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Syntax proposal: let punning #10013

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merged 3 commits into from Dec 8, 2020
Merged

Syntax proposal: let punning #10013

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d65593b
Let-punning: allow "let* x" syntax without an explicit binding.
stedolan Nov 10, 2020
a5ace5c
Support let-punning in let%foo extensions
stedolan Dec 1, 2020
382850e
changes
stedolan Dec 1, 2020
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3 changes: 3 additions & 0 deletions Changes
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Expand Up @@ -3,6 +3,9 @@ Working version

### Language features:

- #??: Let-punning
(Stephen Dolan, review by ??)
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I guess we need to refine this to "binding-operator punning". Could you include an actual example (and its desugaring)?

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Also, are you sure that it is let-punning not let-prunning?

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The name we use really is "punning". I think of it as a "pun" in that we use something for something else (the name of a declaration as its definition). The naming "record field punning" is well-established and more or less (well, remotely) consistent with type punning for example.

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haha, I was always under impression that I was using type pruning)) Good day I have learned something new)


### Runtime system:

### Code generation and optimizations:
Expand Down
13 changes: 9 additions & 4 deletions boot/menhir/menhirLib.ml
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Expand Up @@ -43,7 +43,7 @@ let rec uniq1 cmp x ys =
[]
| y :: ys ->
if cmp x y = 0 then
uniq1 compare x ys
uniq1 cmp x ys
else
y :: uniq1 cmp y ys

Expand Down Expand Up @@ -85,7 +85,6 @@ let rec foldr f xs accu =
accu
| Cons (x, xs) ->
f x (foldr f xs accu)

end
module Convert = struct
(******************************************************************************)
Expand Down Expand Up @@ -3133,8 +3132,14 @@ module Make
type item =
int * int

let low_bits =
10

let low_limit =
1 lsl low_bits

let export t : item =
(t lsr 7, t mod 128)
(t lsr low_bits, t mod low_limit)

let items s =
(* Map [s] to its LR(0) core. *)
Expand Down Expand Up @@ -3513,5 +3518,5 @@ module MakeEngineTable (T : TableFormat.TABLES) = struct
end
end
module StaticVersion = struct
let require_20190924 = ()
let require_20200624 = ()
end
2 changes: 1 addition & 1 deletion boot/menhir/menhirLib.mli
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Expand Up @@ -1701,5 +1701,5 @@ module MakeEngineTable
and type nonterminal = int
end
module StaticVersion : sig
val require_20190924 : unit
val require_20200624: unit
end
5,860 changes: 2,858 additions & 3,002 deletions boot/menhir/parser.ml
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5 changes: 4 additions & 1 deletion parsing/parser.mly
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Expand Up @@ -2485,6 +2485,9 @@ and_let_binding:
letop_binding_body:
pat = let_ident exp = strict_binding
{ (pat, exp) }
| val_ident
(* Let-punning *)
{ (mkpatvar ~loc:$loc $1, mkexpvar ~loc:$loc $1) }
| pat = simple_pattern COLON typ = core_type EQUAL exp = seq_expr
{ let loc = ($startpos(pat), $endpos(typ)) in
(ghpat ~loc (Ppat_constraint(pat, typ)), exp) }
Expand All @@ -2495,7 +2498,7 @@ letop_bindings:
body = letop_binding_body
{ let let_pat, let_exp = body in
let_pat, let_exp, [] }
| bindings = letop_bindings pbop_op = mkrhs(ANDOP) body = let_binding_body
| bindings = letop_bindings pbop_op = mkrhs(ANDOP) body = letop_binding_body
{ let let_pat, let_exp, rev_ands = bindings in
let pbop_pat, pbop_exp = body in
let pbop_loc = make_loc $sloc in
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10 changes: 8 additions & 2 deletions parsing/pprintast.ml
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Expand Up @@ -1314,8 +1314,14 @@ and bindings ctxt f (rf,l) =
(list ~sep:"@," (binding "and" Nonrecursive)) xs

and binding_op ctxt f x =
pp f "@[<2>%s %a@;=@;%a@]"
x.pbop_op.txt (pattern ctxt) x.pbop_pat (expression ctxt) x.pbop_exp
match x.pbop_pat, x.pbop_exp with
| {ppat_desc = Ppat_var { txt=pvar; _ }; ppat_attributes = []; _},
{pexp_desc = Pexp_ident { txt=Lident evar; _}; pexp_attributes = []; _}
when pvar = evar ->
pp f "@[<2>%s %s@]" x.pbop_op.txt evar
| pat, exp ->
pp f "@[<2>%s %a@;=@;%a@]"
x.pbop_op.txt (pattern ctxt) pat (expression ctxt) exp
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Remark: if I understand correctly, we have pretty-printing support for the binding operator form, but not for the extension form. This is just fine as far as I am concerned.


and structure_item ctxt f x =
match x.pstr_desc with
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9 changes: 9 additions & 0 deletions testsuite/tests/parsetree/source.ml
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Expand Up @@ -7414,3 +7414,12 @@ let test = function

let test = function
| (`A | `B) as x | `C -> ()

(* Let-punning *)
module M = struct
let (let*) x f = f x
let (and*) a b = (a, b)
let x = 1 and y = 2 and z = 3
let p =
let* x and* y and* z in (x,y,z)
end