Error Handling

The Chapel language supports throw, try, try!, catch, and throws which are described below. Chapel supports several error handling modes, including a mode suitable for prototype development and a less-permissive mode intended for production code.

Implementation Notes.

Additional information about the current implementation of error handling and the strict error handling mode, which is not defined here, is available in the errorHandling technical note

Throwing Errors

Errors may be thrown from a function to its callee with a throw statement. For a function to throw an error, its signature must include a throws declaration. The declaration is put after the return type and before any where clauses.

The statements following a throw statement in the same block are ignored by the compiler because they cannot be executed.

Only owned instances of a type inheriting from Error can be thrown.

Example (throwing.chpl).

proc canThrow(i: int): int throws {
  if i < 0 then
    throw new owned Error();

  return i + 1;
}

proc alwaysThrows():int throws {
  throw new owned Error();
  // never reached
  return 1;
}

Handling Errors

There are three ways to handle an error:

  • Halt with try!.

  • Handle the error with catch blocks.

  • Propagate the error out of the current function with throws.

Halting on error with try!

If an error is thrown by a call within the lexical scope of a try! block or a try! expression prefix, the program halts.

Example (try-bang.chpl).

proc haltsOnError():int {
  // the try! next to the throwing call
  // halts the program if an error occurs.
  return try! canThrow(0);
}

proc haltsOnErrorBlock() {
  try! {
    canThrow(1);
    canThrow(0);
  }
}

Handling an error with catch

When an error is raised by a call in a try or try! block, the rest of the block is abandoned and control flow is passed to its catch clause(s), if any.

Catch clauses

A try or try! block can have one or more catch clauses.

A catch clause can specify the variable that refers to the caught error within the catch block. If the variable is given a type, for example catch e:SomeError, it is a type filter. The corresponding catch clause matches the errors that are of the class SomeError or its subclass. If no type filter is present on a catch clause, or if no variable is present at all, then it is a catchall clause, which matches all errors.

The type filters are evaluated in the order that the catch clauses appear in the program. If a catch clause’s type filter matches, then its block is executed to the exclusion of the others. Hence there is no notion of best match, only a first match.

If the catch block itself throws an error, it is handled in the same manner as if that error were thrown by a statement adjacent to the try-catch blocks. Otherwise, after the execution of the catch block completes, the program execution proceeds to the next statement after the try-catch blocks.

Example (catching-errors.chpl).

use OS;

proc catchingErrors() throws {
  try {
    alwaysThrows(0);
  } catch {
    writeln("caught an error, unnamed catchall clause");
  }

  try {
    var x = alwaysThrows(-1);
    writeln("never reached");
  } catch e:FileNotFoundError {
    writeln("caught an error, FileNotFoundError type filter matched");
  } catch e {
    writeln("caught an error in a named catchall clause");
  }
}

try! with catch

If an error is thrown within a try! block and none of its catch clauses, if any, match that error, the program halts.

Example (catching-errors-halt.chpl).

use OS;

proc catchingErrorsHalt() {
  try! {
    var x = alwaysThrows(-1);
    writeln("never reached");
  } catch e:FileNotFoundError {
    writeln("caught a file not found error");
  }
  // errors other than FileNotFoundError cause a halt
}

Nested try

If an error is thrown within a try block and none of its catch clauses, if any, match that error, the error is directed to the enclosing try block, when present.

Example (nested-try.chpl).

class DemoError : Error { }

proc nestedTry() {
  try {
    try {
      alwaysThrows(0);
    } catch e: DemoError {
      writeln("caught a DemoError");
    }
    writeln("never reached");
  } catch {
    writeln("caught an Error from inner try");
  }
}

Propagating an error with throws

A function marked throws can pass along an error thrown by a function called within it. This can be done in several ways.

After catch clauses

Propagation can occur when no matching catch clause is found for an error raised in a try block.

Example (catching-errors-propagate.chpl).

use OS;

proc catchingErrorsPropagate() throws {
  try {
    var x = alwaysThrows(-1);
    writeln("never reached");
  } catch e:FileNotFoundError {
    writeln("caught a file not found error");
  }
  // errors other than FileNotFoundError propagate
}

catch-less try

A logical extension of the above is the case where no catch blocks are attached to the try. Here the try keyword marks throwing calls to clarify control flow.

Example (propagates-error.chpl).

proc propagatesError() throws {
  // control flow changes if an error was thrown;
  // could be indicated more clearly with try
  canThrow(0);

  try canThrow(0);

  try {
    canThrow(0);
  }

  var x = try canThrow(1);
  writeln(x);

  return try canThrow(0);
}

try expressions

try and try! are available as expressions to clarify control flow at expression granularity. The expression form may not be used with catch clauses.

Example (expression-try.chpl).

proc expressionTry(): int throws {
  var x = try canThrow(1);
  writeln(x);

  return try canThrow(0);
}

Complete handling

For a function to handle errors from its calls without itself throwing, its try/catch must be complete. This may be accomplished in two ways:

  • A catchall clause on try. This prevents try from propagating the error out of the function as described above.

    Example (warns-on-error.chpl).

    proc warnsOnError(i: int): int {
  try {
    alwaysThrows(i);
  } catch e {
    writeln("Warning: caught a error ", e);
  }
}

  • try! instead of try. This will halt the program if no matching catch clause is found, instead of propagating.

    Example (halts-on-error.chpl).

    class DemoError : Error { }
proc haltsOnError(i: int): int {
  try! {
    canThrow(i);
  } catch e: DemoError {
    writeln("caught a DemoError");
  }
}

Defer statement

When an error is thrown, it is sometimes necessary to clean up state and allocated memory. defer statements facilitate that by running when a scope is exited, regardless of how it is exited.

