package ppx_let
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md5=40d1798d7724816c65eb5cdabd20f150
Description
Part of the Jane Street's PPX rewriters collection.
Published: 22 Mar 2018
README
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 lefthand sides of the let bindings are not available in subsequent righthand 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
.
Dependencies (5)

ppxlib
>= "0.1.0" & < "0.9.0"

ocamlmigrateparsetree
>= "1.0" & < "2.0.0"

jbuilder
>= "1.0+beta18.1"

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

ocaml
>= "4.04.1"
Dev Dependencies
Used by (14)

angstrom
>= "0.15.0"
 dataframe
 finch
 lwt_ppx_let
 mmdb
 noise

obeam
>= "0.1.0"

ocamlprotocplugin
< "1.0.0"
 opamchecknpmdeps
 opine
 orewa

ppx_jane
= "v0.11.0"
 useragentparser
 zeit
Conflicts (1)

jbuilder
= "1.0+beta19"