all: handle <- arrow token for channel operations (#6152)

vlib/v/checker/checker.v

@@ -639,6 +639,17 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) table.Type {
 			}
 			return table.bool_type
 		}
+		.arrow { // `chan <- elem`
+			if left.kind == .chan {
+				elem_type := left.chan_info().elem_type
+				if !c.check_types(right_type, elem_type) {
+					c.error('cannot push `$right.name` on `$left.name`', right_pos)
+				}
+			} else {
+				c.error('cannot push on non-channel `$left.name`', left_pos)
+			}
+			return table.void_type
+		}
 		else {
 			// use `()` to make the boolean expression clear error
 			// for example: `(a && b) || c` instead of `a && b || c`
@@ -2441,6 +2452,14 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 			if node.op == .not && right_type != table.bool_type_idx && !c.pref.translated {
 				c.error('! operator can only be used with bool types', node.pos)
 			}
+			if node.op == .arrow {
+				right := c.table.get_type_symbol(right_type)
+				if right.kind == .chan {
+					return right.chan_info().elem_type
+				} else {
+					c.error('<- operator can only be used with `chan` types', node.pos)
+				}
+			}
 			return right_type
 		}
 		ast.None {

vlib/v/checker/tests/arrow_op_wrong_left_type_err_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/arrow_op_wrong_left_type_err_a.v:4:2: error: cannot push on non-channel `i64`
+    2 |     ch := i64(3)
+    3 |     obj := 5
+    4 |     ch <- obj
+      |     ~~
+    5 | }

vlib/v/checker/tests/arrow_op_wrong_left_type_err_a.vv

@@ -0,0 +1,5 @@
+fn main() {
+	ch := i64(3)
+	obj := 5
+	ch <- obj
+}

vlib/v/checker/tests/arrow_op_wrong_left_type_err_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/arrow_op_wrong_left_type_err_b.v:4:8: error: cannot assign `string` to `obj` of type `int`
+    2 |     ch := chan string{}
+    3 |     mut obj := 9
+    4 |     obj = <-ch
+      |           ~~
+    5 | }

vlib/v/checker/tests/arrow_op_wrong_left_type_err_b.vv

@@ -0,0 +1,5 @@
+fn main() {
+	ch := chan string{}
+	mut obj := 9
+	obj = <-ch
+}

vlib/v/checker/tests/arrow_op_wrong_right_type_err_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/arrow_op_wrong_right_type_err_a.v:4:8: error: cannot push `string` on `chan_u64`
+    2 |     ch := chan u64{cap: 10}
+    3 |     obj := 'test'
+    4 |     ch <- obj
+      |           ~~~
+    5 | }

vlib/v/checker/tests/arrow_op_wrong_right_type_err_a.vv

@@ -0,0 +1,5 @@
+fn main() {
+	ch := chan u64{cap: 10}
+	obj := 'test'
+	ch <- obj
+}

vlib/v/checker/tests/arrow_op_wrong_right_type_err_b.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/arrow_op_wrong_right_type_err_b.v:3:9: error: <- operator can only be used with `chan` types
+    1 | fn main() {
+    2 |     ch := i64(3)
+    3 |     obj := <-ch
+      |            ~~
+    4 |     println(obj)
+    5 | }

vlib/v/checker/tests/arrow_op_wrong_right_type_err_b.vv

@@ -0,0 +1,5 @@
+fn main() {
+	ch := i64(3)
+	obj := <-ch
+	println(obj)
+}

vlib/v/parser/parser.v

@@ -858,7 +858,7 @@ fn (mut p Parser) parse_multi_expr(is_top_level bool) ast.Stmt {
 		return p.partial_assign_stmt(left, left_comments)
 	} else if is_top_level && tok.kind !in [.key_if, .key_match, .key_lock, .key_rlock] &&
 		left0 !is ast.CallExpr && left0 !is ast.PostfixExpr && !(left0 is ast.InfixExpr &&
-		(left0 as ast.InfixExpr).op == .left_shift) && left0 !is ast.ComptimeCall {
+		(left0 as ast.InfixExpr).op in [.left_shift, .arrow]) && left0 !is ast.ComptimeCall {
 		p.error_with_pos('expression evaluated but not used', left0.position())
 	}
 	if left.len == 1 {

vlib/v/parser/pratt.v

@@ -60,7 +60,7 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 			}
 			p.next()
 		}
-		.minus, .amp, .mul, .not, .bit_not {
+		.minus, .amp, .mul, .not, .bit_not, .arrow {
 			// -1, -a, !x, &x, ~x
 			node = p.prefix_expr()
 		}

vlib/v/scanner/scanner.v

@@ -1037,6 +1037,9 @@ fn (mut s Scanner) text_scan() token.Token {
 					}
 					s.pos++
 					return s.new_token(.left_shift, '', 2)
+				} else if nextc == `-` {
+					s.pos++
+					return s.new_token(.arrow, '', 2)
 				} else {
 					return s.new_token(.lt, '', 1)
 				}
@@ -1045,9 +1048,6 @@ fn (mut s Scanner) text_scan() token.Token {
 				if nextc == `=` {
 					s.pos++
 					return s.new_token(.eq, '', 2)
-				} else if nextc == `>` {
-					s.pos++
-					return s.new_token(.arrow, '', 2)
 				} else {
 					return s.new_token(.assign, '', 1)
 				}

vlib/v/token/token.v

@@ -39,7 +39,7 @@ pub enum Kind {
 	comma
 	semicolon
 	colon
-	arrow // =>
+	arrow // <-
 	amp
 	hash
 	dollar
@@ -181,7 +181,7 @@ fn build_token_str() []string {
 	// s[Kind.at] = '@'
 	s[Kind.semicolon] = ';'
 	s[Kind.colon] = ':'
-	s[Kind.arrow] = '=>'
+	s[Kind.arrow] = '<-'
 	s[Kind.assign] = '='
 	s[Kind.decl_assign] = ':='
 	s[Kind.plus_assign] = '+='
@@ -339,6 +339,7 @@ pub fn build_precedences() []Precedence {
 	p[Kind.left_shift] = .product
 	p[Kind.right_shift] = .product
 	p[Kind.amp] = .product
+	p[Kind.arrow] = .product
 	// `+` |  `-` |  `|` | `^`
 	p[Kind.plus] = .sum
 	p[Kind.minus] = .sum
@@ -394,7 +395,7 @@ pub fn (tok Token) is_scalar() bool {
 pub fn (tok Token) is_unary() bool {
 	return tok.kind in [
 	// `+` | `-` | `!` | `~` | `*` | `&`
-	.plus, .minus, .not, .bit_not, .mul, .amp]
+	.plus, .minus, .not, .bit_not, .mul, .amp, .arrow]
 }
 
 pub fn (tok Kind) is_relational() bool {
@@ -412,5 +413,5 @@ pub fn (kind Kind) is_infix() bool {
 	//
 	.key_as, .ge, .le, .logical_or, .xor, .not_in, .key_is, .not_is,
 	//
-	.and, .dot, .pipe, .amp, .left_shift, .right_shift]
+	.and, .dot, .pipe, .amp, .left_shift, .right_shift, .arrow]
 }