[incubator-kie-issues#507] DMN - Add a new "list replace" function

[gitgabrio]

Fixes apache/incubator-kie-issues#507

Also requires kiegroup/dmn-feel-handbook#19

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[tarilabs]

iirc in FEEL you can access by negative index (a-la python so to speak).

I don't have access to the spec, maybe you want to check if you need to support negative indexing here.

[gitgabrio]

@tarilabs
That's true, and TBH this is a tricky part I did not understood:
position must be a non-zero integer (0 scale number) in the range [-L..L], where L is the length of the list
this is the given example
list replace( [2, 4, 7, 8], 3, 6) = [2, 4, 6, 8]
but

1. the position in the example seems to be 1 based
2. what does negative position means ? i.e. in the following example
   list replace( [2, 4, 7, 8], -3, 6) = ... what should be the expected behavior ?
   Thanks for help and suggestion!!!

[tarilabs]

TBH this is a tricky part I did not understood:
integer (0 scale number)

means it's not a floating point, just a int ("zero scale" here I believe they refer as: no decimals)

non-zero integer

the set of integers, except 0

in the range [-L..L], where L is the length of the list

it's an integer, but cannot be 0, in the range from -size to size included.

For a list of 2 elements such as [1,2], L = 2, so position can be [-2..2],
For a list of 3 elements such as [1,2,3], L = 3, so position can be [-3..3],
etc. but in any case L is never 0.

the position in the example seems to be 1 based

Yes as specified, and consistent with:

10.3.2.5 Lists and filters
... The nth element from the beginning can be accessed using L[n], and the nth element from the end can be accessed using L[-n].

taken from the spec, semantic of List

what does negative position means ? i.e. in the following example
list replace( [2, 4, 7, 8], -3, 6) = ... what should be the expected behavior ?

list replace( [2, 4, 7, 8], -3, 6) = [2, 6, 7, 8]

[gitgabrio]

thanks @tarilabs ! This helps a lot

[gitgabrio]

@tarilabs Fixed!

[tarilabs]

so my understanding the semantic of the list replace to take:

• as first argument, the list subject of replacements
• as second argument, a FEEL:number for the position (or a predicate function)
• as third argument, the to-replace value

so either replace-at-position or replace-when-predicate semantic

in case the 2nd arg is a predicate, is a predicate in 2 arguments, the "current" item being processed and the third to-replace value.

There is no doubt for the second argument (a FEEL:number for the position or a predicate function) in which variant of the built-in function you are -- ie it's not like Collections.replaceAll
this is my understanding 👍

[gitgabrio]

@tarilabs
This is from spec:
[..] return a new list where newItem replaced at all positions where the match function returned true
with example
list replace ( [2, 4, 7, 8], function(item, newItem) item < newItem, 5) = [5, 5, 7, 8]
so, I get it as "replace all elements where the match is true" - in the given example: replace all items that are less then 5: does this make sense ? Is that what you meant ?

[tarilabs]

yep, I'm just unsure why they specified the 2nd argument predicate to be a function in two arguments?

[gitgabrio]

I guess beacuse the matching function could depend on both (current) item and newItem: would it be possible to achieve that with a function with only one argument (I just started working on DMN specs)?
Anyway, I also wrote a test where the match function does not use the newItem at all, since, reading the documentation, seems a valid use case (e.g. if a number is even, replace with the newItem)

[tarilabs]

That's indeed what I would ask to the DMN RTF, since IBM is a member.

Here I can only comment personally as an external observer.
See even your example, you wrote:

list replace ( [2, 4, 7, 8], function(item, newItem) item < newItem, 5)

the 3rd argument, the item to be replaced into, is the value 5.
it will always be a value.
and even if you place in there a function invocation, the intent of FEEL/DMN is to be idempotent, so the resulting value should always be the same.

let's now focus on the 2nd argument of list replace(, the predicate-function.
I personally don't see much of a case where you need then to specify newItem as a second parameter of the predicate-function. It will always be the value 5.
So you just could have written it as function(item) item < 5.

It could be either of the two:

• in the DMN 1.5 Beta changebar they had a typo, it was not meant to be signature of function(item, newItem) but it was meant as signature of function(item)
• they really meant function(item, newItem) , and I understand it can also works this way, but to me seems a bit overcomplicated and I didn't understand the reasoning behind the choice --should be asked to the DMN RTF, or maybe someone in the TCK knows (as some also participate in the RTF as well)

[gitgabrio]

@baldimir ^^ ?