Closures are functions; functions are closures

Anywhere that you can pass a closure, whether inline or as a named variable, you can also pass a function defined with the more familiar func statement.

The follow function in the following code makes use of a declared function, audioWisdom, a declared closure, visualWisdom, and an inline closure:

func audioWisdom() { print("Hear no evil") } 
let visualWisdom = { print("See no evil") } 

struct Acolyte 
{ 
  func follow(theWay: Subroutine) 
  { 
    theWay() 
  } 
} 

struct WiseOne 
{ 
  func impartKnowledge(acolyte: Acolyte) 
  { 
    acolyte.follow(theWay: audioWisdom) 
    acolyte.follow(theWay: visualWisdom) 
    acolyte.follow(theWay: { print("Speak no evil") }) 
  } 
}

The higher order follow function has been passed both a function and closures as its theWay parameter.

There are a few differences between a function defined with func and a closure defined with let. The closure cannot be recursive, for example. However, in the majority of cases, the choice between function and closure declaration is a matter of taste and style, guided by convention and the situation at hand. Defining a dozen simple string operations with let will look a lot more concise, and therefore easier to read, than a dozen function declarations.
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