Code Blocks and Identifier Scopes

In a Go project, there are four kinds of code blocks (also called blocks later):

• the universe block contains all project source code.
• each package has a package block containing all source code, excluding the package import declarations in that package.
• each file has a file block containing all the source code, including the package import declarations, in that file.
• generally, a pair of braces {} encloses a local block. However, some local blocks aren't enclosed within {}, such blocks are called implicit local blocks. The local blocks enclosed in {} are called explicit local blocks. The {} in composite literals and type definitions don't form local blocks.

Some keywords in all kinds of control flows are followed by some implicit code blocks.

• An if, switch or for keyword is followed by two nested local blocks. One is implicit, the other is explicit. The explicit one is nested in the implicit one. If such a keyword is followed by a short variable declaration, then the variables are declared in the implicit block.
• An else keyword is followed by one explicit or implicit block, which is nested in the implicit block following its if counterpart keyword. If the else keyword is followed by another if keyword, then the code block following the else keyword can be implicit, otherwise, the code block must be explicit.
• An select keyword is followed by one explicit block.
• Each case and default keyword is followed by one implicit block, which is nested in the explicit block following its corresponding switch or select keyword.

The local blocks which aren't nested in any other local blocks are called top-level (or package-level) local blocks. Top-level local blocks are all function bodies.

Note, the input parameters and output results of a function are viewed as being declared in explicit body code block of the function, even if their declarations stay out of the pair of braces enclosing the function body block.

Code block hierarchies:

• package blocks are nested in the universe block.
• file blocks are also directly nested in the universe block, instead of package blocks. (This explanation is different from the go/* standard packages.)
• each top-level local block is nested in both a package block and a file block. (This explanation is also different from the go/* standard packages.)
• a non-top local block must be nested in another local block.

(The differences to Go specification are to make the below explanations for identifier shadowing simpler.)

Here is a picture shows the block hierarchies in a program:

There are six kinds of source code elements which can be declared:

• package imports.
• defined types and type alias.
• named constants.
• variables.
• functions.
• labels.

Labels are used in the break, continue, and goto statements.

The following table shows which code blocks all kinds of source code elements can be directly declared in:

	the universe block	package blocks	file blocks	local blocks
predeclared (built-in elements) (1)	Yes
package imports			Yes
defined types and type alias (non-builtin)		Yes	Yes	Yes
named constants (non-builtin)		Yes	Yes	Yes
variables (non-builtin) (2)		Yes	Yes	Yes
functions (non-builtin)		Yes	Yes
labels				Yes

⁽¹⁾ predeclared elements are documented in builtin standard package.
⁽²⁾ excluding struct field variables.

So,

• package imports can never be declared in package blocks and local blocks.
• functions can never be declared in local blocks. (Anonymous functions can be enclosed in local blocks but they are not declarations.)
• labels can only be declared in local blocks.

Please note,

• if the innermost containing blocks of two code element declarations are the same one, then the names (identifiers) of the two code elements can't be identical.
• the name (identifier) of a package-level code element declared in a package must not be identical to any package import name declared in any source file in the package (a.k.a., a package import name in a package must not be identical to any package-level code element declared in the package). This rule might be relaxed later.
• if the innermost containing function body blocks of two label declarations are the same one, then the names (identifiers) of the two labels can't be identical.
• the references of a label must be within the innermost function body block containing the declaration of the label.
• some special portions in the implicit local blocks in all kinds of control flows have special requirements. Generally, no code elements are allowed to be directly declared in such implicit local blocks, except some short variable declarations.
  • Each if, switch or for keyword can be closely followed by a short variable declaration.
  • Each case keyword in a select control flow can be closely followed by a short variable declaration.

(BTW, the go/* standard packages think file code blocks can only contain package import declarations.)

The scope of a declared identifier means the identifiable range of the identifier (or visible range).

Without considering identifier shadowing which will be explained in the last section of the current article, the scope definitions for the identifiers of all kinds of source code elements are listed below.

• The scope of a predeclared/built-in identifier is the universe block.
• The scope of the identifier of a package import is the file block containing the package import declaration.
• The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at package level is the package block.
• The scope of an identifier denoting a method receiver, function parameter, or result variable is the corresponding function body (a local block).
• The scope of the identifier of a constant or a variable declared inside a function body begins at the end of the specification of the constant or variable (or the end of the declaration for a short declared variable) and ends at the end of the innermost containing block.
• The scope of the identifier of a type of type alias declared inside a function body begins at the end of the identifier in the corresponding type specification and ends at the end of the innermost containing block.
• The scope of a label is the body of the innermost function body block containing the label declaration but excludes all the bodies of anonymous functions nested in the containing function.
• About the scopes of type parameters, please read the Go generics 101 book.

You may have noticed the minor difference of identifier scope definitions between local type definitions and local variables, local constants and local type aliases. The difference means a defined type may be able to reference itself in its declaration. Here is an example to show the difference.

package main

func main() {
	// var v int = v   // error: v is undefined
	// const C int = C // error: C is undefined
	/*
	type T = struct {
		*T    // error: T uses 
		x []T // error: T uses 
	}
	*/

	// Following type definitions are all valid.
	type T struct {
		*T
		x []T
	}
	type A [5]*A
	type S []S
	type M map[int]M
	type F func(F) F
	type Ch chan Ch
	type P *P

	// ...
	var p P
	p = &p
	p = ***********************p
	***********************p = p

	var s = make(S, 3)
	s[0] = s
	s = s[0][0][0][0][0][0][0][0]

	var m = M{}
	m[1] = m
	m = m[1][1][1][1][1][1][1][1]
}

Note, call fmt.Print(s) and call fmt.Print(m) both panic (for stack overflow).

