This open statement shadows the label

1

Hello,

I often see this warning in my code:

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image823×525 25.6 KB

I don’t quite get why this is problematic and how to avoid this warning.
I think it is pretty common for types to use the same field name.
What am I missing here?

2

If you want to give the latest open higher priority, you can add ! to it.

open! Blah

But this is a last resort, it is better to do the following:

Write the module name in front of the first field of a record to explicitly tell the type system from which module it is imported.

let x = { Meh.data: 7 }
let y = { Blah.data: 9 }

And it is necessary because you may indeed ending up misguiding the type system.

Generally, I’d use open very sparingly. If you have a module with a very long name that you use quite often you can also alias it:

module MyAlias = MyVeryVeryLongModuleName
3

Thanks, but I still don’t quite grasp what the problem space is that two modules expose the same record field? data could not be misused, because I would need a type annotation anyway. And I would assume data on itself cannot be used anywhere else but inside the context of creating a new record instance.

If this were two let bindings with the same name, from different modules, I could understand the compiler warning. But I can’t grasp where the shadowing is happening here.

4

I have another one of these I don’t get:

module DOMAPI = {
  type timeoutId
}

module Global = {
  open DOMAPI

  external setTimeout: (~handler: unit => unit, ~timeout: int=?) => timeoutId =
    "setTimeout"
}

[W] Line 6, column 2:
this open statement shadows the type identifier timeoutId (which is later used)

What is wrong here? How is anything shadowed?

5

this comes from the fact you’re shadowing Stdlib.timeoutId that is globally available.

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