTRL__LABEL(tag##__allowelse_else);		\
-	} else for (;;)							\
-	  if (1) goto MCTRL__LABEL(tag##__allowelse_else_break);	\
-	  else for (;;)							\
-	    if (1) goto MCTRL__LABEL(tag##__allowelse_else_end);	\
-	    else MCTRL__LABEL(tag##__allowelse_else): if (0) for (;;)	\
-	      if (1) goto MCTRL__LABEL(tag##__allowelse_body_end);	\
-	      else MCTRL__LABEL(tag##__allowelse_body):
-#define GOELSE(tag)							\
-	goto MCTRL__LABEL(tag##__allowelse_before_else)
+#define ALLOWELSE(tag)							\
+			MC_GOTO(tag##__body)				\
+  MC_LABEL(tag##__else)	if (0)						\
+  MC_LABEL(tag##__body)
+#define GOELSE(tag)	do MC_GOTO(tag##__else); while (0)
 
 /* @DOWHILE(tag, cond) stmt@
  *
@@ -138,8 +139,9 @@
  * as one would expect.
  */
 #define DOWHILE(tag, cond)						\
-	if (1) goto MCTRL__LABEL(tag##__dowhile_body);			\
-	else while (cond) MCTRL__LABEL(tag##__dowhile_body):
+			MC_GOTO(tag##__body)				\
+			while (cond)					\
+  MC_LABEL(tag##__body)
 
 /* @DECL(tag, decl) stmt@
  *
@@ -150,12 +152,12 @@
  */
 #if __STDC_VERSION__ >= 199901 || defined(__cplusplus)
 #  define DECL(tag, decl)						\
-	for (decl;;)							\
-	  if (1) goto MCTRL__LABEL(tag##__decl_body);			\
-	  else if (1) MCTRL__LABEL(tag##__decl_exit): break;		\
-	  else for (;;)							\
-	    if (1) goto MCTRL__LABEL(tag##__decl_exit);			\
-	    else MCTRL__LABEL(tag##__decl_body):
+			for (decl;;)					\
+			  MC_GOTO(tag##__body)				\
+  MC_LABEL(tag##__end)	  MC_ACT({ break; })				\
+			  for (;;)					\
+			    MC_GOTO(tag##__end)				\
+  MC_LABEL(tag##__body)
 #endif
 
 /*----- That's all, folks -------------------------------------------------*/