How Swift Code Runs

Swift is a compiled language. Before your code runs, the Swift compiler reads all of it, checks it thoroughly, and translates it into machine code your processor executes directly.

Real-world analogy: a compiler is like an editor who reads your whole book and fixes every error before it's printed, rather than a live translator working sentence by sentence as you speak.

swift hello.swift compiles and runs in one step. For bigger projects, you build an executable once (with Xcode or swift build) and run it as many times as you like.

Errors appear before anything runs

Because the compiler checks everything first, many mistakes are reported before a single line runs:

print("This line would run")
let age: Int = "twenty"

hello.swift:2:16: error: cannot convert value of type 'String' to specified type 'Int'

Nothing is printed at all — not even the first line — because the program never compiled. The message gives the file, the line (2), the column (16), and the problem.

This is one of Swift's biggest strengths. A whole class of mistakes that other languages only discover when a user hits them, Swift reports while you're still writing the code.

Runtime errors still exist

Some problems can only be detected while the program runs — for example, reading element 10 of a 3-element array. Swift stops the program immediately with a clear message instead of carrying on with bad data:

Fatal error: Index out of range

Stopping straight away is deliberate: it's safer than continuing with corrupted values.