print and println go variadic, with Clojure's spacing

Arguments print in order with a single space between each pair, println
ending the line; (println) is the newline alone and (print) is nothing.
The checker's arm renders each argument exactly as it did alone, so typed
and dyn values mix in one call, one-argument sites are byte-identical, and
an unprintable argument is still refused at its own span.

FIX.org

@@ -423,3 +423,26 @@ rename. typed-flan branch freezes the static language pre-dyn.
 
 All of it dispatches after the x86-dyn lane lands. The struct dyn-field
 refusal (01e60fa) is the stopgap 2 lifts.
+
+* println is variadic, 2026-09-20
+Author, dogfooding: "println should be variadic" — hit "println takes 1
+argument, given 2".
+
+Semantics chosen: Clojure's. Every argument prints in order, a single space
+between each pair, println ends the line. (println) is the newline alone,
+(print) is nothing. Single-argument call sites are byte-identical to before —
+the space is a separator, never a trailer.
+
+Mechanics: no prelude macro. print/println were never functions — they are
+the checker's structural walk (lib/check.ml, the "print" | "println" arm;
+the walk in lib/render.ml) — so the arm itself went variadic: each argument
+is checked and rendered exactly as it was alone, with a one-byte " " write
+interleaved. Typed and dyn arguments mix in one call because each gets its
+own printer and both sinks share stdio's buffer (flan_write_stdout and
+flan_dyn_print both go through stdout). One generic argument still defers
+the whole call to instantiation. Diagnostics stay on the argument: each is
+checked carrying its own loc and render.ml fails on the expression's loc,
+so an unprintable second argument underlines that argument, not the form —
+pinned in test_flan.ml. Output pinned by test/programs/println-variadic.flan
+and its acceptance row (LLVM), spacing exact, "|" markers so a leaked
+trailing space is a visible red.

lib/check.ml

@@ -5175,12 +5175,18 @@ and named_call ctx ~want loc name args =
      cannot be told from the punctuation. The split is exactly top level vs
      nested, which is why it lives here and not in the walk. *)
   | "print" | "println" ->
-    arity loc name 1 args;
+    (* Variadic, with Clojure's spacing: every argument prints in order with a
+       single space between each pair, and [println] ends the line. No arity
+       check — (println) is the newline alone and (print) is nothing, which is
+       also Clojure's answer. Each argument gets the same walk it would get
+       alone, so one call mixes typed and dyn values freely, and a bad argument
+       is refused at its own location: the arguments are checked one by one
+       below, each carrying its own loc, and render.ml fails on the
+       expression's loc rather than the form's. *)
     (* Printing is a read, not a move: the walk goes over the value and keeps
        nothing. Without this, (println v) would consume a Vec and every
        printing of one would be its last. *)
-    let target = List.hd args in
-    let a = check ctx target in
+    let checked = List.map (fun target -> check ctx target) args in
     (* ── The allow-list, and what it takes to get on it ───────────────
        plan.org names [println] as the one compiler-provided exception — it
        "selects a structural printer at each concrete instantiation" — and
@@ -5223,8 +5229,10 @@ and named_call ctx ~want loc name args =
 
        The node produced here is a unit no-op, thrown away with the rest of
        the abstract pass. The real printer is selected when the copy is
-       checked with [t] concrete. *)
-    if generic_ty a.Tast.ty then
+       checked with [t] concrete. One generic argument defers the whole call:
+       the printers for its neighbours would be re-selected at instantiation
+       anyway, so building them here would be work thrown away twice. *)
+    if List.exists (fun a -> generic_ty a.Tast.ty) checked then
       mk loc Types.Unit Tast.Unit
     else
     let bslice = Types.Slice (Types.Int Types.U8) in
@@ -5264,12 +5272,26 @@ and named_call ctx ~want loc name args =
         ptrs = None;
         alloc = (fun ty -> fresh_slot ctx ty) }
     in
-    let parts =
+    let render_one a =
       match a.Tast.ty with
       | Types.String | Types.Slice (Types.Int Types.U8) ->
         [ write (mk loc bslice (Tast.Prim (Tast.Bytes, [ a ]))) ]
       | _ -> Render.render rc 0 a
     in
+    (* Built fresh per use rather than shared: nothing else in this file puts
+       one node in two places of a tree, and a pass that hangs state off a
+       node would be entitled to assume it appears once. *)
+    let space () =
+      write (mk loc bslice
+               (Tast.Prim (Tast.Bytes, [ mk loc Types.String (Tast.Str " ") ])))
+    in
+    let parts =
+      match checked with
+      | [] -> []
+      | first :: rest ->
+        render_one first
+        @ List.concat_map (fun a -> space () :: render_one a) rest
+    in
     let nl =
       if String.equal name "println" then
         [ write
@@ -5913,12 +5935,14 @@ let builtins : (string * string * string) list =
     ("write-stdout", "write-stdout [[u8]] ()",
      "Writes the bytes to standard output exactly as given: no newline and \
       no formatting.");
-    ("print", "print [T] ()",
-     "The structural printer, selected for the argument's concrete type. A \
-      string prints raw at the top level and quoted inside a structure, and \
-      a Ptr or a Handle prints its address rather than being followed. \
-      Printing is a read, so it does not consume the value.");
-    ("println", "println [T] ()", "print, with a newline after it.");
+    ("print", "print [T ...] ()",
+     "The structural printer, selected for each argument's concrete type; \
+      arguments print in order with a single space between them. A string \
+      prints raw at the top level and quoted inside a structure, and a Ptr \
+      or a Handle prints its address rather than being followed. Printing \
+      is a read, so it does not consume the value.");
+    ("println", "println [T ...] ()",
+     "print, with a newline after it — (println) alone is the newline.");
     ("exit", "exit [i32] never",
      "Ends the process with this status. It has no value, so nothing written \
       after it runs.");

