From 425c9ce4017f10f64aa22d7f5abf4ee089cd977a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 5 Nov 2019 17:15:19 +0300
Subject: [PATCH] a special `last` variable in for loops (experimental)

---
 vlib/builtin/array_test.v | 14 ++++++++++++++
 vlib/compiler/parser.v    |  9 +++++++++
 2 files changed, 23 insertions(+)

diff --git a/vlib/builtin/array_test.v b/vlib/builtin/array_test.v
index cf632f1645..b551cfd785 100644
--- a/vlib/builtin/array_test.v
+++ b/vlib/builtin/array_test.v
@@ -479,3 +479,17 @@ fn test_eq() {
 	assert [5,6,7].eq([6,7]) == false
 	assert [`a`,`b`].eq([`a`,`b`]) == true
 }
+
+fn test_for_last() {
+	numbers := [1, 2, 3, 4]
+	mut s := '['
+	for num in numbers {
+		s += '$num'
+		if !last {
+			s += ', '
+			
+		}	
+	}	
+	s += ']'
+	assert s == '[1, 2, 3, 4]'
+}	
diff --git a/vlib/compiler/parser.v b/vlib/compiler/parser.v
index d2e651f65b..b514b217dc 100644
--- a/vlib/compiler/parser.v
+++ b/vlib/compiler/parser.v
@@ -3377,6 +3377,15 @@ fn (p mut Parser) for_st() {
 		else if is_arr {
 			typ = typ[6..]// all after `array_`
 			p.gen_for_header(i, tmp, typ, val)
+			p.register_var(Var {
+				name: 'last'
+				typ: 'bool'
+				is_mut: false
+				is_used: true
+			})
+			// TODO don't generate if it's not used.
+			// Otherwise it's a C warning + perf.
+			p.genln('bool last = $i == $tmp . len - 1;')
 		}
 		else if is_str {
 			typ = 'byte'