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Support optional left argument in function definitions

--- a/dat.h

+++ b/dat.h

@@ -179,6 +179,7 @@

 	char *name;

 	int nameclass;

 	int prim;

+	int optional; /* optional left arg */

 	Ast *funcname;

 	Ast *funcresult;

--- a/eval.c

+++ b/eval.c

@@ -72,9 +72,11 @@

 		{

 			Function *fn = alloc(DataFunction);

 			fn->ast = a;

-			if(fn->ast->funcleftarg)

+			if(fn->ast->funcleftarg){

 				fn->valence = Dyadic;

-			else if(fn->ast->funcrightarg)

+				if(fn->ast->funcleftarg->optional)

+					fn->valence |= Monadic;

+			}else if(fn->ast->funcrightarg)

 				fn->valence = Monadic;

 			else

 				fn->valence = Niladic;

@@ -347,6 +349,8 @@

 						error(ESyntax, "Function %s does not produce a result", funcname(func));

 				}

 				pushval(values, y);

+				if(func->valence & Dyadic) /* ambivalent function */

+					pushval(values, nil);

 				pushcall(calls, func->code, &c, &o);

 			}else{

 				z = primmonad(func->prim, y);

--- a/parse.c

+++ b/parse.c

@@ -18,6 +18,7 @@

 static Ast *parseprog(TokenList *);

 static Ast *parsefuncdef(TokenList *);

 static Ast *parsefuncheader(TokenList *);

+static Ast *parsefuncleftopt(TokenList *);

 static Ast *parselocals(TokenList *);

 static Ast *parseexpr(TokenList *, Symtab *, Ast *);

 static Ast *parseexprsub(TokenList *);

@@ -202,15 +203,18 @@

 	match(t, TokDel);

+	func->funcleftarg = parsefuncleftopt(t); /* optional left arg */

+

 	func->funcname = parsename(t);

-	if(peek(t) == TokLarrow){

+	if(peek(t) == TokLarrow && !func->funcleftarg){

 		match(t, TokLarrow);

 		func->funcresult = func->funcname;

+		func->funcleftarg = parsefuncleftopt(t);

 		func->funcname = parsename(t);

 	}

 	if(peek(t) == TokName)

 		func->funcrightarg = parsename(t);

-	if(peek(t) == TokName){

+	if(peek(t) == TokName && !func->funcleftarg){

 		func->funcleftarg = func->funcname;

 		func->funcname = func->funcrightarg;

 		func->funcrightarg = parsename(t);

@@ -218,6 +222,19 @@

 	func->funclocals = parselocals(t);

 	return func;

+}

+

+static Ast *

+parsefuncleftopt(TokenList *t)

+{

+	Ast *name = nil;

+	if(peek(t) == TokLbrace){

+		match(t, TokLbrace);

+		name = parsename(t);

+		name->optional = 1;

+		match(t, TokRbrace);

+	}

+	return name;

 }

 static Ast *