package ppx_monad

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A Syntax Extension for all Monadic Syntaxes







Published: 24 Feb 2022


#+TITLE: =ppx_monad=

/An OCaml Syntax Extension for all Monadic Syntaxes./

** Overview

=ppx_monad= aims at providing its users with a set of standard monadic syntaxes as
well as an easy mean to define their own. Concretely, it consists in:

- a library making it easy (-ier) to write syntax extensions for monads,
- a set of pre-defined such extensions, and
- a library providing runtime dependencies for the pre-defined extensions.

One of the simplest examples is the option monad. Using =ppx_monad=, one can
#+BEGIN_SRC ocaml
let add_string_ints (s1 : string) (s2 : string) : string option =
  let%option n1 = int_of_string_opt s1 in
  let%option n2 = int_of_string_opt s2 in
  Some (string_of_int (n1 + n2))

which is then converted to:
#+BEGIN_SRC ocaml
let add_string_ints (s1 : string) (s2 : string) : string option =
  Option.bind (int_of_string_opt s1) @@ fun n1 ->
  Option.bind (int_of_string_opt s2) @@ fun n2 ->
  Some (string_of_int (n1 + n2))

equivalent to the following bind-free code:
#+BEGIN_SRC ocaml
let add_string_ints (s1 : string) (s2 : string) : string option =
  match int_of_string_opt s1 with
  | None -> None
  | Some n1 ->
    match int_of_string_opt s2 with
    | None -> None
    | Some n2 -> Some (string_of_int (n1 + n2))

=ppx_monad= can behave as PPX for the option monad as well as a library to help
someone define similar extensions.

** Installation

The easiest way to install =ppx_monad= is via OPAM:
opam install ppx_monad

Alternatively, one can close this repository, make sure to have Dune and ppxlib
installed and run:
make install

** Using =ppx_monad= as a PPX

Using =ppx_monad= as a PPX is simply a matter of adding a preprocessing step to
your target. With Dune, for instance, this is done by putting:

#+BEGIN_SRC dune
 (name example)
 (preprocess (pps ppx_monad)))

in the appropriate Dune file. With ocamlbuild, this goes by adding the following
to the =_tags= file:

<src/*>: package(ppx_monad)

*** Extensions

=ppx_monad= contains the following syntax extensions:

| Name/Label   | Other Labels            | Error | Since |
| =option=       | =opt=                     | yes   |       |
| =result.ok=    | =res.ok=, =res=, =result=, =ok= | yes   |  4.08 |
| =result.error= | =err=, =error=              | no    |  4.08 |
| =list=         | =lst=                     | no    |       |
| =seq=          |                         | no    |  4.07 |
| =either.right= | =either=, =right=           | yes   |  4.12 |
| =either.left=  | =left=                    | no    |  4.12 |

The second column indicate alternative labels that one can use to trigger the
PPX. for instance, =let%result.ok=, =let%res= and =let%ok= all trigger the same
extension. The third column indicate whether the extension covers an error part
of the monad. This error part gives access to error-related structures such as
=assert= and =try=. For instance, one can right =try%opt f () with () -> Some 7= which
will return =Some x= if =f= returns =Some x= and =Some 7= if =f= returns =None=. On the
other hand, =try%list= does not exist.

In the case of =Result=, the same module gives birth to two monads and therefore
two extensions: =result.ok= and =result.error=. The former is what you expect:
=let%result.ok= (=let%ok= or =let%res= for short) binds on value of the form =Ok x=,
propagating values of the form =Error y=. It features an error part and =try%res f
() with Foo -> Ok 7 | Bar -> Error "no bar"= returns =Ok x= if =f= returns =Ok x=, =Ok 7=
if =f= returns =Error Foo= and =Error "no bar"= if =f= returns =Error Bar=. The latter is
the counterpart which binds on values of the form =Error y=, propagating values of
the form =Ok x=. It does not feature an error part. The same considerations hold
for =Either= which the convention that values of the form =Right x= are the “good”
ones, even if OCaml's standard library specifies that one should only use =Either=
“without assigning a specific meaning to what each case should be.”

*** The =monad= Extension

=ppx_monad= contains one specific extension, =ppx_monad.ppx.monad=, which does not
rely on any particular monad but uses the “current” one. For instance:

#+BEGIN_SRC ocaml
let%monad x = ...some computation... in
...some other computation...

is rewritten into:

#+BEGIN_SRC ocaml
bind ...some computation... (fun x ->
    ...some other computation...)

and it is up to the user to make sure that the =bind= function is in scope. This
can be useful when manipulating modules that define a monad with the usual
keywords. For instance:

#+BEGIN_SRC ocaml
let open Result in
let%m x = ...some computation... in
...some other computation...

will rely on =bind= from the =Result= monad. (Note that =m= can be used as a short
label for =monad.=) This extension will expect the functions =return=, =bind=, =fail=
and =catch= to be defined in order to provide all the possible structures.

