package ppx_let
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sha256=2f4afb3100f4f0ae87be781b0ca6710f0a360ee8edd7eeaeb7574eca4d8be65c
Description
Part of the Jane Street's PPX rewriters collection.
Published: 02 Jun 2023
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
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
.
Function statements
We accept function%bind
and function%map
too.
let f = function%bind
 Some a > g a
 None > h
is equivalent to
let f = fun temp >
match%bind temp with
 Some a > g a
 None > h
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
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)
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)
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
. The intended way to do this is to create a module Let_syntax
with a signature like:
module Let_syntax : sig
module Let_syntax : sig
val bind : ...
val map : ...
...
end
...
end
and then use open Let_syntax
to make the inner Let_syntax
module available.
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.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
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
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
.
Local values
ppx_let
can operate on local values. This requires a compiler that supports the local_
keyword and stack allocation, which as of 202303 is a nonstandard compiler extension.
Use
%mapl
and%bindl
instead of%map
and%bind
.Implement a
Let_syntax
module that matches the following signature:module Let_syntax : sig module Let_syntax : sig val return : local_ 'a > local_ 'a t val map : local_ 'a t > f:local_ (local_ 'a > local_ 'b) > local_ 'b t val both : local_ 'a t > local_ 'b t > local_ ('a * 'b) t module Open_on_rhs : << some signature >> end end
Dependencies (5)
Dev Dependencies
Used by (23)

angstrom
>= "0.15.0"

base_quickcheck
>= "v0.16.0"

bonsai
>= "v0.16.0"

core_bench
>= "v0.16.0"
 finch

gopcamlmodemerlin
< "0.0.6"
 gremlin
 lwt_ppx_let
 mmdb
 noise

obeam
>= "0.1.0"
 opamchecknpmdeps
 openstellina
 opine
 orewa

ppx_css
>= "v0.16.0"

ppx_jane
>= "v0.16.0"

ppx_pattern_bind
>= "v0.16.0"
 pyml_bindgen

pythonlib
>= "v0.16.0"

sexp_grammar
>= "v0.16.0"
 useragentparser
 zeit