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Chapter 12 Questions and Answers

1. Consider these two functions. Will EdgeDB allow both of them to be defined at the same time?

No, because the input signature for both of them is the same:

function gives_number(input: int64) -> int64
  using(input);

function gives_number(input: int64) -> int32
  using(<int32>input);

There is no way to know which of the two you want to use if both could take an int64.

Here's the error:

error: cannot create the `default::gives_number(input: std::int64)` function: a function with the same signature is already defined

2. How about these two functions? Will EdgeDB allow both of them to be defined at the same time?

Yes, they have different signatures because the input is different: one takes an int16 and the other takes an int32. In fact, you can see both of them by using describe. For example, describe function make64 as text gives the following:

{
  'function default::make64(input: std::int16) ->  std::int64 using (input);
   function default::make64(input: std::int32) ->  std::int64 using (input);',
}

Note however that select make64(8); will actually produce an error! The error is:

error: function "make64(arg0: std::int64)" does not exist
  ┌─ query:1:8
  │
1 │ select make64(8);
  │        ^^^^^^^^^ Did you want one of the following functions instead:
default::make64(input: std::int16)
default::make64(input: std::int32)

That's because select make64(8) is giving it an int64, and it doesn't have a signature for that. You would need to cast with select make64(<int32>8); (or <int16>) to make it work.

3. Will select {} ?? {3, 4} ?? {5, 6}; work?

No, because EdgeDB doesn't know what type {} is. But if you cast the {} to an int64:

select <int64>{} ?? {3, 4} ?? {5, 6};

Then it will give the output {3, 4}.

4. Will select <int64>{} ?? <int64>{} ?? {1, 2} work?

Yes, with the output {1, 2}. If you continue to use ??, it will keep going until it hits something that is not empty.

Knowing that, you can probably guess the output of this:

select <int64>{} ?? <int64>{} ?? {1} ?? <int64>{} ?? {5};

The output is {1}, the first not empty set that it sees.

5. Trying to make a single string of everyone's name with select array_join(array_agg(Person.name)); isn't working. What's the problem?

The error message gives a hint:

error: function "array_join(arg0: array<std::str>)" does not exist

That means that the input that it received doesn't match any of its function signatures. And if you check {eql:func}the function itself <docs:std::array_join>, you can see why: it needs a second string:

std::array_join(array: array<str>, delimiter: str) -> str

So we can just change it to select array_join(array_agg(Person.name), ' '); or select array_join(array_agg(Person.name), ' is awesome '); or anything else with a second string and it works.