DMN13 — Need additional FEEL Types

It is possible for a FEEL expression to generate a value with no possible datatype (as they are currently defined). Here are some examples:

1. If x>0 then x else ‘value must be greater than 0’

2. [0,’a’]

3. [0,[1,2]]

In the first case the result is either a string or a number. In the second case it is a list (collection) where the items are of different types. In the third case, which is more borderline, the collection items include a number and a list, again not the same type. The third case also relates, I believe, to DMN12-115.

The rules for item definition, in combination with FEEL base types, do not support any of these cases. That means a variable with these values cannot have a typeRef. Some implementations may depend on the typeRef to compile the expression properly, and/or may prevalidate to ensure that a typeRef is provided.

I suggest one or more of these as possible solutions:

1. Allow item definition to include a union of existing types, e.g. number or string

2. Allow typeRef to be a list, e.g. feel:number, feel:string

3. Add new base type, anyAtomicType, meaning any of the FEEL base types.

We support homogeneous list types. Need better justification for heterogeneous list or union types.

DMN 1.2 adds type lattice, supports homogeneous list types like list<string>, promotes heterogeneous list types to list<Any>, which is good enough, unless more evidence to the contrary is presented.