From 61af4f905bd0de1eb907da336bb48f9f48d7d2b3 Mon Sep 17 00:00:00 2001 From: Andrew Gerrand Date: Wed, 13 Nov 2013 14:40:41 +1100 Subject: [PATCH] [release-branch.go1.2] spec: clarify rules for blank identifiers MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit ««« CL 14415043 / 4daa80747394 spec: clarify rules for blank identifiers This documents the status quo more precisely. Not a language change. Fixes #6006. R=r, rsc, iant, ken CC=golang-dev https://golang.org/cl/14415043 »»» R=golang-dev CC=golang-dev https://golang.org/cl/25790043 --- doc/go_spec.html | 88 +++++++++++++++++++++++++++++++----------------- 1 file changed, 58 insertions(+), 30 deletions(-) diff --git a/doc/go_spec.html b/doc/go_spec.html index 87ee7459ff..bc9ec682a0 100644 --- a/doc/go_spec.html +++ b/doc/go_spec.html @@ -1,6 +1,6 @@ @@ -1456,10 +1456,6 @@ by a value of type T. -

-Any value may be assigned to the blank identifier. -

-

Blocks

@@ -1516,6 +1512,11 @@ No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block.

+

+The blank identifier may be used like any other identifier +in a declaration, but it does not introduce a binding and thus is not declared. +

+ 
 Declaration   = ConstDecl | TypeDecl | VarDecl .
 TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .
@@ -1585,8 +1586,10 @@ the body of any nested function.
 
Blank identifier

 
 

-The blank identifier, represented by the underscore character _, may be used in a declaration like
-any other identifier but the declaration does not introduce a new binding.
+The blank identifier is represented by the underscore character _.
+It serves as an anonymous placeholder instead of a regular (non-blank)
+identifier and has special meaning in declarations,
+as an operand, and in assignments.
 

 
 
@@ -2077,8 +2080,8 @@ operators and functions to operands.
 
 

 Operands denote the elementary values in an expression. An operand may be a
-literal, a (possibly qualified) identifier
-denoting a
+literal, a (possibly qualified)
+non-blank identifier denoting a
 constant,
 variable, or
 function,
@@ -2086,6 +2089,11 @@ a method expression yielding a function,
 or a parenthesized expression.
 

 
+

+The blank identifier may appear as an
+operand only on the left-hand side of an assignment.
+

+
 
 Operand    = Literal | OperandName | MethodExpr | "(" Expression ")" .
 Literal    = BasicLit | CompositeLit | FunctionLit .
@@ -4255,7 +4263,8 @@ assign_op = [ add_op | mul_op ] "=" .
 
 

 Each left-hand side operand must be addressable,
-a map index expression, or the blank identifier.
+a map index expression, or (for = assignments only) the
+blank identifier.
 Operands may be parenthesized.
 

 
@@ -4268,12 +4277,13 @@ a[i] = 23
 
 

 An assignment operation x op=
-y where op is a binary arithmetic operation is equivalent
+y where op is a binary arithmetic operation equivalent
 to x = x op
 y but evaluates x
 only once.  The op= construct is a single token.
 In assignment operations, both the left- and right-hand expression lists
-must contain exactly one single-valued expression.
+must contain exactly one single-valued expression, and the left-hand
+expression must not be the blank identifier.
 

 
 
@@ -4298,21 +4308,26 @@ x, y = f()
 
 

 assigns the first value to x and the second to y.
-The blank identifier provides a
-way to ignore values returned by a multi-valued expression:
-

-
-
-x, _ = f()  // ignore second value returned by f()
-

-
-

 In the second form, the number of operands on the left must equal the number
 of expressions on the right, each of which must be single-valued, and the
 nth expression on the right is assigned to the nth
-operand on the left.
+operand on the left:
 

 
+
+one, two, three = '一', '二', '三'
+

+
+

+The blank identifier provides a way to
+ignore right-hand side values in an assignment:
+

+
+
+_ = x       // evaluate x but ignore it
+x, _ = f()  // evaluate f() but ignore second result value
+

+
 

 The assignment proceeds in two phases.
 First, the operands of index expressions
@@ -4350,16 +4365,29 @@ for i, x[i] = range x {  // set i, x[2] = 0, x[0]
 

 
 

-In assignments, each value must be
-assignable to the type of the
-operand to which it is assigned. If an untyped constant
-is assigned to a variable of interface type, the constant is converted
-to type bool, rune, int, float64,
-complex128 or string
-respectively, depending on whether the value is a
-boolean, rune, integer, floating-point, complex, or string constant.
+In assignments, each value must be assignable
+to the type of the operand to which it is assigned, with the following special cases:
 

 
+
    
+
  1. 
+	If an untyped constant
+	is assigned to a variable of interface type or the blank identifier,
+	the constant is first converted to type
+	bool, rune, int, float64,
+	complex128 or string respectively, depending on
+	whether the value is a boolean, rune, integer, floating-point, complex, or
+	string constant.
+
    2. 
+
+
  2. 
+	
+	If a left-hand side is the blank identifier, any typed or non-constant
+	value except for the predeclared identifier
+	nil
+	may be assigned to it.
+
    3. 
+

 
 
If statements