REPL 與 偵錯器(英文)

The Swift.org community makes use of the LLDB debugger to provide a rich REPL as well as the debugging environment for the Swift Language. Swift is tightly coupled to the version of the Swift compiler embedded in the debugger. Tight integration of compiler and debugger enables accurate inspection of Swift types as well as full-featured expression evaluation in the context of a rapidly evolving language.

However, because of this tight integration, developers must use a matched pair of compiler and debugger built using the same sources. Debugging using any other version of LLDB can lead to unpredictable results.

Why Combine the REPL and Debugger?

Several motivating factors contributed to the decision to use the Swift debugger as a foundation for the Swift REPL.

• Integrated debugging. The most obvious benefit is that the Swift REPL is also a full-featured debugger. It’s trivial to declare a function, set a breakpoint in it, and then call it. When execution stops at a breakpoint the full feature set of the debugger is immediately available.

    1> func answer() -> Int {
    2.     return 42
    3. }
    4> :b 2
    4> answer()
  Execution stopped at breakpoint.  Enter LLDB commands to investigate (type help for assistance.)
     1   	  func answer() -> Int {
  -> 2   	      return 42
     3   	  }
  

• Recovering from failure. Fatal errors in Swift normally result in immediate termination of a process, which makes sense for programmer errors in production code but are undesirable in an interactive context. The Swift REPL supports investigating failures with the full debugger or unwinding for immediate recovery.

    1> ["One", "Two"][2]
  fatal error: Array index out of range
  Execution interrupted. Enter Swift code to recover and continue.
  Enter LLDB commands to investigate (type :help for assistance.)
  

• Robust expression evaluation. Supporting the full range of REPL scenarios in the debugger set a high bar for the expression evaluator. As a result, expressions in the debugger have access to the full range of language features in Swift and can freely declare any valid langauge construct.

• Consistent result formatting. The strategy used for textually representing values in the REPL is shared by the debugger, ensuring consistent output even for user-defined types.