Chapter 12 Language extensions

23 Binding operators

• 23.1 Examples
• 23.2 Conventions
• 23.3 General desugaring rules
• 23.4 Short notation for variable bindings (let-punning)

(Introduced in 4.08.0)

Binding operators offer syntactic sugar to expose library functions under (a variant of) the familiar syntax of standard keywords. Currently supported “binding operators” are let<op> and and<op>, where <op> is an operator symbol, for example and+$.

Binding operators were introduced to offer convenient syntax for working with monads and applicative functors; for those, we propose conventions using operators * and + respectively. They may be used for other purposes, but one should keep in mind that each new unfamiliar notation introduced makes programs harder to understand for non-experts. We expect that new conventions will be developed over time on other families of operator.

23.1 Examples

Users can define let operators:

let ( let* ) o f =
  match o with
  | None -> None
  | Some x -> f x

let return x = Some x

val ( let* ) : 'a option -> ('a -> 'b option) -> 'b option = 
val return : 'a -> 'a option = 

and then apply them using this convenient syntax:

let find_and_sum tbl k1 k2 =
  let* x1 = Hashtbl.find_opt tbl k1 in
  let* x2 = Hashtbl.find_opt tbl k2 in
    return (x1 + x2)

val find_and_sum : ('a, int) Hashtbl.t -> 'a -> 'a -> int option = 

which is equivalent to this expanded form:

let find_and_sum tbl k1 k2 =
  ( let* ) (Hashtbl.find_opt tbl k1)
    (fun x1 ->
       ( let* ) (Hashtbl.find_opt tbl k2)
         (fun x2 -> return (x1 + x2)))

val find_and_sum : ('a, int) Hashtbl.t -> 'a -> 'a -> int option = 

Users can also define and operators:

module ZipSeq = struct

  type 'a t = 'a Seq.t

  open Seq

  let rec return x =
    fun () -> Cons(x, return x)

  let rec prod a b =
    fun () ->
      match a (), b () with
      | Nil, _ | _, Nil -> Nil
      | Cons(x, a), Cons(y, b) -> Cons((x, y), prod a b)

  let ( let+ ) f s = map s f
  let ( and+ ) a b = prod a b

end

module ZipSeq :
  sig
    type 'a t = 'a Seq.t
    val return : 'a -> 'a Seq.t
    val prod : 'a Seq.t -> 'b Seq.t -> ('a * 'b) Seq.t
    val ( let+ ) : 'a Seq.t -> ('a -> 'b) -> 'b Seq.t
    val ( and+ ) : 'a Seq.t -> 'b Seq.t -> ('a * 'b) Seq.t
  end

to support the syntax:

open ZipSeq
let sum3 z1 z2 z3 =
  let+ x1 = z1
  and+ x2 = z2
  and+ x3 = z3 in
    x1 + x2 + x3

val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = 

which is equivalent to this expanded form:

open ZipSeq
let sum3 z1 z2 z3 =
  ( let+ ) (( and+ ) (( and+ ) z1 z2) z3)
    (fun ((x1, x2), x3) -> x1 + x2 + x3)

val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = 

23.2 Conventions

An applicative functor should provide a module implementing the following interface:

module type Applicative_syntax = sig
  type 'a t
  val ( let+ ) : 'a t -> ('a -> 'b) -> 'b t
  val ( and+ ): 'a t -> 'b t -> ('a * 'b) t
end

where (let+) is bound to the map operation and (and+) is bound to the monoidal product operation.

A monad should provide a module implementing the following interface:

module type Monad_syntax = sig
  include Applicative_syntax
  val ( let* ) : 'a t -> ('a -> 'b t) -> 'b t
  val ( and* ): 'a t -> 'b t -> ('a * 'b) t
end

where (let*) is bound to the bind operation, and (and*) is also bound to the monoidal product operation.

23.3 General desugaring rules

The form

let<op0>
  x1 = e1
and<op1>
  x2 = e2
and<op2>
  x3 = e3
in e

desugars into

( let<op0> )
  (( and<op2> )
    (( and<op1> )
      e1
      e2)
    e3)
  (fun ((x1, x2), x3) -> e)

This of course works for any number of nested and-operators. One can express the general rule by repeating the following simplification steps:

• The first and-operator in
  let<op0> x1 = e1 and<op1> x2 = e2 and... in e
  can be desugared into a function application
  let<op0> (x1, x2) = ( and<op1> ) e1 e2 and... in e.
• Once all and-operators have been simplified away, the let-operator in
  let<op> x1 = e1 in e
  can be desugared into an application
  ( let<op> ) e1 (fun x1 -> e).

Note that the grammar allows mixing different operator symbols in the same binding (<op0>, <op1>, <op2> may be distinct), but we strongly recommend APIs where let-operators and and-operators working together use the same symbol.

23.4 Short notation for variable bindings (let-punning)

(Introduced in 4.13.0)

When the expression being bound is a variable, it can be convenient to use the shorthand notation let+ x in ..., which expands to let+ x = x in .... This notation, also known as let-punning, allows the sum3 function above can be written more concisely as:

open ZipSeq
let sum3 z1 z2 z3 =
  let+ z1 and+ z2 and+ z3 in
  z1 + z2 + z3

val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = 

This notation is also supported for extension nodes, expanding let%foo x in ... to let%foo x = x in .... However, to avoid confusion, this notation is not supported for plain let bindings.