Chapter 7. Conditions

Table of Contents


Dylan offers sophisticated exception handling, allowing programs to recover smoothly from error conditions. Like C++, Dylan represents errors with objects. Dylan also supports advisory warnings and potentially correctable errors.

When something unusual happens, a program can signal a condition. Handlers specify how to react to various sorts of conditions.


A block is a group of statements. As with other control structures, it may return a value. A simple block might appear as follows:

block ()
  1 + 1;
end; // returns 2

Blocks also support non-local exits. These allow a block to exit at any time, optionally returning a value. In some ways, they are similar to goto statements or the POSIX longjmp function. To use them, specify a name in the parentheses following a block statement. Dylan binds this name to an exit function which can be called from anywhere within the block or the functions it calls. The following block returns either "Weird!" or "All's well.", depending on the color of the sky.

block (finished)
	if (sky-is-green())
	"All's well."
end block;

Many programs need to dispose of resources or perform other cleanup work, regardless of how a block is exited. Blocks may contain an optional cleanup clause, which doesn't affect the return value of the block and will always be executed.

let fd = open-input-file();
block (return)
	let (errorcode, data) = read-data(fd);
	if (errorcode)
	end if;