janestreet / Ppx_sexp_conv

#+TITLE: ppx_sexp_conv

• [@@deriving sexp]

=ppx_sexp_conv= is a PPX syntax extension that generates code for converting OCaml types to and from s-expressions, as defined in the [[https://github.com/janestreet/sexplib][=sexplib=]] library. S-expressions are defined by the following type:

#+begin_src ocaml type sexp = Atom of string | List of sexp list #+end_src

and are rendered as parenthesized lists of strings, /e.g./ =(This (is an) (s expression))=.

=ppx_sexp_conv= fits into the [[https://github.com/whitequark/ppx_deriving][=ppx_deriving=]] framework, so you can invoke it the same way you invoke any other deriving plug-in. Thus, we can write

#+begin_src ocaml type int_pair = (int * int) [@@deriving sexp] #+end_src

to get two values defined automatically, =sexp_of_int_pair= and =int_pair_of_sexp=. If we only want one direction, we can write one of the following.

#+begin_src ocaml type int_pair = (int * int) [@@deriving sexp_of] type int_pair = (int * int) [@@deriving of_sexp] #+end_src

These sexp-converters depend on having a set of converters for basic values (/e.g./, =int_of_sexp=) already in scope. This can be done by writing:

#+begin_src ocaml open Sexplib.Std #+end_src

If you're using [[https://github.com/janestreet/core][=Core=]] or [[https://github.com/janestreet/core_kernel][=Core_kernel=]], you can get the same effect with =open Core= or =open Core_kernel=.

It's also possible to construct converters based on type expressions, /i.e./:

#+begin_src ocaml [%sexp_of: (int * string) list] [1,"one"; 2,"two"] |> Sexp.to_string;; => "((1 one) (2 two))"

[%sexp_of: (int * string) list] [1,"one"; 2,"two"] |> [%of_sexp: (int * string) list];; => [1,"one"; 2,"two"] #+end_src

For =%sexp_of=, we can also omit the conversion of some types by putting underscores for that type name.

#+begin_src ocaml [%sexp_of: (int * _) list] [1,"one"; 2,"two"] |> Sexp.to_string;; => "((1 _)(2 _))" #+end_src

• [@@deriving sexp_grammar]

If =ppx_sexp_conv= can derive =of_sexp=, it can also generate a description of the sexps that the resulting =t_of_sexp= would accept. This is the sexp grammar. See =Sexplib0.Sexp_grammar= for details.

It is possible to construct sexp grammars directly from type expressions, e.g.,

#+BEGIN_SRC ocaml [%sexp_grammar: (int, bool array) Either.t Base.Map.M(String).t] #+END_SRC

** Conversion rules

In the following, we'll review the serialization rules for different OCaml types.

*** Basic types

Basic types are represented as atoms. For numbers like =int=, =int32=, =int64=, =float=, the string in the atom is what is accepted the standard ocaml functions =int_of_string=, =Int32.of_string=, etc. For the types =char= or =string=, the string in the atom is respectively a one character string or the string itself.

*** Lists and arrays

OCaml-lists and arrays are represented as s-expression lists.

*** Tuples and unit

OCaml tuples are treated as lists of values in the same order as in the tuple. The type =unit= is treated like a 0-tuple. /e.g./:

#+begin_src ocaml (3.14, "foo", "bar bla", 27) => (3.14 foo "bar bla" 27) #+end_src

*** Options

With options, =None= is treated as a zero-element list, and =Some= is treated as a singleton list, as shown below.

#+begin_src ocaml None => () Some value => (value) #+end_src

We also support reading options following the ordinary rules for variants /i.e./:

#+begin_src ocaml None => None Some value => (Some value) #+end_src

The rules for variants are described below.

