Brent Traut Brent Traut - 3 months ago 9
Swift Question

Does swift allow code blocks without conditions/loops to reduce local variable scope?

In languages with block level scope, I sometimes create arbitrary blocks just so I can encapsulate local variables and not have them pollute their parents' scope:

func myFunc() {
// if statements get block level scope
if self.someCondition {
var thisVarShouldntExistElsewhere = true
self.doSomethingElse(thisVarShouldntExistElsewhere)
}

// many languages allow blocks without conditions/loops/etc
{
var thisVarShouldntExistElsewhere = false
self.doSomething(thisVarShouldntExistElsewhere)
}
}


When I do this in Swift, it thinks I'm creating a closure and doesn't execute the code. I could create it as a closure and immediately execute, but that seems like it would come with execution overhead (not worth it just for code cleanliness).

func myFunc() {
// if statements get block level scope
if self.someCondition {
var thisVarShouldntExistElsewhere = true
self.doSomethingElse(thisVarShouldntExistElsewhere)
}

// converted to closure
({
var thisVarShouldntExistElsewhere = false
self.doSomething(thisVarShouldntExistElsewhere)
})()
}


Is there support for something like this in Swift?

Answer

You can use a do statement to create arbitrary scope in Swift. For example:

func foo() {
    let x = 5

    do {
        let x = 10
        print(x)
    }
}

foo() // prints "10"

As per The Swift Programming Language:

The do statement is used to introduce a new scope and can optionally contain one or more catch clauses, which contain patterns that match against defined error conditions. Variables and constants declared in the scope of a do statement can be accessed only within that scope.

A do statement in Swift is similar to curly braces ({}) in C used to delimit a code block, and does not incur a performance cost at runtime.

Ref: The Swift Programming Language - Language Guide - Statements - Do Statement

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