[Haskell-cafe] Concept for "equal but unknown"

martin martin.drautzburg at web.de
Fri Oct 2 14:37:16 UTC 2015

Am 10/02/2015 um 04:02 PM schrieb Stefan Höck:
>> how can I express that two things are equal to each other without knowing the exact value and
>> how do I manage to make both assume a value once one of them becomes known.
> I'm sure there are fancier ways to express such things, but you could
> use a placeholder (an Integer for instance, wrapped in a newtype)
> for the room reservation, together with a Map from Integer to Room to look
> up the room assigned to each reservation. As long as no room is assigned
> to a reservation number, a lookup in the map yields Nothing. Once a room
> is assigned, a lookup yields the room. All this should probably be
> wrapped up in the State Monad (the Map with the reservations being
> the state), so you don't have to carry around the Map explicitly.

Yes, this is what I would have done in imperative code and from your suggestion I understand that in haskell I'll have
to carry that map around. So far so good.

Here is another thought:

In math it is terribly easy to express what I am after a simple "x=y" states that x and y are equal, but unknown. As
soon as I add x=25, there is no more choice on the value of y.

I guess I am after values which become more and more defined in the course of computation.

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