If you have written go code much, you should have known that we can't use arbitrary code styles in Go programming. Specifically speaking, we can't break a code line at an arbitrary space character position. The remaining of this article will list the detailed line break rules in Go.
{) of any explicit code block on a new line.
For example, the following for loop code block fails to compile.
for i := 5; i > 0; i--
{ // unexpected newline, expecting { after for clause
}
for i := 5; i > 0; i-- {
}
However, there are some exceptions for the rule mentioned above. For example, the following bare
for loop block compiles okay.
for
{
// do something ...
}
Then, what are the fundamental rules to do line breaks in Go programming?
Before answering this question, we should know a fact that the formal Go grammar
uses semicolons ; as terminators of code lines.
However, we seldom use semicolons in our Go code. Why?
The reason is most semicolons are optional and can be omitted.
Go compilers will insert the omitted semicolons
for us automatically in compiling.
package main;
import "fmt";
func main() {
var (
i int;
sum int;
);
for i < 6 {
sum += i;
i++;
};
fmt.Println(sum);
};
Assume the above program is stored in a file named semicolons.go,
we can run go fmt semicolons.go to remove all the unnecessary
semicolons from that file.
Compilers will insert the removed semicolons back (in memory) automatically
in compiling the source code.
What are the semicolons insertion rules in Go? Let's read the semicolon rules listed in Go specification.
The formal grammar uses semicolons ";" as terminators in a number of productions. Go programs may omit most of these semicolons using the following two rules:
break, continue,
fallthrough, or return
++,
--, ),
], or }
) or }.
For the scenarios listed in the first rule, surely, we can also insert the semicolons manually, just like the semicolons in the last code example. In other words, these semicolons are optional in programming.
The second rule means the last semicolon in a multi-item declaration before
the closing sign ) and the last semicolon within a code block or
a (struct or interface) type declaration before the closing sign
} are optional.
If the last semicolon is absent, compilers will automatically insert it back.
import (_ "math"; "fmt")
var (a int; b string)
const (M = iota; N)
type (MyInt int; T struct{x bool; y int32})
type I interface{m1(int) int; m2() string}
func f() {print("a"); panic(nil)}
var (a int; b string;);
const (M = iota; N;);
type (MyInt int; T struct{x bool; y int32;};);
type I interface{m1(int) int; m2() string;};
func f() {print("a"); panic(nil);};
Compilers will not insert semicolons for any other scenarios. We must insert the semicolons manually as needed for other scenarios. For example, the first semicolon at each line in the last code example are all required. The semicolons in the following example are also required.
var a = 1; var b = true
a++; b = !b
print(a); print(b)
From the two rules, we know that a semicolon will never be inserted
just after the for keyword.
This is why the bare for loop example shown above is valid.
func f() {
a := 0
// The following two lines both fail to compile.
println(a++) // unexpected ++, expecting comma or )
println(a--) // unexpected --, expecting comma or )
}
func f() {
a := 0;
println(a++;);
println(a--;);
}
Another consequence of the semicolon insertion rules is we can't break a line before the dot
. in a selector.
We can only break a line after the dot, as the following code shows
anObject.
MethodA().
MethodB().
MethodC()
anObject
.MethodA()
.MethodB()
.MethodC()
anObject;
.MethodA();
.MethodB();
.MethodC();
The semicolon insertion rules make us write cleaner code. They also make it is possible to write some valid but a little weird code. For example,
package main
import "fmt"
func alwaysFalse() bool {return false}
func main() {
for
i := 0
i < 6
i++ {
// use i ...
}
if x := alwaysFalse()
!x {
// do something ...
}
switch alwaysFalse()
{
case true: fmt.Println("true")
case false: fmt.Println("false")
}
}
All the three control flow blocks are valid. Compilers will insert a semicolon at the end of each of line 9, 10, 15 and 20.
Please note, theswitch-case block in the above example
will print a true instead of a false.
It is different from
switch alwaysFalse() {
case true: fmt.Println("true")
case false: fmt.Println("false")
}
go fmt command to format the former one,
a semicolon will be appended automatically after the
alwaysFalse() call, so it will become to
switch alwaysFalse();
{
case true: fmt.Println("true")
case false: fmt.Println("false")
}
switch alwaysFalse(); true {
case true: fmt.Println("true")
case false: fmt.Println("false")
}
That is why it will print a true.
It is a good habit to run go fmt
and go vet often for your code.
func f(x int) {
switch x {
case 1:
{
goto A
A: // compiles okay
}
case 2:
goto B
B: // syntax error: missing statement after label
case 0:
goto C
C: // compiles okay
}
}
The compilation error message indicates that there must be a statement following a label declaration. But it looks none label in the above example is followed by a statement. Why is only the
B: label declaration invalid?
The reason is, by the second semicolon insertion rule mentioned above,
compilers will insert a semicolon before each of the } characters
following the A: and C: label declarations.
As the following code shows.
func f(x int) {
switch x {
case 1:
{
goto A
A:
;} // a semicolon is inserted here
case 2:
goto B
B: // syntax error: missing statement after label
case 0:
goto C
C:
;} // a semicolon is inserted here
}
A solo semicolon represents a blank statement actually,
which is why the A: and C: label declarations are both valid.
On the other hand, the B: label declaration is followed by case 0:,
which is not a statement, so the B: label declaration is invalid.
We can manually insert a semicolon (a blank statement) at the end of
each of the B: label declaration to make it compile okay.
,) Will Not Be Inserted AutomaticallyIn some syntax forms containing multiple alike items, commas are used as separators, such as composite literals, function argument lists, function parameter lists and function result lists. In such a syntax form, the last item can always be followed by a comma. If the following comma is the last effective character in its respective code line, then the comma is required, otherwise, it is optional. Compilers will not insert commas automatically for any cases.
For example, the following code snippet is valid.func f1(a int, b string,) (x bool, y int,) {
return true, 789
}
var f2 func (a int, b string) (x bool, y int)
var f3 func (a int, b string, // the last comma is required
) (x bool, y int, // the last comma is required
)
var _ = []int{2, 3, 5, 7, 9,} // the last comma is optional
var _ = []int{2, 3, 5, 7, 9, // the last comma is required
}
var _ = []int{2, 3, 5, 7, 9}
var _, _ = f1(123, "Go",) // the last comma is optional
var _, _ = f1(123, "Go", // the last comma is required
)
var _, _ = f1(123, "Go")
// The same for explicit conversions.
var _ = string(65,) // the last comma is optional
var _ = string(65, // the last comma is required
)
unexpected newline syntax errors.
func f1(a int, b string,) (x bool, y int // error
) {
return true, 789
}
var _ = []int{2, 3, 5, 7 // error: unexpected newline
}
var _, _ = f1(123, "Go" // error: unexpected newline
)
At the end, let's describe the line break rules in Go according to the above explanations.
break, continue and return,
or after any of the three keywords which are not followed by labels or return results;
Like some other design details in Go, there are both praises and criticisms for the semicolon insertion rules. Some programmers don't like the rules, for they think the rules limit the freedom of code styles. Praisers think the rules make code compile faster, and make the code written by different programmers look similar, so that it is easy to understand the code written by each other.
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reflect standard package.sync standard package.sync/atomic standard package.