support new "~" conditional: useful for implying default parameters in macros

2 files changed, 44 insertions, 14 deletions

diff --git a/doc/sciteco.7.template b/doc/sciteco.7.template
index 340f58b..42d1425 100644
--- a/doc/sciteco.7.template
+++ b/doc/sciteco.7.template
@@ -1004,8 +1004,8 @@ They correspond to structured IF-THEN-ELSE statements
 in other imperative languages.
 The general syntax of conditionals is as follows:
 .br
-\# n"c if [| else ]'
-\fIn\fB"\fIc if \fR[\fB|\fI else \fR] \fB'\fR
+\# [n]"c if [| else ]'
+[\fIn\fP]\fB"\fIc if \fR[\fB|\fI else \fR] \fB'\fR
 .br
 Whereas \fIn\fP is a value on the stack to test,
 \fIc\fP is a case-insignificant character identifying a
@@ -1013,12 +1013,31 @@ condition to check for,
 \fIif\fP is a code block to execute if the condition applies and
 \fIelse\fP is an optional code block to execute if the condition
 does not apply.
-The first command in the \fIif\fP block will not see
-any arguments on the expression stack but values accumulated
-on the expression stack by the \fIif\fP or \fIelse\fP blocks
-are not discarded by the terminating single-quote.
-In other words, conditionals may return values.
-The possible conditions are defined in the following list:
+The conditional pops at most one argument from the expression
+stack \(em the remaining arguments may be evaluated by the
+\fIif\fP or \fIelse\fP blocks.
+Values accumulated on the expression stack by the \fIif\fP or \fIelse\fP
+blocks are not discarded by the terminating single-quote.
+In other words, conditionals may also return values.
+The possible conditions are defined in the following list.
+Unless \fIn\fP is defined optional, the conditionals described
+below yield an error if \fIn\fP is omitted:
+.TP
+\# [n]"~
+[\fIn\fP]\fB\(dq~\fP
+Applies if \fIn\fP is \fBnot\fP given (i.e. the argument
+stack is empty).
+If \fIn\fP is given (i.e. the stack is not empty), it is \fBnot\fP
+removed from the expression stack.
+This construct is especially useful in macros to imply default
+parameter values.
+For instance the following macro inserts \fIn\fP tab characters
+(one by default):
+.EX
+@^Ut{
+  "~1'<@^I//>
+}
+.EE
 .TP
 .IB n \(dqA
 Applies if \fIn\fP is the code of an alphabetic character.
diff --git a/src/parser.cpp b/src/parser.cpp
index 1510352..9f5be4c 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -1594,17 +1594,30 @@ StateCondCommand::custom(gchar chr)
 
 	switch (mode) {
 	case MODE_PARSE_ONLY_COND:
-		undo.push_var<gint>(nest_level);
-		nest_level++;
+		undo.push_var(nest_level)++;
 		break;
+
 	case MODE_NORMAL:
+		expressions.eval();
+
+		if (chr == '~')
+			/* don't pop value for ~ conditionals */
+			break;
+
+		if (!expressions.args())
+			throw ArgExpectedError('"');
 		value = expressions.pop_num_calc();
 		break;
+
 	default:
 		break;
 	}
 
 	switch (g_ascii_toupper(chr)) {
+	case '~':
+		BEGIN_EXEC(&States::start);
+		result = !expressions.args();
+		break;
 	case 'A':
 		BEGIN_EXEC(&States::start);
 		result = g_ascii_isalpha((gchar)value);
@@ -1663,11 +1676,9 @@ StateCondCommand::custom(gchar chr)
 		throw Error("Invalid conditional type \"%c\"", chr);
 	}
 
-	if (!result) {
+	if (!result)
 		/* skip to ELSE-part or end of conditional */
-		undo.push_var<Mode>(mode);
-		mode = MODE_PARSE_ONLY_COND;
-	}
+		undo.push_var(mode) = MODE_PARSE_ONLY_COND;
 
 	return &States::start;
 }