Easily define PPX derivers without boilerplate and runtime overhead.
opam install ppx_easy_derivingThe following shows multiple ways of defining a PPX deriver for a standard equal function with various trade-offs.
The simplest way is to define equal (here called easy_equal2) for binary product and sum types, i.e. pairs and eithers.
Using an isomorphism, all algebraic datatypes can be handled using the simple binary constructs and their base cases (unit and empty type).
This is very similar to the simple derivers of ppx_type_directed_value, but with lower runtime overhead.
module EasyEqual2Arg: Simple.Variant.S =
struct
(** Name of deriver. *)
let name = "easy_equal2"
(** Type of derived [easy_equal2] function for type [t]. *)
let typ ~loc t = [%type: [%t t] -> [%t t] -> bool]
(** [easy_equal2] function for [unit] type. *)
let unit ~loc = [%expr fun () () -> true]
(** [easy_equal2] function for a pair of types with given [easy_equal2] functions [e1] and [e2]. *)
let both ~loc e1 e2 = [%expr fun (l1, l2) (r1, r2) -> [%e e1] l1 r1 && [%e e2] l2 r2]
(** [easy_equal2] function for the empty type. *)
let empty ~loc = [%expr fun _ _ -> true]
(** [easy_equal2] function for an either of types with given [easy_equal2] functions [e1] and [e2]. *)
let either ~loc e1 e2 =
[%expr fun l r ->
match l, r with
| Either.Left l, Either.Left r -> [%e e1] l r
| Either.Right l, Either.Right r -> [%e e2] l r
| _, _ -> false
]
(** Apply isomorphism to given [easy_equal2] function.
This allows {!both} and {!either} to be applied to tuples, records and (polymorphic) variants of arbitrary size. *)
let apply_iso ~loc easy_equal2 ~f ~f':_ =
[%expr fun l r -> [%e easy_equal2] ([%e f] l) ([%e f] r)]
end
(* Create full deriver from simple deriver and register it with ppxlib. *)
module EasyEqual2Deriver = Deriver.Make (Simple.Variant.Reduce (EasyEqual2Arg))
let _ = EasyEqual2Deriver.register ()The isomorphisms of simple deriver definitions still incur some runtime overhead.
A more efficient way to define equal (here called easy_equal) is to define it for entire products and sums in one go, instead of decomposing them to binary reductions.
These definitions are more involved, especially variant since it also handles inline records in constructor arguments.
However, the deriver defined this way is as efficient as a dedicated equal deriver, e.g. from ppx_deriving.eq or ppx_compare.
module EasyEqualArg: Product.Variant.S =
struct
(** Name of deriver. *)
let name = "easy_equal"
(** Type of derived [easy_equal] function for type [t]. *)
let typ ~loc t = [%type: [%t t] -> [%t t] -> bool]
(** [easy_equal] function body for a product type with list of given [easy_equal] functions for the elements. *)
let product_body ~loc es pel per =
let body =
let esl = Pat_exp.to_exps ~loc pel in
let esr = Pat_exp.to_exps ~loc per in
Util.map3 (fun e l r ->
(* Apply [easy_equal] function per element. *)
[%expr [%e e] [%e l] [%e r]]
) es esl esr
in
(* Reduce per-element [easy_equal] results to a result for the entire product. *)
Util.reduce ~unit:[%expr true] ~both:(fun acc x ->
[%expr [%e acc] && [%e x]]
) body
(** [easy_equal] function for a product type with list of given [easy_equal] functions for the elements.
[pe_create] allows creating patterns and expressions of the actual product type. *)
let product ~loc ~pe_create es =
let pel = pe_create ~prefix:"l" in
let per = pe_create ~prefix:"r" in
let body = product_body ~loc es pel per in
let pl = Pat_exp.to_pat ~loc pel in
let pr = Pat_exp.to_pat ~loc per in
[%expr fun [%p pl] [%p pr] -> [%e body]]
(** [easy_equal] function for a (polymorphic) variant type with list of given [easy_equal] functions for the (polymorphic) variant constructors. *)
let variant ~loc ces =
let cases = List.map (fun (c, c2, _es, es2) ->
let pel = c ~prefix:"l" in
let per = c ~prefix:"r" in
let pel2 = c2 ~prefix:"l" in
let per2 = c2 ~prefix:"r" in
let body = product_body ~loc es2 pel2 per2 in
let pa = Pat_exp.to_pat ~loc pel in
let pb = Pat_exp.to_pat ~loc per in
(* Equality case for one constructor with given product argument. *)
case ~lhs:[%pat? [%p pa], [%p pb]]
~guard:None
~rhs:body
) ces
in
let fallback =
(* Inequality case as fallback. *)
case ~lhs:[%pat? _, _]
~guard:None
~rhs:[%expr false]
in
let body = pexp_match ~loc [%expr l, r] (cases @ [fallback]) in
[%expr fun l r -> [%e body]]
end
(* Create full deriver from product deriver and register it with ppxlib. *)
module EasyEqualDeriver = Deriver.Make (Product.Variant.Make (EasyEqualArg))
let _ = EasyEqualDeriver.register ()These two easy derivers are benchmarked against ppx_deriving.eq, ppx_compare, ppx_type_directed_value and refl. The benchmark involves an equality check on a 60-field record where only the last fields differ.
The table shows the product version (ppx_easy_deriving) being on par with ppx_deriving and ppx_compare. The simple version (ppx_easy_deriving2) is notably slower but still faster than ppx_type_directed_value (ppx_type_directed_equal) and refl.
| Rate | refl | ppx_type_directed_equal | ppx_easy_deriving2 | ppx_deriving | ppx_compare | ppx_easy_deriving | |
|---|---|---|---|---|---|---|---|
| refl | 605433/s | -- | -73% | -91% | -97% | -97% | -97% |
| ppx_type_directed_equal | 2245269/s | 271% | -- | -65% | -90% | -90% | -90% |
| ppx_easy_deriving2 | 6412029/s | 959% | 186% | -- | -71% | -72% | -72% |
| ppx_deriving | 22347280/s | 3591% | 895% | 249% | -- | -1% | -1% |
| ppx_compare | 22595512/s | 3632% | 906% | 252% | 1% | -- | -0% |
| ppx_easy_deriving | 22639836/s | 3639% | 908% | 253% | 1% | 0% | -- |
See the full benchmark in example/ppx_easy_equal_bench/.
example/ppx_lattice/ contains examples of additional product deriver helper functors by defining a deriver for product lattices with the following signature:
module type Lattice =
sig
type t
(* reduce2-like function *)
val leq: t -> t -> bool (* reduce by conjunction *)
(* map2-like function *)
val join: t -> t -> t
(* create-like function *)
val bot: unit -> t
(* reduce-like function *)
val is_bot: t -> bool (* reduce by conjunction *)
(* map-like function *)
val relift: t -> t (* not really a lattice operation *)
end- ppx_type_directed_value — has runtime overhead on every call of derived function.
- ppx_derive_at_runtime — also has runtime overhead (TODO: include in benchmarks).
- refl — requires runtime representations of types.