Invalid ppx annotation does not fail :o

1

Now I need to pinch myself. Is it really true that calling an non-existing ppx by writing

@asdadasda

does not cause compilation error?!! Wut…

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2

if you mean the @ident syntax.
They are not ppx annotations. they are annotations (or attributes, decorators).
Annotations let you add information to different items in your code. they can be processed/used by the compiler (@unboxed, @unwrap …) or other exetrnal tools (gentype, reanalyze, …).
You can even make internal tools (used by your team) to make use of annotations.

3

Oookay. I see. And fwiw I think it’s very cool it’s possible to define custom annotations. But the compiler should surely know what annotations it can handle and what not, right?

I just caught a bug in my code where I used React-Intl like this (no error):

module Messages  = {
  @@intl.messages
  let title = { .. }
}

I’m just saying that if critical code is generated with annotations (e.g. @react.component, @genType, @intl.messages, etc etc), it will be a colossal source of bugs if typos in those are not caught in compile time. Or am I missing something relevant here?

4

in case of @gentype and @react.component you’d get type erros at usage time.
for example if you mis-type @react.component on top of your make function you will get error when you try to use the component.

module Foo = {
  @react.comonen
  let make = () => React.null
}

let a = <Foo />

gentype has similiar story.

I don’t remember if there was a way to whitelist annotations. it’s not a bad idea to have this feature , maybe via configuration (though it might get complicated since they can accpet arguments).

anyway it’s an advanced feature and should be used with care.

5

Yeah, you’re right that some typos get immediately caught because immediate problems follow from them (like with @react.component. But it would be dangerous to assume that’s always the case.

Now I wonder what is the role of ppx-flags in bsconfig. I assumed that was the white list. It wasn’t? Or has it changed in ReScript?

6

According to the configuration-schema docs, the ppx-flags option is used to hook up ppx binaries:

image

PPXes only search the AST for their specific decorators. There is no tracked list of all hooked up decorators in the compiler afaik (and I think this is also not really possible, bc ppxes are independent binaries).

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7

ppx's usually operate on extension points such as %graphql("..."). If they are not being handled there is an error. Annotations are not required to be handled by a ppx :slight_smile:

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