PowerQuery List.Generate: Order of Argument Evaluation? - powerbi

Question

Regarding PowerQuery's List.Generate function.
Could someone please help me understand why the first version of this List.Generate call returns 2 elements (desired result), while the second returns only 1 element (only the initial value - an empty list)? The only difference in the two calls is found on line 3.
Working query
Source =
List.Generate(
()=>[Page = 1, NextPage = "next", Response = [next = "next", results = {}]],
each [Page] <= MaxPages and NextPage <> null,
each[
Page = [Page] + 1,
Response = Json.Document(Web.Contents("https://platform.myapi.com/public_api?",
[
Query=
[
page=Text.From([Page]),
page_size=Text.From(1000)
],
Headers=[#"x-api-key"=#"ApiKey"],
RelativePath=relativePath
]
)
),
NextPage = [Response][next]
],
each [Response][results]
), ...
Correct. 2 lists - the initial, empty list, and the results of the API call
Query I think should work, but doesn't
List.Generate(
()=>[Page = 1, NextPage = "next", Response = [next = "next", results = {}]],
each [Page] <= MaxPages and [Response][next] <> null, // only change
each[
Page = [Page] + 1,
Response = Json.Document(Web.Contents("https://platform.myapi.com/public_api?",
[
Query=
[
page=Text.From([Page]),
page_size=Text.From(1000)
],
Headers=[#"x-api-key"=#"ApiKey"],
RelativePath=relativePath
]
)
),
NextPage = [Response][next]
],
each [Response][results]
),...
Incorrect. Only the initial empty list
For clarity, here is sample output from the API. It’s a paginated API with 1,000 records per page. So, since there are only 697 records returned, there is no further page to pull, and the next element is returned as null. There are 697 elements in the results array.

Answer 1

In you data, as you state, Response[next] is null (since you only have 687 items).
So the condition:
each [Page] <= MaxPages and [Response][next] <> null,
Is never met on line 3 of query 2 and no further processing happens.
In the first query, NextPage = "next", so this works.