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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
.
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 withoutbind
.
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 useLet_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 ofLet_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 submoduleLet_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. Formatch%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.