Use constant in attribute

Learning
1

Hello, I am building a Restful API, so I need to validate some data. I used validate package, but I thinks that’s irrelevant.

I have a “schema” with attributes that validate the data, which perfectly works:

type login = {
    username : string;  [@regex  "^[a-zA-Z0-9]{4,16}$"]
    ...
} [@@deriving yojson, validate]

I’d like to move all my regex string costants in a separate file, to reuse them in various schema (username is both in login and register requests, so in both schemas):

(* regex.ml file *)
let username = "^[a-zA-Z0-9]{4,16}$"

(* login.ml file *)
  type login = {
    username : string; [@regex  Regex.username]
    ...
  } [@@deriving yojson, validate]

That doesn’t work: Error: constant expected. My basic understanding is that attributes must have an “hardcoded” value. Is there any way to make this work?

2

The value must be usable by the PPX which is a purely syntactic transformer, so references to identifiers are out of this scope because that would require typechecking, as well as some module-aware smart code to extract the actual string literal, when that’s even possible.

To do what you want, it seems you need a preceding preprocessing step to insert the string, either using another PPX, or some kind of macro system like M4. Intuitively, I would strongly go with a PPX, although even that seems a bit overkill/brittle.

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