爲什麼在if條件中添加括號會導致編譯錯誤？

Question

以下Go代碼運行正常：

package main 

import "fmt" 

func main() { 
    if j := 9; j > 0 { 
     fmt.Println(j) 
    } 
} 

但添加在條件括號後：

package main 

import "fmt" 

func main() { 
    if (j := 9; j > 0) { 
     fmt.Println(j) 
    } 
} 

有編譯錯誤：

.\Hello.go:7: syntax error: unexpected :=, expecting) 
.\Hello.go:11: syntax error: unexpected } 

爲什麼編譯器抱怨呢？

Answer 1

答案不是簡單的 「因爲圍棋不需要括號」;看到下面的例子是一個有效的圍棋語法：

j := 9 
if (j > 0) { 
    fmt.Println(j) 
} 

Go Spec: If statements:

IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" (IfStmt | Block) ] . 

我的例子之間的區別和你的是，我的例子只包含表達塊。可以根據需要將表達式加括號（它不會很好格式化，但這是另一個問題）。

在您的示例中，您同時指定了簡單語句和表達式塊。如果你把整成小括號，編譯器會嘗試將整個解釋爲Expression Block to which this does not qualify：

Expression = UnaryExpr | Expression binary_op UnaryExpr . 

j > 0是有效的表達式，j := 9; j > 0不是有效的表達。

即使j := 9本身並不是一個表達式，它是一個Short variable declaration。此外，簡單的賦值（例如j = 9）不是Go中的表達式，而是語句（Spec: Assignments）。請注意，賦值通常是C，Java等其他語言中的表達式）。這就是爲什麼例如下面的代碼也是無效的原因：

x := 3 
y := (x = 4) 

Answer 2

因爲那是Go語法defines和if聲明。

IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" (IfStmt | Block) ] .

而且從Effective Go：

Parentheses

Go needs fewer parentheses than C and Java: control structures (if, for, switch) do not have parentheses in their syntax.

and：

Control structures

The control structures of Go are related to those of C but differ in important ways. There is no do or while loop, only a slightly generalized for; switch is more flexible; if and switch accept an optional initialization statement like that of for; break and continue statements take an optional label to identify what to break or continue; and there are new control structures including a type switch and a multiway communications multiplexer, select. The syntax is also slightly different: there are no parentheses and the bodies must always be brace-delimited.

（強調）