Question

Could you tell me when to use throw, exit and error?

1> catch throw ({aaa}).
{aaa}
2> catch exit ({aaa}).
{'EXIT',{aaa}}
3> catch gen_server:call(aaa,{aaa}).
{'EXIT',{noproc,{gen_server,call,[aaa,{aaa}]}}}
4> catch exit("jaj")
{'EXIT',"jaj"}

Answer 1

There are 3 classes which can be caught with a try ... catch: throw, error and exit.

• throw is generated using throw/1 and is intended to be used for non-local returns and does not generate an error unless it is not caught (when you get a nocatch error).

• error is generated when the system detects an error. You can explicitly generate an error using error/1. The system also includes a stacktrace in the generated error value, for example {badarg,[...]}.

• exit is generated using exit/1 and is intended to signal that this process is to die.

The difference between error/1 and exit/1 is not that great, it more about intention which the stacktrace generated by errors enhances.

The difference between them is actually more noticeable when doing catch ...: when throw/1 is used then the catch just returns the thrown value, as is expected from a non-local return; when an error/1 is used then the catch returns {'EXIT',Reason} where Reason contains the stacktrace; while from exit/1 catch also returns {'EXIT',Reason} but Reason only contains the actual exit reason. try ... catch looks like it equates them, but they are/were very different.