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README.org

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 =sexplib= library. S-expressions are defined by the following type:

type sexp = Atom of string | List of sexp list

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

ppx_sexp_conv fits into the =ppx_deriving= framework, so you can invoke it the same way you invoke any other deriving plug-in. Thus, we can write

type int_pair = (int * int) [@@deriving sexp]

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.

type int_pair = (int * int) [@@deriving sexp_of]
type int_pair = (int * int) [@@deriving of_sexp]

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:

open Sexplib.Std

If you’re using =Core= or =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.:

[%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"]

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

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

[@@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.Raw_grammar for details.

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

[%sexp_grammar: (int, bool array) Either.t Base.Map.M(String).t]

The syntax for grammars of polymorphic types is nonstandard:

[%sexp_grammar: < for_all : 'a 'b . ('a, 'b) Either.t > ]

We represent polymorphic types as object types because we cannot deserialize objects from sexps, and object method types are one of the few places we can use OCaml’s syntax for explicit type variables.

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.:

(3.14, "foo", "bar bla", 27)  =>  (3.14 foo "bar bla" 27)

Options

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

None        =>  ()
Some value  =>  (value)

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

None        =>  None
Some value  =>  (Some value)

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.:

{ foo = (3,4);
  bar = "some string"; }
=> ((foo (3 4)) (bar "some string"))

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

type t =
  { x : int option;
    y : int option [@sexp.option];
  } [@@deriving sexp]

The following examples show how this works.

{ x = Some 1; y = Some 2; } => ((x (1)) (y 2))
{ x = None  ; y = None;   } => ((x ()))

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.

type t = { enabled : bool [@sexp.bool] } [@@deriving sexp]

{ enabled = true } => ((enabled))
{ enabled = false } => ()

Defaults

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

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]

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 @sexp_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. @sexp_drop_if is like @sexp_drop_default, except that it lets you specify the condition under which the field is dropped. Finally, @sexp.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:

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]

Allowing extra fields

The @sexp.allow_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.

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}

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:

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))

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.

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)

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:

type ab = [ `A | `B ] [@@deriving sexp]
type cd = [ `C | `D ] [@@deriving sexp]
type abcd = [ ab | cd ] [@@deriving sexp]

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`):

type ab = [ `A | `B ] [@@deriving sexp]
type alias_of_ab = ab [@@deriving sexp_poly]
type abcd = [ ab | `C | `D ] [@@deriving sexp]

Polymorphic values

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

type 'a t = A | B of 'a [@@deriving sexp]
type foo = int t [@@deriving sexp]

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:

sexp_of_t sexp_of_a v

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.:

type foo = int * (stuff [@sexp.opaque]) [@@deriving sexp]

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:

(42, some_stuff)  =>  (42 <opaque>)

Exceptions

S-expression converters for exceptions can be automatically registered.

module M = struct
  exception Foo of int [@@deriving sexp]
end

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.:

((foo 42) (bar 3))

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:

type 'a t [@@deriving sexp]

will generate:

include Sexpable.S1 with type 'a t := 'a t

instead of:

val t_of_sexp : (Sexp.t -> 'a) -> Sexp.t -> 'a t
val sexp_of_t : ('a -> Sexp.t) -> 'a t -> Sexp.t

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:

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]

will give an error looking like:

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.

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:

type t = int sexp_opaque list
[@@deriving sexp]

After:

type t = (int [@sexp.opaque]) list
[@@deriving sexp]

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:

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

After:

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

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:

type t = A of int sexp_list
[@@deriving sexp]

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

After:

type t = A of int list [@sexp.list]
[@@deriving sexp]

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

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Generation of S-expression conversion functions from type definitions

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