lib/prelude.ml

@@ -19,8 +19,10 @@
 
     No printing function is here at all any more. [print] and [println] are
     the whole printing surface, and neither is a function: both are
-    compiler-provided and structural, a walk over the concrete type at the
-    call site (check.ml, and the walk itself in render.ml). That is plan.org's
+    compiler-provided and structural, a walk over each argument's concrete
+    type at the call site (check.ml, and the walk itself in render.ml). Both
+    are variadic with Clojure's spacing -- arguments in order, one space
+    between each pair, [println] ending the line. That is plan.org's
     Milestone 5 item, and it needed none of the rest of milestone 5 -- there
     is nothing to dispatch on at run time and no user-supplied printer to
     choose between, so no type variables are involved. The earlier note here

test/programs/println-variadic.flan

@@ -0,0 +1,33 @@
+;;;; print and println are variadic with Clojure's spacing: every argument
+;;;; prints in order, a single space between each pair, println ending the
+;;;; line. (println) alone is the newline; (print) alone is nothing.
+;;;;
+;;;; What the pins are for. The one-argument lines must be byte-identical to
+;;;; what they printed before the forms went variadic — no trailing space is
+;;;; the thing an eye cannot see in a diff, so every print line ends in a "|"
+;;;; marker that would move if a space leaked. And the arguments are typed and
+;;;; dyn in the same call: each gets the walk it would get alone, the typed
+;;;; ones selected at check time and the dyn ones handed to the runtime, so
+;;;; the interleaving on one line is what proves the two sinks share a buffer.
+
+(defvar boxed dyn 21)
+(defvar dlabel dyn "mid")
+
+(defn main [] ()
+  ;; println at every arity
+  (println)                              ; bare newline
+  (println "one")                        ; the old single-argument form, unchanged
+  (println "a" "b")
+  (println "x:" 5 true)
+  ;; mixed machine types in one call
+  (println 1 2.5 "s" false)
+  ;; dyn and typed in the same call, dyn in the middle
+  (println "pre" boxed "post")
+  (println dlabel 7)
+  ;; print: zero args is nothing, and no separator ever trails
+  (print)
+  (print "|")
+  (print "p")
+  (print "|")
+  (print "q" "r" 3)
+  (println "|"))

test/test_acceptance.ml

@@ -3291,6 +3291,16 @@ level "1"
        there. 1005 is the defer, 6 is the value the restart produced. *)
     outputs "dyn: a defer on the transfer path" "programs/dyn-defer.flan"
       "1005\n6\n";
+    (* print and println are variadic with Clojure's spacing — arguments in
+       order, one space between each pair, println ending the line, (println)
+       the newline alone and (print) nothing. The program's comment says what
+       each line pins; the string here is the whole of the spacing rule, and
+       the "|" markers are what make a leaked trailing space a visible red
+       rather than an invisible byte. The dyn lines are typed and dyn
+       arguments in one call, which is the two printers sharing one buffer. *)
+    outputs "print and println are variadic"
+      "programs/println-variadic.flan"
+      "\none\na b\nx: 5 true\n1 2.5 s false\npre 21 post\nmid 7\n|p|q r 3|\n";
     outputs ~opt:"-O0" "dyn: a defer on the transfer path, -O0"
       "programs/dyn-defer.flan" "1005\n6\n";
     (* And through the other backend, where the defer and the roots meet in a

test/test_flan.ml

@@ -2785,6 +2785,32 @@ let () =
     "(defn show [x $t] () {:where (equal? $t)} (println x))";
   accepts "and so is print"
     "(defn show [x $t] () {:where (equal? $t)} (print x))";
+  (* One generic argument defers the whole call, neighbours included. *)
+  accepts "a variadic println with a type variable among the arguments"
+    "(defn show [x $t] () {:where (equal? $t)} (println \"x:\" x 1))";
+
+  (* Variadic print/println: any number of arguments, zero included. The
+     acceptance program pins what comes out; these pin only what the checker
+     admits, and — the part a program cannot show — where a refusal lands. *)
+  accepts "println with no arguments" "(defn f [] () (println))";
+  accepts "print with no arguments" "(defn f [] () (print))";
+  accepts "println with three arguments of three types"
+    "(defn f [] () (println \"x:\" 5 true))";
+  (* An unprintable argument is refused at its own span, not the form's:
+     each argument is checked carrying its own loc, and render.ml fails on
+     the expression's loc. Column 43 is the [m], not the [(println]. *)
+  (let name = "an unprintable argument is refused at the argument" in
+   match checked "(defn f [m (Map i32 i32)] () (println \"x\" m))" with
+   | _ ->
+     incr failures;
+     Printf.printf "FAIL  %s: expected a type error\n" name
+   | exception Loc.Error { Loc.dloc; dmsg; _ } ->
+     let got = Loc.to_string dloc in
+     if got <> "<test>:1:43" || not (contains dmsg "no printer for") then begin
+       incr failures;
+       Printf.printf "FAIL  %s\n      wanted: %s (no printer for)\n         got: %s (%s)\n"
+         name "<test>:1:43" got dmsg
+     end);
 
   (* A predicate a body relies on has to be carried by every signature between
      it and the call site, or the refusal moves into code the caller did not