From 65180a2d3e00931226d6c90a3b7ccf9db14722ae Mon Sep 17 00:00:00 2001
From: Li Jin <dragon-fly@qq.com>
Date: Fri, 14 Feb 2020 17:03:53 +0800
Subject: add placeholder support for backcall operator.

---
 spec/inputs/backcall.moon   |  8 +++++++-
 src/MoonP/ast.hpp           | 12 ++++++++++++
 src/MoonP/moon_compiler.cpp | 22 +++++++++++++++++++---
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/spec/inputs/backcall.moon b/spec/inputs/backcall.moon
index a65bb0a..f5d0046 100644
--- a/spec/inputs/backcall.moon
+++ b/spec/inputs/backcall.moon
@@ -30,8 +30,14 @@ f = ->
 998 |> func2 "abc", 233 |> func0 |> func1
 998 |> func0("abc", 233) |> func1 |> func2
 
+1 |> f 2, 3, 4, 5
+val(2) |> f 1, _, 3, 4, 5
+arr[3] |> f 1, 2, _, 4, 5
+
+a = {"1","2","3"} |> table.concat("") |> tonumber |> f1(1, 2, 3, _) |> f2(1, _, 3)
+
 do
-	(x)<-map {1,2,3}
+	(x)<- map {1,2,3}
 	x * 2
 
 do
diff --git a/src/MoonP/ast.hpp b/src/MoonP/ast.hpp
index 8bcfc21..b56cfde 100644
--- a/src/MoonP/ast.hpp
+++ b/src/MoonP/ast.hpp
@@ -399,6 +399,18 @@ public:
 		node->release();
 	}
 
+	bool swap(ast_node* node, ast_node* other) {
+		for (auto it = m_objects.begin(); it != m_objects.end(); ++it) {
+			if (*it == node) {
+				*it = other;
+				other->retain();
+				node->release();
+				return true;
+			}
+		}
+		return false;
+	}
+
 	 const node_container& objects() const {
 		return m_objects;
 	}
diff --git a/src/MoonP/moon_compiler.cpp b/src/MoonP/moon_compiler.cpp
index 30adbae..d009fc9 100644
--- a/src/MoonP/moon_compiler.cpp
+++ b/src/MoonP/moon_compiler.cpp
@@ -29,7 +29,7 @@ inline std::string s(std::string_view sv) {
 }
 
 const char* moonScriptVersion() {
-	return "0.5.0-r0.1.0";
+	return "0.5.0-r0.1.1";
 }
 
 class MoonCompiler {
@@ -1528,10 +1528,26 @@ private:
 						}
 						if (isChainValueCall(chainValue)) {
 							auto last = chainValue->items.back();
+							_ast_list* args = nullptr;
 							if (auto invoke = ast_cast<InvokeArgs_t>(last)) {
-								invoke->args.push_front(arg);
+								args = &invoke->args;
 							} else {
-								ast_to<Invoke_t>(last)->args.push_front(arg);
+								args = &(ast_to<Invoke_t>(last)->args);
+							}
+							bool findPlaceHolder = false;
+							for (auto a : args->objects()) {
+								auto name = singleVariableFrom(a);
+								if (name == "_"sv) {
+									if (!findPlaceHolder) {
+										args->swap(a, arg);
+										findPlaceHolder = true;
+									} else {
+										throw std::logic_error(_info.errorMessage("backcall placeholder can be used only in one place."sv, a));
+									}
+								}
+							}
+							if (!findPlaceHolder) {
+								args->push_front(arg);
 							}
 						} else {
 							auto invoke = x->new_ptr<Invoke_t>();
-- 
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