*** The =do= Extension

Additionally, =ppx_monad= contains a specific extension, that
implements a Haskell-like =do=-notation. For instance, the following code:

#+BEGIN_SRC ocaml
let add_string_ints (s1 : string) (s2 : string) : string option =
  begin%do [@monad Option]
    n1 <- int_of_string_opt s1;
    n2 <- int_of_string_opt s2;
    Some (string_of_int (n1 + n2))

is rewritten to:

#+BEGIN_SRC ocaml
let add_string_ints (s1 : string) (s2 : string) : string option =
  Option.bind int_of_string_opt s1 @@ fun n1 ->
  Option.bind int_of_string_opt s2 @@ fun n2 ->
  Some (string_of_int (n1 + n2))

This notation supports the use of a sequence when no value should be bound. It
does not, however, support patterns on the left-hand side of =<-= but one can
recover this functionality with =<--= which supports basic patterns with the
peculiarity that the wildcard =_= has to be written =__=, without which the compiler
will complain with =Syntax error: wildcard "_" not expected=. If one needed to use
an actual operator =<-= or =<--= or a usual OCaml sequence, they should wrap those
inside a =let () = ... in=.

Finally, this notation supports two optional attributes, =@bind= and =@monad=. =@bind=
allows to provide a custom =bind= function; =@monad= allows to specify a module in
which to find the =bind= function. Without attributes, the PPX simply uses =bind=.

Note that this extension has little to with the others because it only requires
a bind function. It is however compatible with them and it is possible to use
eg. =match%res= inside a do notation.

** Defining Custom Monadic Syntaxes

Defining a new monadic syntax consists in writing a rewriting library calling
the following endpoint at top-level:

#+BEGIN_SRC ocaml
val register :
  (* Easiest *)
  ?monad:string ->
  ?monad_error:string ->
  (* Finer grained *)
  ?mk_return:(loc:Location.t -> expression -> expression) ->
  ?mk_bind:  (loc:Location.t -> expression -> expression -> expression) ->
  ?mk_fail:  (loc:Location.t -> expression -> expression) ->
  ?mk_catch: (loc:Location.t -> expression -> expression -> expression) ->
  (* Bookkeeping *)
  ?applies_on:string ->
  string ->

The easiest way to create an extension is by only providing the =~monad= argument
as well as a name. For instance, one can redefine =ppx_monad.ppx.option=, an
extension for the option monad, with:

#+BEGIN_SRC ocaml
let () = Ppx_monad.register "option" ~monad:"Stdlib.Option"

The =~monad= argument tells =ppx_monad= to go look for the functions =return=, =bind=,
=fail= and =catch= in the given module name. (Note that the actual definition of
=ppx_monad.ppx.option= also makes use of the =~applies_on= argument explained
below.) Sometimes, on might want to define an error side monad, where =let%ext=
would be extended to =catch= instead of =bind=. This is the purpose of the
=~monad_error= argument. For instance, one can redefine =ppx_monad.ppx.result=, an
extension for the error monad, both its normal and error sides, with:

#+BEGIN_SRC ocaml
let () = Ppx_monad.register "result" ~monad:"Stdlib.Result"
let () = Ppx_monad.register "error" ~monad_error:"Stdlib.Result"

The last two arguments of =register= are the name of the PPX and what it applies
on. By default, this is also the labels on which the PPX applies: a PPX of name
=result= will apply on =let%result=, =match%result=, etc. Additionally, one can
provide =applies_on=, another string describing on which labels the PPX applies.
This string can contain simple regular expressions using grouping with =(...)=,
optional parts with =?= and choices with =|=. For instance, =ok|res(ult)?(.ok)?=
matches exactly all of the following: =ok=, =res=, =result=, =res.ok=, =result.ok=.

*** Finer-grained Syntax With the =mk_*= Functions

The functions =mk_return=, =mk_bind=, =mk_fail= and =mk_catch= take precedence over
 =~monad= and =~monad_error=. They are here to build the corresponding monadic
 expressions. =mk_bind=, for instance, takes two OCaml expressions representing
 the two usual arguments to a =bind= function and builds an expression
 representing the application of such a =bind= function to the arguments. For
 instance, one could define it as:

#+BEGIN_SRC ocaml
let mk_bind ~loc e f = [%expr bind [%e e] [%e f]]

The syntax extensions =[%expr ...]= and =[%e ...]= are helpers provided by ppxlib to
manipulate OCaml expression. =[%expr ...]= allows to write OCaml code and to get
an OCaml syntax tree out of it. This OCaml code can contain holes which =[%e ...]=
allows to fill with a syntax tree. /For more information about this, check out
[[][the documentation of ppxlib's Metaquot]]./

The functions =mk_return=, =mk_bind=, =mk_fail= and =mk_catch= are all optional but
providing more of them allows =ppx_monad= to provide more syntactic structures.
For instance, =mk_bind= will suffice for =ppx_monad= to provide a simple =let ... in=,
but =mk_return= is necessary for =let ... and ... in=. As another example, most
structures can be defined using only =mk_return= and =mk_bind=, but =mk_fail= and
=mk_catch= are necessary to define =assert=, =try= or =match= with exception patterns.

