Difficulty when constructing a nested data structure - data-structures

Question

While trying to create a JSON message for an API, I found myself struggling to do something that I thought would be simple. I needed to create a message like the following:
{ "list": [ { "foo": 1, "bar": 2 } ] }
However, my first attempt did not work:
say to-json { foo => [ { a => 1, b => 2 } ] };
# {"foo":[{"a":1},{"b":2}]}
Trying to simplify things further confused me more:
say { foo => [ { a => 1 } ] };
# {foo => [a => 1]}
# Note that this is not JSON, but I expected to see curly braces
Then I tried to use some temporary variables, and that worked:
my #list = { a => 1 };
say to-json { foo => #list };
# {"foo":[{"a":1}]}
my %hash = ( a => 1 );
say to-json { foo => [ %hash ] };
# {"foo":[{"a":1}]}
What's going on here?
And is there a way I can achieve my desired output without an extra temporary variable?

Answer 1

You've discovered the single argument rule. Numerous constructs in Raku will iterate the argument they are provided with. This includes the [...] array composer. This is why when we say:
say [1..10];
We get an array that contains 10 elements, not 1. However, it also means that:
say [[1,2]];
Iterates the [1,2], and thus results in [1,2] - as if the inner array were not there. A Hash iterates to its pairs, thus:
{ foo => [ { a => 1, b => 2 } ] }
Actually produces:
{ foo => [ a => 1, b => 2 ] }
That is, the array has the pairs. The JSON serializer then serializes each pair as a one-element object.
The solution is to produce a single-element iterable. The infix , operator is what produces lists, so we can use that:
say to-json { foo => [ { a => 1, b => 2 }, ] };
# note the , here ^
Then the single argument to be iterated is a 1-element list with a hash, and you get the result you want.
Easy way to remember it: always use trailing commas when specifying the values of a list, array or hash, even with a single element list, unless you actually are specifying the single iterable from which to populate it.