Swift Tutorial - Tutorialspoint

Type Safety

Swift is a type-safe language which means if a part of your code expects a String, you
can't pass it an Int by mistake.

As Swift is type-safe, it performs type-checks when compiling your code and flags any
mismatched types as errors.

import Cocoa

var varA = 42

varA = "This is hello"

println(varA)

When we compile the above program, it produces the following compile time error.

Playground execution failed: error: :6:6: error: cannot assign to 'let' value

'varA'

varA = "This is hello"

Type Inference

Type inference enables a compiler to deduce the type of a particular expression
automatically when it compiles your code, simply by examining the values you provide.
Swift uses type inference to work out the appropriate type as follows.

import Cocoa

// varA is inferred to be of type Int

var varA = 42

println(varA)

// varB is inferred to be of type Double

var varB = 3.14159

println(varB)

// varC is also inferred to be of type Double

var varC = 3 + 0.14159

println(varC)