Hello to the kadena pact developer community
I was looking at some basic code examples and as I wanted to play around with the functionality to develop a better grasp for it, I got curios about the following:
We see that some capabilities as defined in example code test for values within a row inside a table. Is there a way one could simply test for a key and fail the predicate if the key is not present in the table?
Thank you for your insight.
While this may not be the most efficient way, I have found a solution to the question.
The pact syntax for testing membership is with the function call to 'contains'
Now we want to know whether a key exists within our table. In order to do this we can use the built-in function 'keys' 
This will return a list of strings (i.e. our keys) and will let us query via the use of 'contains' whether the key in question exists as a key in our table, or: is X a member in our table.
Since this requires us to get a complete list of keys just to see whether the particular key is within our table, this is where my concern regarding performance comes in. I wanted to share this with everyone, regardless, but in certain circumstances it may be better to just let the transaction fail instead of enforcing membership explicitly like this.
Edit: I used some code previously to show how to achieve this, but it was faulty code. If you need a membership test, you can do it within the context of an if statement, but not with (enforce ) as enforce will only allow "pure" expressions (i.e. expressions that can be evaluated on the spot and do not involve database lookups like the 'keys' function).
Enforcing a test outcome that requires database transaction will return an error like
Error from (api.testnet.chainweb.com): : Failure: Database exception: : Failure: Illegal database access attempt (keys)