And the scope difference between package-level and local declarations:

package main

// Here the two identifiers at each line are the
// same one. The right ones are both not the
// predeclared identifiers. Instead, they are
// same as respective left one. So the two
// lines both fail to compile.
/*
const iota = iota // error: constant definition loop
var true = true   // error: typechecking loop
*/

var a = b // can reference variables declared later
var b = 123

func main() {
	// The identifiers at the right side in the
	// next two lines are the predeclared ones.
	const iota = iota // ok
	var true = true   // ok
	_ = true

	// The following lines fail to compile, for
	// c references a later declared variable d.
	/*
	var c = d
	var d = 123
	_ = c
	*/
}

Ignoring labels, an identifier declared in an outer code block can be shadowed by the same identifier declared in code blocks nested (directly or indirectly) in the outer code block.

Labels can’t be shadowed.

If an identifier is shadowed, its scope will exclude the scopes of its shadowing identifiers.

Below is an interesting example. The code contains 6 declared variables named x. A x declared in a deeper block shadows the xs declared in shallower blocks.

package main

import "fmt"

var p0, p1, p2, p3, p4, p5 *int
var x = 9999 // x#0

func main() {
	p0 = &x
	var x = 888  // x#1
	p1 = &x
	for x := 70; x < 77; x++ {  // x#2
		p2 = &x
		x := x - 70 //  // x#3
		p3 = &x
		if x := x - 3; x > 0 { // x#4
			p4 = &x
			x := -x // x#5
			p5 = &x
		}
	}

	// 9999 888 77 6 3 -3
	fmt.Println(*p0, *p1, *p2, *p3, *p4, *p5)
}

Another example: the following program prints Sheep Goat instead of Sheep Sheep. Please read the comments for explanations.

package main

import "fmt"

var f = func(b bool) {
	fmt.Print("Goat")
}

func main() {
	var f = func(b bool) {
		fmt.Print("Sheep")
		if b {
			fmt.Print(" ")
			f(!b) // The f is the package-level f.
		}
	}
	f(true) // The f is the local f.
}

If we modify the above program as the following shown, then it will print Sheep Sheep.

func main() {
	var f func(b bool)
	f = func(b bool) {
		fmt.Print("Sheep")
		if b {
			fmt.Print(" ")
			f(!b) // The f is also the local f now.
		}
	}
	f(true)
}

For some circumstances, when identifiers are shadowed by variables declared with short variable declarations, some new gophers may get confused about whether a variable in a short variable declaration is redeclared or newly declared. The following example (which has bugs) shows the famous trap in Go. Almost every gopher has ever fallen into the trap in the early days of using Go.

package main

import "fmt"
import "strconv"

func parseInt(s string) (int, error) {
	n, err := strconv.Atoi(s)
	if err != nil {
		// Some new gophers may think err is an
		// already declared variable in the following
		// short variable declaration. However, both
		// b and err are new declared here actually.
		// The new declared err variable shadows the
		// err variable declared above.
		b, err := strconv.ParseBool(s)
		if err != nil {
			return 0, err
		}

		// If execution goes here, some new gophers
		// might expect a nil error will be returned.
		// But in fact, the outer non-nil error will
		// be returned instead, for the scope of the
		// inner err variable ends at the end of the
		// outer if-clause.
		if b {
			n = 1
		}
	}
	return n, err
}

func main() {
	fmt.Println(parseInt("TRUE"))
}

The output:

1 strconv.Atoi: parsing "TRUE": invalid syntax

Go only has 25 keywords. Keywords can't be used as identifiers. Many familiar words in Go are not keywords, such as int, bool, string, len, cap, nil, etc. They are just predeclared (built-in) identifiers. These predeclared identifiers are declared in the universe block, so custom declared identifiers can shadow them. Here is a weird example in which many predeclared identifiers are shadowed. Its compiles and runs okay.

package main

import (
	"fmt"
)

// Shadows the built-in function identifier "len".
const len = 3
// Shadows the built-in const identifier "true".
var true = 0
// Shadows the built-in variable identifier "nil".
type nil struct {}
// Shadows the built-in type identifier "int".
func int(){}

func main() {
	fmt.Println("a weird program")
	var output = fmt.Println

	// Shadows the package import "fmt".
	var fmt = [len]nil{{}, {}, {}}
	// Sorry, "len" is a constant.
	// var n = len(fmt)
	// Use the built-in cap function instead, :(
	var n = cap(fmt)

	// The "for" keyword is followed by one
	// implicit local code block and one explicit
	// local code block. The iteration variable
	// "true" shadows the package-level variable
	// "true" declared above.
	for true := 0; true < n; true++ {
		// Shadows the built-in const "false".
		var false = fmt[true]
		// The new declared "true" variable
		// shadows the iteration variable "true".
		var true = true+1
		// Sorry, "fmt" is an array, not a package.
		// fmt.Println(true, false)
		output(true, false)
	}
}

The output:

a weird program
1 {}
2 {}
3 {}

Yes, this example is extreme. It contains many bad practices. Identifier shadowing is useful, but please don't abuse it.

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