*** Records

Records are represented as lists of lists, where each inner list is a key-value pair. Each pair consists of the name of the record field (first element), and its value (second element). /e.g./:

#+begin_src ocaml { foo = (3,4); bar = "some string"; } => ((foo (3 4)) (bar "some string")) #+end_src

Type specifications of records allow the use of several attributes. The attribute =sexp.option= indicates that a record field should be optional. /e.g./:

#+begin_src ocaml type t = { x : int option; y : int option [@sexp.option]; } [@@deriving sexp] #+end_src

The following examples show how this works.

#+begin_src ocaml { x = Some 1; y = Some 2; } => ((x (1)) (y 2)) { x = None ; y = None; } => ((x ())) #+end_src

Note that, when present, an optional value is represented as the bare value, rather than explicitly as an option.

The attribute =sexp.bool= indicates that a boolean record field is shown as either present or absent, but not as containing a value.

#+begin_src ocaml type t = { enabled : bool [@sexp.bool] } [@@deriving sexp]

{ enabled = true } => ((enabled)) { enabled = false } => () #+end_src

**** Defaults

More complex default values can be specified explicitly using several constructs, /e.g./:

#+begin_src ocaml type t = { a : int [@default 42]; b : int [@default 3] [@sexp_drop_default (=)]; c : int [@default 3] [@sexp_drop_if fun x -> x = 3]; d : int Queue.t [@sexp.omit_nil] } [@@deriving sexp] #+end_src

The =@default= annotation lets one specify a default value to be selected if the field is not specified, when converting from an s-expression. The [email protected]_drop_default= annotation implies that the field will be dropped when generating the s-expression if the value being serialized is equal to the default according to the specified equality function. [email protected]_drop_if= is like [email protected]_drop_default=, except that it lets you specify the condition under which the field is dropped. Finally, [email protected]omit_nil= means to treat a missing field as if it has value =List []= when reading, and drop the field if it has value =List []= when writing.

***** Specifying equality for [@sexp_drop_default]

The equality used by [@sexp_drop_default] is customizable. There are several ways to specify the equality function:

#+begin_src ocaml type t = { a : u [@default u0] [@sexp_drop_default (=)]; (* explicit user-provided function ) b : u [@default u0] [@sexp_drop_default.compare]; ( uses [%compare.equal: u] ) c : u [@default u0] [@sexp_drop_default.equal]; ( uses [%equal: u] ) d : u [@default u0] [@sexp_drop_default.sexp]; ( compares sexp representations ) e : u [@default u0] [@sexp_drop_default]; ( deprecated. uses polymorphic equality. *) } [@@deriving sexp] #+end_src

**** Allowing extra fields

The [email protected]_extra_fields= annotation lets one specify that the sexp-converters should silently ignore extra fields, instead of raising. This applies only to the record to which the annotation is attached, and not to deeper sexp converters that may be called during conversion of a sexp to the record.

#+begin_src ocaml type t = { a: int } [@@deriving sexp] ((a 0)(b b)) => exception

type t = { a: int } [@@deriving sexp] [@@sexp.allow_extra_fields] ((a 0)(b b)) => {a = 0}

type t = A of { a : int } [@sexp.allow_extra_fields] [@@deriving sexp] (A (a 0)(b b)) => A {a = 0} #+end_src

*** Variants

Constant constructors in variants are represented as strings. Constructors with arguments are represented as lists, the first element being the constructor name, the rest being its arguments. Constructors may also be started in lowercase in S-expressions, but will always be converted to uppercase when converting from OCaml values.

For example:

#+begin_src ocaml type t = A | B of int * float * t [@@deriving sexp] B (42, 3.14, B (-1, 2.72, A)) => (B 42 3.14 (B -1 2.72 A)) #+end_src

The above example also demonstrates recursion in data structures.

Variants support the attribute =sexp.list= when a clause has a single list as its argument.

#+begin_src ocaml type t = | A of int list | B of int list [@sexp.list]

A [1; 2; 3] => (A (1 2 3)) B [1; 2; 3] => (B 1 2 3) #+end_src

**** Inline records

Constructors with inline records are represented as lists, the first element being the constructor name, the rest being the record fields, represented the same way as in record types, but without being wrapped in an extra layer of parentheses.

