A ppx rewriter for monadic and applicative let bindings, match expressions, and
if expressions.

Overview

The aim of this rewriter is to make monadic and applicative code look nicer by
writing custom binders the same way that we normally bind variables. In OCaml,
the common way to bind the result of a computation to a variable is:

let VAR = EXPR in BODY

ppx_let simply adds two new binders: let%bind and let%map. These are
rewritten into calls to the bind and map functions respectively. These
functions are expected to have

val map  : 'a t -> f:('a -> 'b)   -> 'b t
val bind : 'a t -> f:('a -> 'b t) -> 'b t

for some type t, as one might expect.

These functions are to be provided by the user, and are generally expected to be
part of the signatures of monads and applicatives modules. This is the case for
all monads and applicatives defined by the Jane Street's Core suite of
libraries. (see the section below on getting the right names into scope).

Parallel bindings

ppx_let understands parallel bindings as well. i.e.:

let%bind VAR1 = EXPR1 and VAR2 = EXPR2 and VAR3 = EXPR3 in BODY

The and keyword is seen as a binding combination operator. To do so it expects
the presence of a both function, that lifts the OCaml pair operation to the
type t in question:

val both : 'a t -> 'b t -> ('a * 'b) t

Some applicatives have optimized map functions for more than two arguments.
These applicatives will export functions like map4 shown below:

val map4: 'a t -> 'b t -> 'c t -> 'd t -> f:('a -> 'b -> 'c -> 'd -> 'r) -> 'r t

In order to use these optmized functions, ppx_let provides the let%mapn
syntax, which picks the right map{n} function to call based on the amount of
applicatives bound by the syntax.

Match statements

We found that this form was quite useful for match statements as well. So for
convenience ppx_let also accepts %bind and %map on the match keyword.
Morally match%bind expr with cases is seen as let%bind x = expr in match x with cases.

If statements

As a further convenience, ppx_let accepts %bind and %map on the if
keyword. The expression if%bind expr1 then expr2 else expr3 is morally
equivalent to let%bind p = expr1 in if p then expr2 else expr3.

While statements

We also expand while%bind expr1 do expr2 done as

let rec loop () =
  if%bind expr1
  then (
    let%bind () = expr2 in
    loop ())
  else return ()
in loop ()

Note that this form will (potentially) evaluate the textual form of
expr1 multiple times!

We do not support while%map, as that cannot be implemented without
bind.

Syntactic forms and actual rewriting

ppx_let adds seven syntactic forms

let%bind P = M in E

let%map  P = M in E

let%sub P = M in E

match%bind M with P1 -> E1 | P2 -> E2 | ...

match%map  M with P1 -> E1 | P2 -> E2 | ...

if%bind M then E1 else E2

if%map  M then E1 else E2

while%bind M do E done

that expand into

bind M ~f:(fun P -> E)

map  M ~f:(fun P -> E)

sub M ~f:(fun P -> E)

bind M ~f:(function P1 -> E1 | P2 -> E2 | ...)

map  M ~f:(function P1 -> E1 | P2 -> E2 | ...)

bind M ~f:(function true -> E1 | false -> E2)

map  M ~f:(function true -> E1 | false -> E2)

let rec loop () = bind M ~f:(function true -> bind E ~f:loop | false -> return ()) in loop ()

respectively.

As with let, let%bind and let%map also support multiple parallel
bindings via the and keyword:

let%bind P1 = M1 and P2 = M2 and P3 = M3 and P4 = M4 in E

let%map  P1 = M1 and P2 = M2 and P3 = M3 and P4 = M4 in E

that expand into

let x1 = M1 and x2 = M2 and x3 = M3 and x4 = M4 in
bind
  (both x1 (both x2 (both x3 x4)))
  ~f:(fun (P1, (P2, (P3, P4))) -> E)

let x1 = M1 and x2 = M2 and x3 = M3 and x4 = M4 in
map
  (both x1 (both x2 (both x3 x4)))
  ~f:(fun (P1, (P2, (P3, P4))) -> E)

respectively. (Instead of x1, x2, ... ppx_let uses variable names that are
unlikely to clash with other names)

Unlike let%map and let%bind, let%sub does not permit
multiple bindings via the and keyword.

As with let, names introduced by left-hand sides of the let bindings are not
available in subsequent right-hand sides of the same sequence.

Getting the right names in scope

The description of how the %bind and %map syntax extensions expand left out
the fact that the names bind, map, both, and return are not used
directly., but rather qualified by Let_syntax. For example, we use
Let_syntax.bind rather than merely bind.

This means one just needs to get a properly loaded Let_syntax module
in scope to use %bind and %map.

Alternatively, the extension can use values from a Let_syntax module
other than the one in scope. If you write %map.A.B.C instead of
%map, the expansion will use A.B.C.Let_syntax.map instead of
Let_syntax.map (and similarly for all extension points).

For monads, Core.Monad.Make produces a submodule Let_syntax of the
appropriate form.

For applicatives, the convention for these modules is to have a submodule
Let_syntax of the form:

module Let_syntax : sig
  val return : 'a -> 'a t
  val map    : 'a t -> f:('a -> 'b) -> 'b t
  val both   : 'a t -> 'b t -> ('a * 'b) t
  module Open_on_rhs : << some signature >>
end

The Open_on_rhs submodule is used by variants of %map and %bind called
%map_open and %bind_open. It is locally opened on the right hand sides of
the rewritten let bindings in %map_open and %bind_open expressions. For
match%map_open and match%bind_open expressions, Open_on_rhs is opened for
the expression being matched on.

Open_on_rhs is useful when programming with applicatives, which operate in a
staged manner where the operators used to construct the applicatives are
distinct from the operators used to manipulate the values those applicatives
produce. For monads, Open_on_rhs contains return.

let%sub

let%sub is a form equivalent to let%bind but calling a function called
[sub] instead of [bind]. The intended use case is for things which have a
"bind-like" operation with a type like:

val sub : 'a t -> f:('a s -> 'b t) -> 'b t

(e.g. a relative monad) The name comes from the quintessential example
of such an operation: substitution of terms for variables. We didn't
want to just use [let%bind] for such functions as it might confuse
people.