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

Question

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?

These are my favorites: https://gist.github.com/fluidityt/a87a3d0d74660786e466ea59d1e2d39f — Fluidity, Aug 23 '16 at 20:40

Jack Lawrence · Accepted Answer · 2016-08-23T20:01:54.140

12

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

edited Aug 23 '16 at 20:01

answered Aug 23 '16 at 19:57

Jack Lawrence

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Oh, perfect! I've only ever used those with repeat conditions so I didn't think of it. – Brent Traut Aug 23 '16 at 19:58
1

Damn, I didn't know about this. I would've thought to use an anon closure: `_ = { print("foo") }()` – Alexander Aug 23 '16 at 19:59

score 1 · Answer 2 · answered Aug 23 '16 at 20:00

1

An alternative to @Jack Lawrence's answer is to use blocks; similar to the block in your first code snippet.

func foo () {
    let x = 5

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

    block()
}

foo()

answered Aug 23 '16 at 20:00

keithbhunter

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This isn't really different from the OP's blocks solution. You're just assigning the block to a variable before executing it. – Caleb Aug 23 '16 at 20:07

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

2 Answers2

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