*** Redefining =ppx_monad_result=

Let us now re-implement the PPX for =Result=, available through
=ppx_monad.ppx.result= or =ppx_monad=. We can do this with the following file, of
only 21 lines:

#+BEGIN_SRC ocaml
open Ppxlib

let mk_return ~loc x =
  [%expr Result.ok [%e x]]

let mk_bind ~loc e f =
  [%expr Result.bind [%e e] [%e f]]

let mk_fail ~loc y =
  [%expr Result.error [%e y]]

let mk_catch ~loc e f =
  [%expr (fun e f -> match e with
           | Ok x -> Ok x
           | Error y -> f y) [%e e] [%e f]]

let () =
  Ppx_monad.register "result"
    ~mk_return ~mk_bind
    ~mk_fail ~mk_catch

It is then only a matter of building this file as a library that depends on
=ppx_monad= and gets pre-processed by ppxlib's Metaquot. For instance, with Dune,
assuming that our library is called =ppx_result=:

#+BEGIN_SRC dune
 (name ppx_result)
 (public_name ppx_result)
 (libraries ppx_monad)
 (preprocess (pps ppxlib.metaquot))
 (kind ppx_rewriter))

You can have a look at how =ppx_monad.ppx.result= [[./src/ppx/result/][is actually defined in this

*** Sanitising Variable Names

For most of the functions in the example above (=mk_return=, =mk_bind= and =mk_fail=),
there exists an implementation in the =Result= module which we can use directly.
This is however not the case for =mk_catch= and we had to implement it by hand.
The way we wrote it might feel weird and we might be tempted to write it as
either of the following:

#+BEGIN_SRC ocaml
let incorrect_mk_catch ~loc e f =
  [%expr let catch e f = match e with
           | Ok x -> Ok x
           | Error y -> f y
         in catch [%e e] [%e f]

let incorrect_mk_catch ~loc e f =
  [%expr match [%e e] with
         | Ok x -> Ok x
         | Error y -> [%e f] y]

These implementations, however, are incorrect, because they bind variables in
the scope of =e= and =f=. For instance, in the first implementation, if =e= or =f= were
to contain the free variable =catch=, it would . The same issue is present in the
second implementation if =f= were to contain the free variable =y=. If one wants to
go down this road, the proper way is to ensure that the variable names are
unique. Luckily, =ppx_monad= includes a mechanism for this. A proper way to write
the above functions would be the following:

#+BEGIN_SRC ocaml
let mk_catch ~loc e f =
  let (pcatch, catch) = Ppx_monad.fresh_variable () in
  [%expr let [%p pcatch] e f = match e with
           | Ok x -> Ok x
           | Error y -> f y
         in [%e catch] [%e e] [%e f]]

let mk_catch ~loc e f =
  let (py, y) = Ppx_monad.fresh_variable () in
  [%expr match [%e e] with
         | Ok x -> Ok x
         | Error [%p py] -> [%e f] [%e y]]

=[%p ...]= is similar to =[%e ...]= for wholes in pattern positions.
=Ppx_monad.fresh_variable= returns a pair of a pattern and an expression, the
former binding a unique variable name which the latter mentions.

** Related Works

This section attempts to list all works that provide similar features as
=ppx_monad=. We consider not mentioning such a project here a bug and welcome any
[[][issue]] or [[][pull request]] aiming at fixing this.

- [[][OCaml's binding operators]]:
  - pure OCaml

- [[][zepalmer]]/[[][ocaml-monadic]], a “lightweight PPX extension for OCaml to support
  natural monadic syntax.”
  - last updated in 2021.

- [[][marigold-dev]]/[[][ppx_let_binding]], an “OCaml syntax extension for monads in the
  style of ReasonML.”
  - very new (to be followed),
  - not documented yet, and
  - not published on OPAM yet.

- [[][kandu]]/[[][ppx_ok_monad]], “a ppx syntax extension for monad syntax sugar.”
  - last updated 2 years ago.

- [[][foretspaisibles]]/[[][ppx_monad]], “a monad syntax extension for OCaml, that provides
  two major monad syntaxes: clean but incomplete Haskell-style monad syntax and
  verbose but complete let monad syntax.”
  - last updated in 2017.

- [[][rizo]]/[[][ppx_monad]], a “minimalistic monad syntax for OCaml.”
  - last updated in 2017.

- [[][danmey]]/[[][omonad]], a “monad syntax using ppx extensions.”
  - last updated in 2013,
  - no documentation, and
  - no OPAM package.

- [[][pippijn]]/[[][pa_monad_custom]]:
  - based on Camlp4,
  - last updated in 2013, and
  - no OPAM package.

Dependencies (4)

  1. ppxlib >= "0.9.0"
  2. dune-configurator
  3. ocaml >= "4.08.0"
  4. dune >= "2.7"

Dev Dependencies (1)

  1. odoc with-doc

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