Ltac2: add ikfprintf API to do conditional printing

Close coq#18292

doc/changelog/06-Ltac2-language/18311-ltac2-ikfprintf.rst

@@ -0,0 +1,6 @@
+- **Added:**
+  `Ltac2.Message.Format.ikfprintf` useful to implement conditional printing
+  efficiently (i.e. without building an unused message when not printing)
+  (`#18311 <https://github.com/coq/coq/pull/18311>`_,
+  fixes `#18292 <https://github.com/coq/coq/issues/18292>`_,
+  by Gaëtan Gilbert).

plugins/ltac2/tac2core.ml

@@ -299,17 +299,21 @@ let () =
   define "format_alpha" (format @-> ret format) @@ fun s ->
   Tac2print.FmtAlpha :: s
 
-let () =
-  define "format_kfprintf" (closure @-> format @-> tac valexpr) @@ fun k fmt ->
+let arity_of_format fmt =
   let open Tac2print in
   let fold accu = function
-  | FmtLiteral _ -> accu
-  | FmtString | FmtInt | FmtConstr | FmtIdent -> 1 + accu
-  | FmtAlpha -> 2 + accu
+    | FmtLiteral _ -> accu
+    | FmtString | FmtInt | FmtConstr | FmtIdent -> 1 + accu
+    | FmtAlpha -> 2 + accu
   in
+  List.fold_left fold 0 fmt
+
+let () =
+  define "format_kfprintf" (closure @-> format @-> tac valexpr) @@ fun k fmt ->
+  let open Tac2print in
   let pop1 l = match l with [] -> assert false | x :: l -> (x, l) in
   let pop2 l = match l with [] | [_] -> assert false | x :: y :: l -> (x, y, l) in
-  let arity = List.fold_left fold 0 fmt in
+  let arity = arity_of_format fmt in
   let rec eval accu args fmt = match fmt with
   | [] -> apply k [of_pp accu]
   | tag :: fmt ->
@@ -344,6 +348,13 @@ let () =
   if Int.equal arity 0 then eval []
   else return (Tac2ffi.of_closure (Tac2ffi.abstract arity eval))
 
+let () =
+  define "format_ikfprintf" (closure @-> valexpr @-> format @-> tac valexpr) @@ fun k v fmt ->
+  let arity = arity_of_format fmt in
+  let eval _args = apply k [v] in
+  if Int.equal arity 0 then eval []
+  else return (Tac2ffi.of_closure (Tac2ffi.abstract arity eval))
+
 (** Array *)
 
 let () = define "array_empty" (unit @-> ret valexpr) (fun _ -> v_blk 0 [||])

test-suite/ltac2/printf.v

@@ -29,3 +29,20 @@ Fail Ltac2 Eval printf "%I" "foo".
 Fail Ltac2 Eval printf "%t" "foo".
 Fail Ltac2 Eval printf "%a" (fun _ _ => ()).
 Fail Ltac2 Eval printf "%a" (fun _ i => Message.of_int i) "foo".
+
+Import Message.
+
+Ltac2 print_if b fmt :=
+  if b then Format.kfprintf Message.to_string fmt
+  else Format.ikfprintf Message.to_string (Message.of_string "") fmt.
+
+Ltac2 Notation "print_if" b(tactic(0)) fmt(format) := print_if b fmt.
+
+Ltac2 Eval Control.assert_true (String.equal "hello friend" (print_if true "hello %s" "friend")).
+
+Ltac2 Eval Control.assert_true (String.equal "" (print_if false "hello %s" "friend")).
+
+Fail Ltac2 Eval print_if true "%a" (fun _ => Control.throw Assertion_failure) ().
+
+(* ikfprintf doesn't run the closure *)
+Ltac2 Eval print_if false "%a" (fun _ => Control.throw Assertion_failure) ().

user-contrib/Ltac2/Message.v

@@ -55,4 +55,7 @@ Ltac2 @ external alpha : ('a, 'b, 'c, 'd) format ->
 Ltac2 @ external kfprintf : (message -> 'r) -> ('a, unit, message, 'r) format -> 'a :=
   "coq-core.plugins.ltac2" "format_kfprintf".
 
+Ltac2 @ external ikfprintf : ('v -> 'r) -> 'v -> ('a, unit, 'v, 'r) format -> 'a :=
+  "coq-core.plugins.ltac2" "format_ikfprintf".
+
 End Format.