#+begin_src ocaml type t = A of { x : int }

A { x = 8 } => (A (x 8)) #+end_src

*** Polymorphic variants

Polymorphic variants behave almost the same as ordinary variants. The notable difference is that polymorphic variant constructors must always start with an either lower- or uppercase character, matching the way it was specified in the type definition. This is because OCaml distinguishes between upper and lowercase variant constructors. Note that type specifications containing unions of variant types are also supported by the S-expression converter, for example as in:

#+begin_src ocaml type ab = [ A |B ] [@@deriving sexp] type cd = [ C |D ] [@@deriving sexp] type abcd = [ ab | cd ] [@@deriving sexp] #+end_src

However, because ppx_sexp_conv needs to generate additional code to support inclusions of polymorphic variants, ppx_sexp_conv needs to know when processing a type definition whether it might be included in a polymorphic variant. ppx_sexp_conv will only generate the extra code automatically in the common case where the type definition is syntactically a polymorphic variant like in the example above. Otherwise, you will need to indicate it by using [@@deriving sexp_poly] (resp of_sexp_poly) instead of [@@deriving sexp] (resp of_sexp):

#+begin_src ocaml type ab = [ A |B ] [@@deriving sexp] type alias_of_ab = ab [@@deriving sexp_poly] type abcd = [ ab | C |D ] [@@deriving sexp] #+end_src

*** Polymorphic values

There is nothing special about polymorphic values as long as there are conversion functions for the type parameters. /e.g./:

#+begin_src ocaml type 'a t = A | B of 'a [@@deriving sexp] type foo = int t [@@deriving sexp] #+end_src

In the above case the conversion functions will behave as if =foo= had been defined as a monomorphic version of =t= with ='a= replaced by =int= on the right hand side.

If a data structure is indeed polymorphic and you want to convert it, you will have to supply the conversion functions for the type parameters at runtime. If you wanted to convert a value of type ='a t= as in the above example, you would have to write something like this:

#+begin_src ocaml sexp_of_t sexp_of_a v #+end_src

where =sexp_of_a=, which may also be named differently in this particular case, is a function that converts values of type ='a= to an S-expression. Types with more than one parameter require passing conversion functions for those parameters in the order of their appearance on the left hand side of the type definition.

*** Opaque values

Opaque values are ones for which we do not want to perform conversions. This may be, because we do not have S-expression converters for them, or because we do not want to apply them in a particular type context. /e.g./ to hide large, unimportant parts of configurations. To prevent the preprocessor from generating calls to converters, simply apply the attribute =sexp.opaque= to the type, /e.g./:

#+begin_src ocaml type foo = int * (stuff [@sexp.opaque]) [@@deriving sexp] #+end_src

Thus, there is no need to specify converters for type =stuff=, and if there are any, they will not be used in this particular context. Needless to say, it is not possible to convert such an S-expression back to the original value. Here is an example conversion:

#+begin_src ocaml (42, some_stuff) => (42 ) #+end_src

*** Exceptions

S-expression converters for exceptions can be automatically registered.

#+begin_src ocaml module M = struct exception Foo of int [@@deriving sexp] end #+end_src

Such exceptions will be translated in a similar way as sum types, but their constructor will be prefixed with the fully qualified module path (here: =M.Foo=) so as to be able to discriminate between them without problems.

The user can then easily convert an exception matching the above one to an S-expression using =sexp_of_exn=. User-defined conversion functions can be registered, too, by calling =add_exn_converter=. This should make it very convenient for users to catch arbitrary exceptions escaping their program and pretty-printing them, including all arguments, as S-expressions. The library already contains mappings for all known exceptions that can escape functions in the OCaml standard library.

*** Hash tables

The Stdlib's Hash tables, which are abstract values in OCaml, are represented as association lists, /i.e./ lists of key-value pairs, /e.g./:

#+begin_src scheme ((foo 42) (bar 3)) #+end_src

Reading in the above S-expression as hash table mapping strings to integers (=(string, int) Hashtbl.t=) will map =foo= to =42= and =bar= to =3=.

