`Error: request to generate code for .compileTime proc`

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I have a small helper proc that is supposed to tell me at compile-time whether a type is an object-type or not.

func isObject*[T](val: typedesc[T]): bool {.compileTime.} = T is (object or ref object)

However, when I call this proc with a simple echo to see whether it works, I receive an error:

type A = object
echo isObject(A)

Error: request to generate code for .compileTime proc: isObject

Why is that? It should be perfectly valid to just call this, isObject should just compile to true and in the end what's written there is echo true, why does this cause this cryptic error?

nim-lang compile-time
1 Answers

The problem here is that runtime code (The echo call) is trying to work with a compiletime proc.

That is not valid, as the compiler would not replace the function-call with its result, but try to actually call the function at runtime instead. The compiler knows this is invalid behaviour and thus prohibits it by throwing an error, albeit one that isn't that useful.

The only way this can be allowed is if you store the result of the compile-time proc in a compile-time variable, aka a const. These are allowed to be used at runtime.

So the calling code would look more like this instead:

type A = object
const x = isObject(A)
echo x

EDIT: As Elegantbeef pointed out on nim's discord:

Another alternative is to just do what I thought would happen initially and have that isObject(A) call evaluate fully at compile-time, so that at runtime it goes away and all that's left is it's result, true.

To do so, just use static:

type A = object
echo static(isObject(A))
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