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

Stefan Höck efasckenoth at gmail.com
Fri Oct 2 14:02:57 UTC 2015

> 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.

Hope this is helps.

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