► |  | Guide: PLT Scheme

1 | Welcome to PLT Scheme
2 | Scheme Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units (Components)
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Performance
18 | Running and Creating Executables
19 | Compilation and Configuration
20 | More Libraries
21 | Dialects of Scheme
| Bibliography
| Index

▼ | 10 | Exceptions and Control

10.1 | Exceptions
10.2 | Prompts and Aborts
10.3 | Continuations

Version: 4.1.4
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#### 10.2`` Prompts and Aborts

When an exception is raised, control escapes out of an arbitrary deep evaluation context to the point where the exception is caught – or all the way out if the expression is never caught:

> (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (/ 1 0)))))))
/: division by zero

But if control escapes “all the way out,” why does the REPL keep going after an error is printed? You might think that it’s because the REPL wraps every interaction in a with-handlers form that catches all exceptions, but that’s not quite the reason.

The actual reason is that the REPL wraps the interaction with a prompt, which effectively marks the evaluation context with an escape point. If an exception is not caught, then information about the exception is printed, and then evaluation aborts to the nearest enclosing prompt. More precisely, each prompt has a prompt tag, and there is a designated default prompt tag that the uncaught-exception handler uses to abort.

The call-with-continuation-prompt function installs a prompt with a given prompt tag, and then it evaluates a given thunk under the prompt. The default-continuation-prompt-tag function returns the default prompt tag. The abort-current-continuation function escapes to the nearest enclosing prompt that has a given prompt tag.

> (define (escape v)
(abort-current-continuation
(default-continuation-prompt-tag)
(lambda () v)))

> (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (escape 0)))))))
0

> (+ 1
(call-with-continuation-prompt
(lambda ()
(+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (escape 0))))))))
(default-continuation-prompt-tag)))

1

In escape above, the value v is wrapped in a procedure that is called after escaping to the enclosing prompt.

Prompts and aborts look very much like exception handling and raising. Indeed, prompts and aborts are essentially a more primitive form of exceptions, and with-handlers and raise are implemented in terms of prompts and aborts. The power of the more primitive forms is related to the word “continuation” in the operator names, as we discuss in the next section.

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