Example (defer.chpl).

proc deferredDelete(i: int) {
  try {
    var huge = allocateLargeObject();
    defer {
      delete huge;
      writeln("huge has been deleted");
    }

    canThrow(i);
    processObject(huge);
  } catch {
    writeln("no memory leaks");
  }
}

It is not possible to throw errors out of a defer statement because the atomicity of all defer statements must be guaranteed, and the handling context would be unclear.

Errors also cannot be thrown by deinit() for similar reasons.

Errors and Methods

Errors can be thrown by methods, just as with any other function. An overriding method must throw if the overridden method throws, or not throw if the overridden method does not throw.

Example (throwing-methods.chpl).

class ThrowingObject {
  proc f() throws {
    throw new owned Error();
  }
}

class SubThrowingObject : ThrowingObject {
  // must be marked throws even though it doesn't throw
  proc f() throws {
    writeln("this version doesn't throw");
  }
}

Errors and Multilocale

Errors can be thrown within on statements. In that event, the error will be propagated out of the on statement.

Example (handle-from-on.chpl).

proc handleFromOn() {
  try {
    on Locales[0] {
      canThrow(1);
    }
  } catch {
    writeln("caught from Locale 0");
  }
}

Errors and Parallelism

TaskErrors

TaskErrors class helps coordinate errors among groups of tasks by collecting them for centralized handling. It can be iterated on and filtered for different kinds of errors. See also the module documentation for TaskErrors.

Nested coforall statements do not produce nested TaskErrors. Instead, the nested errors are flattened into the TaskErrors error thrown by the outer loop.

Errors and begin

Errors can be thrown within a begin statement. In that event, the error will be propagated to the sync statement that waits for that task.

Example (handle-from-begin.chpl).

proc handleFromBegin() {
  try! {
    sync {
      begin canThrow(0);
      begin canThrow(1);
    }
  } catch e: TaskErrors {
    writeln("caught from Locale 0");
  }
}

Errors and coforall and cobegin

Errors can be thrown from coforall and cobegin statements and handled as TaskErrors. The nested coforall loops will emit a flattened TaskErrors error.

Example (handle-from-coforall.chpl).

proc handleFromCoforall() {
  try! {
    writeln("before coforall block");
    coforall i in 1..2 {
      coforall j in 1..2 {
        throw new owned DemoError();
      }
    }
    writeln("after coforall block");
  } catch errors: TaskErrors { // not nested
    // all of e will be of runtime type DemoError in this example
    for e in errors {
      writeln("Caught task error e ", e!.message());
    }
  }
}

Example (handle-from-cobegin.chpl).

proc handleFromCobegin() {
  try! {
    writeln("before cobegin block");
    cobegin {
      throw new owned DemoError();
      throw new owned DemoError();
    }
    writeln("after cobegin block");
  } catch errors: TaskErrors {
    for e in errors {
      writeln("Caught task error e ", e!.message());
    }
  }
}

Errors and forall

Errors can be thrown from forall loops, too. Although the forall may execute serially within a single task, it will always throw a TaskErrors error if error(s) are thrown in the loop body.

Example (handle-from-forall.chpl).

proc handleFromForall() {
  try! {
    writeln("before forall block");
    forall i in 1..2 {
      throw new owned DemoError();
    }
    writeln("after forall block");
  } catch errors: TaskErrors {
    for e in errors {
      writeln("Caught task error e ", e!.message());
    }
  }
}

Creating New Error Types

Errors in Chapel are implemented as classes, with a base class Error defined in the standard modules. Error may be used directly, and new subclass hierarchies may be created from it. See also the module documentation for Errors.

A hierarchy for system errors is included in the OS module, accessed with a use statement. See also the module documentation for OS.

Example (defining-errors.chpl).

use OS;

class DemoError : Error { }

class DemoSysError : SystemError { }

Error Handling Modes

Certain error handling details depend on the error handling mode:

Code that is legal in the production mode is always legal in the prototype mode.

Prototype Mode

In the prototype mode, it is not necessary to explicitly handle errors from a function that throws. If an error is thrown and the calling function throws, the error will be propagated out of the function. However, if an error is thrown and the calling function does not include a throws declaration, the program will halt.

In the following example, the code is in an implicit module. It is legal in the prototype mode:

Example (fatal-mode.chpl).

canThrow(1); // handling can be omitted; halts if an error occurs

proc throwsErrorsOn() throws {
  // error propagates out of this function
  canThrow(-1);
}

proc doesNotThrowErrorsOn() {
  // causes a halt if called
  alwaysThrows();
}

The following module is explicitly marked as a prototype module, so the prototype mode applies here, too.

Example (PrototypeModule.chpl).

prototype module PrototypeModule {

  canThrow(1); // handling can be omitted; halts if an error occurs

  proc throwsErrorsOn() throws {
    // error propagates out of this function
    alwaysThrows();
  }

  proc doesNotThrowErrorsOn() {
    // causes a halt if called
    alwaysThrows();
  }

  proc canThrow(i: int): int throws {
    if i < 0 then
      throw new owned Error();

    return i + 1;
  }
}

Production Mode

In the production mode, it is necessary to handle errors if the calling function does not throw. If the calling function does throw, then the error will be propagated out, as with the prototype mode.

Example (ProductionModule.chpl).

module ProductionModule {
  // This would cause a compilation error since the error is not handled:
  // canThrow(1);

  proc throwsErrorsOn() throws {
    // any error thrown by alwaysThrows will propagate out
    alwaysThrows();
  }

  // this function does not compile because the error is not handled
  // proc doesNotThrowErrorsOn() {
  //   alwaysThrows();
  // }
}