[leprechaun1066]

Most applications backed kdb+ do just this. It comes with its own parser and you can query tables using something like an ast.

For example the user might ask for data with the constraint

  where TradingDesk=`Eq, AvgPx>500.0

which kdb+ parses into

  ((=;`TradingDesk;(),`Eq);(>;`AvgPx;500.0))

As a dev on the system I can then have a function which takes in this constraint and a list of clients that I want to restrict the result to. That list of clients could come from another function related to the entitlements of the user who made the request:

  applyClientRestriction:{[constraint;clients] constraint,enlist(in;`Client;enlist clients)}

Which results in an extension of the constraint like this for two clients A and B:

  q)applyClientRestriction[((=;`TradingDesk;(),`Eq);(>;`AvgPx;500.0));`ClientA`ClientB]
  ((=;`TradingDesk;enlist`Eq);(>;`AvgPx;500.0);(in;`Client;enlist`ClientA`ClientB))

Then that gets passed to the function which executes the query on the table (kdb+ supports querying tables in a functional manner as well as with a structured query language) and the result has the restrictions applied.

It's really nice because, once parsed, it's list processing like in a lisp and not string processing which is a pain.