fmt: support `select` expressions (#6460)

vlib/v/fmt/fmt.v

@@ -937,7 +937,42 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 			f.expr(node.high)
 		}
 		ast.SelectExpr {
-			// TODO: implement this
+			f.writeln('select {')
+			f.indent++
+			for branch in node.branches {
+				if branch.comment.text != '' {
+					f.comment(branch.comment, {
+						inline: true
+					})
+					f.writeln('')
+				}
+				if branch.is_else {
+					f.write('else {')
+				} else {
+					if branch.is_timeout {
+						f.write('> ')
+					}
+					f.single_line_if = true
+					match branch.stmt as stmt {
+						ast.ExprStmt { f.expr(stmt.expr) }
+						else { f.stmt(branch.stmt) }
+					}
+					f.single_line_if = false
+					f.write(' {')
+				}
+				if branch.stmts.len > 0 {
+					f.writeln('')
+					f.stmts(branch.stmts)
+				}
+				f.writeln('}')
+				if branch.post_comments.len > 0 {
+					f.comments(branch.post_comments, {
+						inline: true
+					})
+				}
+			}
+			f.indent--
+			f.write('}')
 		}
 		ast.SelectorExpr {
 			f.expr(node.expr)

vlib/v/fmt/tests/select_keep.vv

@@ -0,0 +1,128 @@
+import time
+import sync
+
+struct St {
+	a int
+}
+
+fn getint() int {
+	return 8
+}
+
+fn f1(ch1 chan int, ch2 chan St, ch3, ch4, ch5 chan int, sem sync.Semaphore) {
+	mut a := 5
+	select {
+		// pre comment
+		a = <-ch3 {
+			a = 0
+		}
+		b := <-ch2 {
+			a = b.a
+		}
+		ch3 <- 5 {
+			a = 1
+		}
+		ch2 <- St{
+			a: 37
+		} {
+			a = 2
+		}
+		// another comment
+		ch4 <- (6 + 7 * 9) {
+			a = 8
+		}
+		ch5 <- getint() {
+			a = 9
+		}
+		> 300 * time.millisecond {
+			a = 3
+		}
+		// post comment
+	}
+	assert a == 3
+	sem.post()
+}
+
+fn f2(ch1 chan St, ch2 chan int, sem sync.Semaphore) {
+	mut r := 23
+	for i in 0 .. 2 {
+		select {
+			b := <-ch1 {
+				r = b.a
+			}
+			ch2 <- r {
+				r = 17
+			}
+		}
+		if i == 0 {
+			assert r == 17
+		} else {
+			assert r == 13
+		}
+	}
+	sem.post()
+}
+
+fn test_select_blocks() {
+	ch1 := chan int{cap: 1}
+	ch2 := chan St{}
+	ch3 := chan int{}
+	ch4 := chan int{}
+	ch5 := chan int{}
+	sem := sync.new_semaphore()
+	mut r := false
+	t := select {
+		b := <-ch1 {
+			println(b)
+		}
+		else {
+			// no channel ready
+			r = true
+		}
+	}
+	assert r == true
+	assert t == true
+	go f2(ch2, ch3, sem)
+	n := <-ch3
+	assert n == 23
+	ch2 <- St{
+		a: 13
+	}
+	sem.wait()
+	stopwatch := time.new_stopwatch({})
+	go f1(ch1, ch2, ch3, ch4, ch5, sem)
+	sem.wait()
+	elapsed_ms := f64(stopwatch.elapsed()) / time.millisecond
+	assert elapsed_ms >= 295.0
+	ch1.close()
+	ch2.close()
+	mut h := 7
+	mut is_open := true
+	if select {
+		b := <-ch2 {
+			h = 0
+		}
+		ch1 <- h {
+			h = 1
+		}
+		else {
+			h = 2
+		}
+	} {
+		panic('channel is still open')
+	} else {
+		is_open = false
+	}
+	// no branch should have run
+	for select {
+		b := <-ch2 {
+			h = 0
+		}
+	} {
+		println('ch2 open')
+	}
+	println('ch2 closed')
+	assert h == 7
+	// since all channels are closed `select` should return `false`
+	assert is_open == false
+}