On Sun, Nov 29, 2009 at 11:08 PM, Malcolm Wallace
Are you sure that there can be no error recovery, to continue with events after a mal-formed event has been discarded? In many cases, it is possible. However, if you really want to terminate the stream at the first error, and to reflect this in the type, then I guess you can define your own list type:
data ListThenError e a = Cons a (ListThenError e a) | Error e
Of course this has the disadvantage that then your consumer must change to use this type too.
If it is correct, there is no disadvantage. Using a list when it is not the appropriate structure will make both the producer and the consumer code uglier. You might gain a little notational convenience, but you bubble implicit assumptions, such as an error terminates a stream, through the code where they can not be checked. Of course, when you have a stream from which errors can be recovered, do not use a type that terminates with errors. Everything cleans up so nicely when your model perfectly captures your problem. Luke