extern "C" {
#endif
+/*----- Notes on the control operator machinery ---------------------------*
+ *
+ * These macros owe an obvious and immense debt to Simon Tatham's article
+ * `Metaprogramming custom control structures in C', available from
+ * https://www.chiark.greenend.org.uk/~sgtatham/mp/. The basic tricks are
+ * all Tatham's, as are most of the provided operators. The focus on
+ * @MC_ACT@ as a significant primitive is probably my main original
+ * contribution.
+ */
+
/*----- Header files ------------------------------------------------------*/
#ifndef MLIB_MACROS_H
*/
#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);@
+/* @MC_ACT(stmt)@
+ *
+ * @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
+
+/* @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 MC_LABEL(tag) MCTRL__LABEL(tag):
+#define MC_GOTO(tag) MC_ACT(goto MCTRL__LABEL(tag))
+
+/* @MC_TARGET(tag, stmt) body@
+ * @MC_GOTARGET(tag);@
+ *
+ * Executing @TARGET@ statement executes the @body@, as if the @TARGET@
+ * weren't there. Executing a @GOTARGET@ transfers control to the @stmt@
+ * (but not normally the @body@). In either case, the normal flow of control
+ * is then to the following statement.
+ */
+#define MC_TARGET(tag, stmt) \
+ MC_GOTO(tag##__body) \
+ MC_LABEL(tag##__tgt) MC_ACT(stmt) \
+ MC_LABEL(tag##__body)
+#define MC_GOTARGET(tag) do MC_GOTO(tag##__tgt); while (0)
+
+/* @MC_BEFORE(tag, stmt) body@
+ *
+ * Execute @stmt@ and then @body@.
+ */
+#define MC_BEFORE(tag, stmt) \
+ MC_ACT(stmt; MC_GOTO(tag##__body)) \
+ MC_LABEL(tag##__body)
+
+/* @MC_AFTER(tag, stmt) body@
*
- * 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.
+ * Execute @body@ and then @stmt@. If @body@ invokes @break@ then control
+ * immediately transfers to the statement following @MC_AFTER@. If @body
+ * invokes @continue@, then control returns to @stmt@.
*/
-#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_AFTER(tag, stmt) \
+ MC_GOTO(tag##__body) \
+ MC_LABEL(tag##__end) MC_ACT(stmt) \
+ for (;;) \
+ MC_GOTO(tag##__end) \
+ MC_LABEL(tag##__body)
-/* @BEFORE(tag, stmt_0) stmt_1@
+/* @MC_DOWHILE(tag, cond) body@
*
- * Execute @stmt_0@ and then @stmt_1@.
+ * Repeatedly execute @body@ until @cond@ evaluates to zero. The @body@ is
+ * executed once before @cond@ is checked the first time. The @break@ and
+ * @continue@ statements work within @body@ as one would expect.
*/
-#define BEFORE(tag, stmt) \
- if (1) { stmt goto MCTRL__LABEL(tag##__before_body); } \
- else MCTRL__LABEL(tag##__before_body):
+#define MC_DOWHILE(tag, cond) \
+ MC_GOTO(tag##__body) \
+ while (cond) \
+ MC_LABEL(tag##__body)
-/* @AFTER(tag, stmt_0) stmt_1@
+/* @MC_ALLOWELSE(tag) apodosis_body [else haeresis_body]@
+ * @MC_GOELSE(tag);@
*
- * Execute @stmt_1@ and then @stmt_0@. If either statement invokes @break@
- * then control immediately transfers to the statement following @AFTER@. If
- * either invokes @continue@, then control returns to @stmt_0@.
+ * Executing @MC_ALLOWELSE@ executes @apodosis_body@, but not
+ * @haeresis_body@. If @MC_GOELSE(tag)@ is executed, then control continues
+ * from @haeresis_body@.
*/
-#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):
+#define MC_ALLOWELSE(tag) \
+ MC_GOTO(tag##__body) \
+ MC_LABEL(tag##__else) if (0) \
+ MC_LABEL(tag##__body)
+#define MC_GOELSE(tag) do MC_GOTO(tag##__else); while (0)
-/* @WRAP(tag, before, onend, onbreak) stmt@
+/* @MC_WRAP(tag, before, onend, onbreak) body@
*
- * Execute the @before@ statement, followed by @stmt@. If @stmt@ invokes
- * @break@, then @onbreak@ is immediately executed; if @stmt@ completes
+ * Execute the @before@ statement, followed by @body@. If @body@ invokes
+ * @break@, then @onbreak@ is immediately executed; if @body@ completes
* normally, or invokes @continue@ then @onend@ is immediately executed.
* Any @break@ and @continue@ in the @before@, @onend@, and @onbreak@
* 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]@
- * @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.
- */
-#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)
-
-/* @DOWHILE(tag, cond) stmt@
- *
- * Repeatedly execute @stmt@ until @cond@ evaluates to zero. Execute @stmt@
- * at least once. The @break@ and @continue@ statements work within @stmt@
- * as one would expect.
