[Haskell-cafe] Trying to Express Constraints using a data structure

[GüŸnther Schmidt]

Hi all,

I'm trying to express a constraint using a data structure.

Let's say I'd want to express a "mapping" of a to b, c to b, d to b and 
e to f.

A mapping can also be from a to a, b to b and so on.

The constraint is that one cannot map a to b if b was already "mapped" 
to let's say c.

I'd like to express this constraint in a data structure, but haven't 
figured out how, yet.

Any suggestions?

Günther