Unintelligible error message with @return(nullable)

[cknitt]

When you have @return(nullable) on an external definition for a function that does not return option, e.g.,

@return(nullable)
external someFunc: unit => bool = "someFunc"

you get the following rather incomprehensible error message:

%@return directive *_to_opt expect return type to be 
syntax wise `_ option` for safety

[cristianoc]

Not sure we're going to explore this. The annotation is not useful with recent changes that allow nullable to be expressed natively and pattern match on.

[cristianoc]

How about just deprecating this?

[cknitt]

Here is an example where this is used in Core:

@return(nullable) @send external toJSON: t => option<string> = "toJSON"

If this feature is deprecated and eventually removed, are you proposing to change the above definition to

@send external toJSON: t => Nullable.t<string> = "toJSON"

and have the user call Nullable.toOption himself (or alternatively pattern match directly on the nullable if the definition of Js.Nullable.t is changed to an untagged variant)?

Or that the code in Core is changed to

@send external toJSON: t => Nullable.t<string> = "toJSON"
let toJSON = x => x->toJSON->Nullable.toOption

for convenience and to keep the existing API?

This is going to affect not just Core, but many other existing bindings/libraries.

[cristianoc]

It seems better to bring Core to a more modern state than improving an outdated mechanism that will be deprecated.

[cristianoc]

CC @zth what do you think?

[github-actions]

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