Serializing a record to JSON with ppx_yojson_conv?

Learning
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So I’m a complete n00b to OCaml (but not a beginner programmer, the ML style is a bit confusing coming from self teaching) and I’ve been searching endlessly and reading source code to figure out how to serialize a record type into a string… What am I missing? The word serialize never appears in any docs, anywhere. There’s either no examples or no info on how to use the functions themselves, or they’re parsing not generating JSON.

I need to know if I’m just dumb or not, [@@deriving yojson] compiles just fine but I have absolutely no idea how to actually get it to return back a string or accept the type. Do I need to use the deriving in an interface file? Where did any of you learn how to do this?

If I’m dumb please delete this, thanks!

PS: I can also get s-expressions to work fine with my derived structs, I think I’m missing some etiquette for how ppx_deriving works?

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Hi @SeedyROM,

Using ppx_yojson_conv to go to & from JSON strings should look something like this:

type t = { foo : string; bar : int } [@@deriving yojson]

let to_json_string t = Yojson.Safe.to_string (yojson_of_t t)
let of_json_string s = t_of_yojson (Yojson.Safe.from_string s)

You’ll also need to tell your build system about the PPX in some way. If you’re using Dune:

(executable  ;; or 'library', or 'test'
 ...
 (preprocess (pps ppx_yojson_conv)))

Unfortunately, if you don’t declare a dependency on some ppx_deriving PPX then the [@@deriving ...] will just silently fail to generate anything. If you’re getting errors of the form "Unbound value: yojson_of_t" then that’s probably the what’s going on.

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@CraigFe
Interesting! I wasn’t getting silent failures, since I was using the ppx_jane pps. However the part about defining the to and of functions (edit: rather the code inside those methods) is what I was missing I think? I’m not at home right now so I can’t check, but I think I was expecting something more like Rust’s derive which will code gen those types for me.

I’m gonna check in a bit, thanks for quick response! I also think I’m gonna make a blog post on some of these gaps in docs coming from other langs and misconceptions I’ve had.

Thanks so much.

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Right, the PPX extension codegens the t_of_yojson and yojson_of_t for you, to-and-from the type of Yojson values. @CraigFe is just adding a nice wrapper, to go back-and-forth to strings. There are functions in the Yojson package to go back-and-forth to other sources/sinks (e.g. channels).

P.S. the “t” in t_of_yojson comes from the name of the type in the typedef, which was “t”.

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Yup, it was those generated functions I was looking for, the wrapper finally got it to click for me. My brain went blank on the t meaning my type as well.

Thanks to both of ya’ll for helping me out! Cheers.

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Imho, this is a slightly bad naming convention. When generating the functions for a type t, the t_of/of_t parts should be left out from the names. For other names, it’s reasonable to put the type names in the functions.

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Yawar, I’m curious why you think this. What naming convention would you think is better?

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I think this is better:

If the type is called t , the functions generated are {of,to}_yojson instead of t_{of,to}_yojson .

From: GitHub - ocaml-ppx/ppx_deriving_yojson: A Yojson codec generator for OCaml.

The reason is, if you have a type t, it’s certainly going to be in a module named something meaningful i.e. the actual name of the domain entity that the type represents. E.g. module Person = .... Then the conversion functions will be accessible as Person.of_yojson and Person.to_yojson, instead of Person.t_of_yojson/Person.t_to_yojson.

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Ah, I see it now. You don’t mean “in general”, but rather specifically for the case of a type “t”. Gotit. Thank you for clarifying. I buy your argument.

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