Get function name, module and arity from function capture

Not sure I follow. You have the module, the function name and the arity beforehand, why use the result of Function.capture result at all? Why not cut out the middleman and just store those 3 pieces in the DB?

Not sure if I understood you correctly, but here is the clarification. I plan to use Functions.capture to “load function” from the database. My problem is how to “store function” to the database from the Elixir code, i.e. I want to write a function function_to_string(f) which can be called like this function_to_string(&MyModule.my_function/0) and would return [module: "MyModule", name: "my_function", arity: 0]" and this result I would store in the database.

So to answer your question - I agree I just need to store these 3 pieces in the DB, however, how to obtain them from a function capture.

Hey @stjefim, are you aware of Kernel.apply/3?

I don’t think there is a general solution as function can be anonymous. Also, storing function in a db may not be the best idea. Code will change, db record last for a long time. Also, think of the security implication.

Thank you, it will be useful after “loading function” - no need to do Functions.capture.

You want to store the function’s bytecode itself in the DB?

I still don’t get your requirement.

I agree with all your points:

• I will not support anonymous functions.
• Haven’t thought about versioning yet, but not a big problem for me.
• Agreed - but can do some validation to allow only certain functions to be called.

No, I will not store byte code in DB. My requirement is to convert function capture, e.g. &MyModule.my_function/0 into module name as string, function name as string and function arity as integer.

That’s not possible. Because you can’t turn a function value back to MFA, just like you can’t turn a value back to the variable name.

You can store the function value itself though:

binary = :erlang.term_to_binary(&MyModule.my_function/0)
# store in db
fun = :erlang.binary_to_term(binary)
fun.()

However, as I mentioned above, that’s a big foot gun.

The underlying Erlang function docs says it’s “mainly” for debugging:

This BIF is mainly intended for debugging, but it can sometimes be useful in library functions that need to verify, for example, the arity of a fun.

That said, the :telemetry library uses it to verify you supply an external function as a handler.