Adding conditions to the language

[StrykerKKD]

Variables could have conditions/validations in them, which could be used for expressing conditions like a > 5 or a!=5 or a > b or a!=b. Basically this would be Logical's defacto constraint system and would make it possible to implement negation in this way. (So much for being negation free :D)

A condition in this case would be a function with the following signature:
state -> value_a -> value_b -> state option
This would mean that a condition can remove a state if it thinks that it's faulty and we would run this check during the unify process.

Using two variables in a condition is a little bit tricky, because in this case the two variables must share the condition, because we can't be sure which variable will be the last to get a value a be able to check the conditions.

Another tricky part is that a condition should pass when it can't evaluate the expression, because one or both variables doesn't have a value.

Conditions are the same type wise as unify, so maybe we can make this whole concept more general by making unify into a condition also. Although unify is a very basic building block of the system.

[StrykerKKD]

Conditions can help us removing the usage State.value_of, which makes the goal dependent of where it's used, which is something we should avoid.

[StrykerKKD]

This issue is basically about creating a constraint system. Right now I can only see two kind of constraint system:

• Local constraint system that puts the constraints beside the variable in a state
• Global constraint system that has a separate global store for constraints that is shared by everyone

The proposed one is a local one and I'm thinking of sticking to it for this issue, but I'm keeping the global one in mind.