Note that the order of elements in the list may matter, because the OCaml-implementation of hash tables keeps duplicates. Bindings will be inserted into the hash table in the order of appearance. Therefore, the last binding of a key will be the "visible" one, the others are "hidden". See the OCaml documentation on hash tables for details.

** A note about signatures

In signatures, =ppx_sexp_conv= tries to generate an include of a named interface, instead of a list of value bindings. That is:

#+begin_src ocaml type 'a t [@@deriving sexp] #+end_src

will generate:

#+begin_src ocaml include Sexpable.S1 with type 'a t := 'a t #+end_src

instead of:

#+begin_src ocaml val t_of_sexp : (Sexp.t -> 'a) -> Sexp.t -> 'a t val sexp_of_t : ('a -> Sexp.t) -> 'a t -> Sexp.t #+end_src

There are however a number of limitations:

• the type has to be named t
• the type can only have up to 3 parameters
• there shouldn't be any constraint on the type parameters

If these aren't met, then =ppx_sexp_conv= will simply generate a list of value bindings.

*** Weird looking type errors

In some cases, a type can meet all the conditions listed above, in which case the rewriting will apply, but lead to a type error. This happens when the type [t] is an alias to a type which does have constraints on the parameters, for instance:

#+begin_src ocaml type 'a s constraint 'a = [> `read ] val sexp_of_s : ... val s_of_sexp : ... type 'a t = 'a s [@@deriving_inline sexp] include Sexpable.S1 with type 'a t := 'a t [@@@end] #+end_src

will give an error looking like:

#+begin_src Error: In this with' constraint, the new definition of t does not match its original definition in the constrained signature: Type declarations do not match: type 'a t = 'a t constraint 'a = [>read ] is not included in type 'a t File "sexpable.mli", line 8, characters 21-58: Expected declaration Their constraints differ. #+end_src

To workaround that error, simply copy the constraint on the type which has the =[@@deriving]= annotation. This will force generating a list of value bindings.

** Deprecated syntax

Originally, ppx_sexp_conv used special types instead of attributes. Those types are now deprecated. Here are the appropriate conversions to update from code using now-deprecated types to the newer attributes.

*** Opaque types

Convert uses of sexp_opaque to uses of [@sexp.opaque]. The [@sexp.opaque] attribute usually needs explicit parentheses to clarify what type it annotate.

Before:

#+begin_src ocaml type t = int sexp_opaque list [@@deriving sexp] #+end_src

After:

#+begin_src ocaml type t = (int [@sexp.opaque]) list [@@deriving sexp] #+end_src

*** Record fields

Convert uses of sexp_option, sexp_list, sexp_array, and sexp_bool to uses of [@sexp.option], [@sexp.list], [@sexp.array], and [@sexp.bool] as appropriate. The attribute only specifies the modification, not the type, so you will need to use the regular types option, list, array, and/or bool as well. Unlike [@sexp.opaque], these attributes do not need extra parentheses.

Before:

#+begin_src ocaml type t = { a : int sexp_option ; b : int sexp_list ; c : int sexp_array ; d : sexp_bool } [@@deriving sexp] #+end_src

After:

#+begin_src ocaml type t = { a : int option [@sexp.option] ; b : int list [@sexp.list] ; c : int array [@sexp.array] ; d : bool [@sexp.bool] } [@@deriving sexp] #+end_src

*** Variant constructors

Convert uses of sexp_list in variants and polymorphic variants to uses of [@sexp.list]. You need to add the regular type list as well. Unlike [@sexp.opaque], this attribute does not need extra parentheses.

Before:

#+begin_src ocaml type t = A of int sexp_list [@@deriving sexp]

type u = [`B of int sexp_list] [@@deriving sexp] #+end_src

After:

#+begin_src ocaml type t = A of int list [@sexp.list] [@@deriving sexp]

type u = [`B of int list [@sexp.list]] [@@deriving sexp] #+end_src