package ppx_sexp_conv

README.org

#+TITLE: ppx_sexp_conv
#+PARENT: ../README.md

* [@@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.Std=.

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

** 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 a special type
=sexp_option= which indicates that a record field should be
optional. /e.g./:

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

The type =sexp_option= is equivalent to ordinary options, but is
treated specially by the code generator.  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, on optional value is represented as the bare
value, rather than explicitly as an option.

The types =sexp_list= and =sexp_array= can be used in ways similar to
the type =sexp_option=.  They assume the empty list and empty array
respectively as default values.

These types need to be already in scope in order to use them. This can be done by
writing:

#+begin_src ocaml
open Sexplib.Conv
#+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 =@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 polymorphic
equality.  =@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. It is a generalization of =sexp_array= and
=sexp_list=.

**** 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.

#+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.

*** 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 qualifier =sexp_opaque= as if it were a
type constructor, /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 <opaque>)
#+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.