Error Handling

The Problem

I want deeply detailed information about exceptions, as a generic facility. The information should be useful for programmatic and human consumption.

Assumptions / Constraints

• Integrate well with platform
• Work ok for interop callers
• Exceptions are not for flow control

Phase One

Provide built-in Clojure exception types that are richer than Java exceptions, and eliminate the need for app developers to gen-class their own exception types. Since some information is costly to provide, provide a *debug* flag that allows the costly stuff to be disabled in production.

Debug Builds

• the following group of features will be enabled when *debug* is set
  • assert
  • *unchecked-math* math becomes checked math
    • explicit unchecked does not change (might be for semantic reasons)
  • include locals (and bindings ??) in all thrown errors (or in assertions if this is too hard)
    • is conditional in throw path too expensive?
    • otherwise compile throw differently based on flag
• unify with Java assertions?
  • Not now. this can be done as a separate enhancement later
  • see discussion at http://dev.clojure.org/jira/browse/CLJ-250
• Need separate assert-like constructs that never get turned off
  • typical name: verify
  • for preconditions: prefix either verify- or assert- to :pre and :post ??
    • what is the default ??

Errors as Data

Provide built-in types than extend Throwable and implement IPersistentMap. Goal: The new types are capable enough that Clojure programmers can be out of the business of making new Throwable subclasses.

• expose error info as part of the map
• clojure owns unnamespaced keywords
• include :namespace key
  • will be a great match target, alleviate one want for typed exceptions
• no need to include :fn key
  • not a good match target
  • available for human consumption in context of stacktrace
• need a few different types
  • error, runtimeexception, assertion?
• need names
  • unworkable: Clojure, PersistentMap,
  • maybe: Condition (but don't want to suggest non-error flow control)
• provide fn that parses Java exceptions into similar maps
  • e.g. clj-stacktrace
• REPL exception help works in terms of maps, not Java exceptions