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6895 :: Reference to tuple type

[2020-10-06T17:05:02+02:00]

kamilchm (orginal) [2020-10-06T17:05:02+02:00] view original
Hi, I try to use a ref to a tuple type, but I'm getting an error which I don't understand: Error: () must have a tuple type. An example code:
```
type MyTuple = tuple
  a: string
  b: int

proc newMyTuple(a: string, b: int): ref MyTuple =
  new(result)
  result[] = (a, b)

const t = newMyTuple("aa", 7)
```
Playground link

How can I make it work?

Yardanico (orginal) [2020-10-06T17:28:23+02:00] view original

It seems like you can't make a ref tuple constant (even if you could, it's kinda hard to understand what exactly it will give), so as a workaround you can replace const with let.

DeletedUser (orginal) [2020-10-06T17:31:12+02:00] view original

If you don't need to use const here, then don't use it and use let, this is a compile time VM specific bug and the compile time VM doesn't handle stuff with pointers/references well.

kamilchm (orginal) [2020-10-06T17:34:26+02:00] view original

Thx! I'll use let.

slonik_az (orginal) [2020-10-06T19:36:18+02:00] view original

Since it is a bug,...

It is not a bug. You cannot use reference types in expressions executed at compile time. From Nim's official documentation:

 Restrictions on Compile-Time Execution

Nim code that will be executed at compile time cannot use the following language features:

methods
closure iterators
the cast operator
reference (pointer) types
the FFI

Araq (orginal) [2020-10-06T20:49:54+02:00] view original

The example screams "use an object type instead" to me.

mratsim (orginal) [2020-10-07T11:52:46+02:00] view original

I think that paragraph is a bit confusing, ref objects work at compile-time, ptr object do not.

Araq (orginal) [2020-10-07T12:11:56+02:00] view original

You cannot store a type that has a ref into a const, but you can use them inside compile-time procs in intermediate steps.