On 22 Nov 2009, at 22:06, samppi wrote:
> Yes, I see. I'm going to guess that the parser-m that I give above has
> no possible m-zero, so I think I'll have to rethink how I'm going to
> approach this problem. I probably am going to just define failure in
> another way. (The reason why I can't use nil is because I need to
> store metadata on the object representing failure. I'm sure I'll
> figure something out.) Thanks for your help.
Using another value than nil (e.g. a keyword) is doable:
(state-t (maybe-t identity-m ::failure))
instead of
(state-t maybe-m)
However, it seems to me (after a quick look at your initial message)
that you want m-zero to be a pair containing a keyword plus a state
value. That is not possible because m-zero has to be a constant. You
can't pack additional information on m-zero. Using metadata on m-zero
looks risky as well (but I didn't think it through to the end) because
metadata is easily lost in operations that don't expect it to be there.
Konrad.
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