#### ppx_let

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
```

#### 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`

.

### Syntactic forms and actual rewriting

`ppx_let`

adds six syntactic forms

```
let%bind P = M in E
let%map 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
```

that expand into

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

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)

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`

.

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`

.

>= "0.1.0" & < "0.9.0"

>= "1.0" & < "2.0.0"

>= "1.0+beta18.1"

>= "v0.11" & < "v0.12"

>= "4.04.1"