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Calls to ref function inside of box functions prints a missleading error
Howdy,
In a small project using Pony, I stumbled across a simple issue, which actually took me a while to figure out: I was trying to call a ref function inside of a box function, though the compiler gave me an error which made it hard for me to find the actual error. Let me phrase this with an example:
actor Main
new create(env: Env) =>
let fox = recover Fox(env) end
let wolf = Wolf(env, consume fox)
wolf()
wolf()
class Wolf
let _env: Env
let fox: Fox ref
new create(env: Env, fox': Fox iso) =>
_env = env
fox = consume fox'
// v-- inserting a "ref" here fixes the issue
fun apply() =>
_env.out.print("I'm in a box function! 🐺")
fox()
class Fox
let _env: Env
var _x: U32 = 0
new create(env: Env) =>
_env = env
fun ref apply() =>
_x = _x + 1
_env.out.print("I'm in a ref function, and look at my number: " + _x.string() + " 🦊")
You would have expected the compiler to output an error similar to the one you get when trying to write to a field in a box function:
Cannot write to a field in a box function. If you are trying to change state in a function use fun ref
Though the error I get when running the code looks like this:
/.../project/main.pony:19:8: receiver type is not a subtype of target type
fox()
^
Info:
/.../project/main.pony:19:5: receiver type: this->Fox ref
fox()
^
/.../project/main.pony:28:3: target type: Fox ref
fun ref apply() =>
^
/.../project/main.pony:10:12: Fox box is not a subtype of Fox ref: box is not a subcap of ref
let fox: Fox ref
My understanding:
This error message states that Fox box is not a subtype of Fox ref, where no references to any Fox box are made in the above infos.
Either the first line of the info is wrong, or it is missing a line stating that the call is being made from withing a box function, meaning that the fox.apply() can only be ran if it would have been a ref function.
I would suggest having an error message saying cannot call a ref method from a field in a box function, use "fun ref" if you are trying to change state in a function.
Additional informations:
Running
0.25.0 [release]
compiled with: llvm 6.0.1 -- cc (GCC) 8.2.1 20180831
Defaults: pic=true ssl=openssl_1.1.0
On Anarchy Linux x86_64; ponyc downloaded from the AUR, version 0.25.0-1
I totally agree with you. The error message is confusing and not very helpful. We should make a special case in the error handling where we check both the receiver capability (of this) inside the current method, the viewpoint-adapted capability of the field ehose method is called and the original capability of that field.
That will turn out very helpful, thanks for taking the time to report this one.