- */
-#define DOWHILE(tag, cond) \
- if (1) goto MCTRL__LABEL(tag##__dowhile_body); \
- else while (cond) MCTRL__LABEL(tag##__dowhile_body):
-
-/* @DECL(tag, decl) stmt@
- *
- * Execute @stmt@ with @decl@ in scope. If @stmt@ completes or invokes
- * @break@ or @continue@ then control continues with the statement following
- * @DECL@. Internally, this uses @for@, so it only works in C99 or later, or
- * C++.
+#define MC_WRAP(tag, before, onend, onbreak) \
+ 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)
+
+/* @MC_FINALLY(tag, cleanup) body@
+ *
+ * Execute @cleanup@ when @body@ completes or ends with @break@. In the
+ * latter case, propagate the @break@ to the enclosing context -- for which
+ * it must be syntactically appropriate.
+ *
+ * The @cleanup@ code is duplicated. If it arrange to have private long-term
+ * state, e.g, by declaring @static@ variables, then the two copies will not
+ * share the same state, so probably don't do this.
+ */
+#define MC_FINALLY(tag, cleanup) \
+ MP_WRAP(tag##__final, { ; } cleanup, { cleanup break; })
+
+/* @MC_DECL(tag, decl) body@
+ *
+ * Execute @body@ with @decl@ in scope. If @body@ completes or invokes
+ * @continue@ then control continues with the statement following @MC_DECL@;
+ * if it invokes @break@ then it will be restarted without leaving the scope
+ * of @decl@. Internally, this uses @for@, so it only works in C99 or later,
+ * or C++.
*/
#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):
+# define MC_DECL(tag, decl) \
+ for (decl;;) \
+ MC_GOTO(tag##__body) \
+ MC_LABEL(tag##__exit) MC_ACT(break) \
+ for (;;) \
+ MC_GOTO(tag##__exit) \
+ MC_LABEL(tag##__body)
#endif
+/* @MC_LOOPELSE(tag, head) loop_body [else else_body]@
+ *
+ * Python-like looping with optional @else@ clause. @head loop_body@ must be
+ * a syntactically valid @for@, @while@, or @MC_DOWHILE@ loop; if the loop
+ * exits because of @break@ then control continues in the usual way;
+ * otherwise, the @else_body@ (if any) is executed.
+ */
+#define MC_LOOPELSE(tag, head) \
+ MC_TARGET(tag##__exit, { ; }) \
+ MC_ALLOWELSE(tag##__else) \
+ MC_AFTER(tag##__after, { MC_GOELSE(tag##__else); }) \
+ head \
+ MC_WRAP(tag##__body, { ; }, { ; }, { MC_GOTARGET(tag##__exit); })
+
+/* @MC_LOOPBETWEEN(tag, setup, cond, step) loop_body [else else_body]@
+ *
+ * This is essentially a @for@ loop with a twist. The @setup@, @cond@, and
+ * @step@ arguments are the parts of the @for@ head clause; because of the
+ * limitations of the C macro syntax, they're separated by commas rather than
+ * semicolons.
+ *
+ * The twist is that, once the @loop_body@ has finished, the @step@
+ * expression evaluated, and the @cond@ evaluated and determined to be
+ * nonzero, the @else_body@ (if any) is executed before re-entering the
+ * @loop_body@. This makes it a useful place to insert any kind of
+ * interstitial material, e.g., printing commas between list items.
+ *
+ * The @cond@ is textually duplicated. You'll get some code bloat if the
+ * condition is very complex. If it somehow arranges to have private
+ * long-term state (e.g., as a result of declaring static variables inside
+ * GCC statement expressions), then the two copies will not share this state,
+ * so probably don't do this.
+ *
+ * Note that by the time that the @else_body@ is executed, the decision has
+ * already been made that another iteration will be performed, and, in
+ * particular, the @step@ has occurred. The @else_body@ is therefore looking
+ * at the next item to be processed, not the item that has just finished
+ * being processed.
+ */
+#define MC_LOOPBETWEEN(tag, setup, cond, step) \
+ for (setup;;) \
+ if (!(cond)) break; else \
+ MC_TARGET(tag##__exit, { break; }) \
+ for (;;) \
+ MC_WRAP(tag##__tailwrap, { ; }, \
+ { ; }, \
+ { MC_GOTARGET(tag##__exit); }) \
+ MC_ALLOWELSE(tag##__tail) \
+ MC_WRAP(tag##__bodywrap, { ; }, \
+ { if ((step), !(cond)) \
+ MC_GOTARGET(tag##__exit); \
+ else \
+ MC_GOELSE(tag##__tail); }, \
+ { MC_GOTARGET(tag##__exit); })
+
/*----- That's all, folks -------------------------------------------------*/
#ifdef __cplusplus
~mdw / mLib / blobdiff