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Error handling

FEEL doesn't define any error handling. Instead, it follows one simple rule: if something goes wrong, return null.

Null-friendly​

If an expression can't be evaluated successfully, it returns null.

For example, in the following cases:

  • No variable exists with the given name
  • No context entry exists with the given key
  • No function exists with the given name
  • A function can't be invoked successfully with the given arguments
  • A value is compared to another value of a different type
  • An operation is not defined for the given values

Handle null values​

Expressions and operators can deal with null values. In many cases, they result also in null.

a.b > 10
// null

[{a: 1}, {b: 2}].a
// [1, null]

[{a: 1}, {b: 2}][a < 10]
// [{a: 1}]

To handle null values explicitly, use a null-check or the function get or else().

a != null and a.b > 10

get or else(a, "prio-99")

Assertions​

By default, the evaluation of an expression doesn't fail but returns null. If there is a special need to fail the evaluation under a certain condition, use the function assert().

assert(a, a != null)
// returns a if a is not null
// fails if a is null

assert(b, b >= 0, "'b' should be positive")