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Code style checker for Jane Street Packages







ppx_js_style - Enforce Jane Street coding styles

ppx_js_style is an identity ppx rewriter that enforces Jane Street coding

Coding rules

The following rules are enforced by ppx_js_style:


[@@deprecated] attributes must contain the date of deprecation, using the
format "[since MM-YYYY] ..."

N.B. this check, on by default at Jane Street, but off by default externally. It
can also be disabled with the flag -no-dated-deprecation.


Ignored expressions must come with a type annotation, such as:

ignore (expr : typ)
let (_ : type) = expr

Note that aliases need not be annotated:

let _ = Foo.bar in

This check is enabled by default, and can be disabled by passing the flag.


Comments in mli must either be documentation comments or explicitly "ignored":

(** documentation comment *)
(*_ ignored comment *)

Normal (* comment *) comments are disallowed.

This flag additionally enables warning 50, which checks the placement of
documentation comments.

Finally, doc comments are checked to be syntactically valid.


Checks that calling ocamlc on the input would produce bytecode that works on 32
bits architectures (including js_of_ocaml), ie that all constant are
representable on 32 bits architectures. Compared to the compiler flag by the
same name, it allows to perform this check without building any bytecode.


This check is enabled by default, and can be disabled by passing the flag.

Check for misleading usage of underscores in number literals. Currrently it
means that underscores must be:

  • in positions that are multiple of 3 for numbers in decimal notation.

  • in positions that are multiple of 2 for numbers in hexadecimal, octal or
    binary notation.

Two consecutive underscores in a number disables the check for that number. This
is useful to turn off the check for integers that are really fixed point
decimals, like 1_000__0000 to represent 1000.0 with four implied decimal

Binding operators

Since 4.08 one can define
binding operators.
Which provide some of the features of ppx_let, but not all of them.
For that reason, we have made the choice to keep using ppx_let internally and
to forbid the use of binding operators, to keep a consistent style across our

So uses of these operators are rejected by default at Jane Street, but allowed
This behavior can be changed with the flags:

  • -allow-let-operators

  • -forbid-let-operators


This check, disabled by default, rejects all uses of the [@inline never].

The compiler attribute [@inline never] prevents a function from being inlined.
In most cases, when a user marks a function with [@inline never] it is because
the function is cold and they would be better off using [@cold], provided
by ppx_cold, which prevents other optimisations as well as inlining.

Setting the flags in a jbuild file

These flags are typically passed to the ppx via the ppx_jane pps. If a developer
wanted to allow unannotated ignores, they might add the following preprocess
clause to their jbuild file:

 ((names (example_application))
  (libraries ())
  (preprocess (pps (ppx_jane -allow-unannotated-ignores)))