What is FEEL?
FEEL (Friendly Enough Expression Language) is a part of the DMN specification of the Object Management Group (OMG). It is designed to write expressions for decision tables and literal expressions in a way that is easily understood by business professionals and developers.
Unary-tests vs. expressions
FEEL has two types of expressions for different use cases:
A unary-tests expression is a special kind of boolean expression. It should be used for the input entries of a decision table (i.e. the conditions of a rule).
// checks if the input value is less than 7
// checks if the input value is neither 2 nor 4
// checks if the input value is between '2015-09-17' and '2015-09-19'
// checks if the input value is less than or equal to one day
General expressions that can return values of different types. They can be used everywhere; for example, in a decision table as an input expression or as an output entry.
applicant.monthly.income * 12
if applicant.maritalStatus in ("M","S") then "valid" else "not valid"
sum( [applicant.monthly.repayments, applicant.monthly.expenses] )
sum( credit_history[record_date > date("2011-01-01")].weight )
some ch in credit_history satisfies ch.event = "bankruptcy"