Asking for the other target, and being told where it cannot go

--target= carries a value, so the flag test becomes a prefix match and the
residual-argument filter uses the same test — otherwise -o out --target=X fell
into the usage error. A wasm build defaults to a .wasm name, since the
extension is what tells a runtime, and a reader, what the file is.

flan run refuses the flag by name. It builds and execs, a cross-built module is
not something this host execs, and choosing a runtime for it is not a decision
this command should be making quietly.

bin/main.ml

@@ -46,6 +46,24 @@ let dev_flag = "--dev"
 
 let flags = [ no_checks_flag; dev_flag ]
 
+(* [--target=wasm32-wasi], the one cross target. Unlike the flags above it
+   carries a value, so it is matched by prefix and stripped from the residual
+   arguments by the same test — otherwise [-o out --target=X] falls into the
+   usage error. *)
+let target_prefix = "--target="
+
+let is_flag a =
+  List.mem a flags || String.starts_with ~prefix:target_prefix a
+
+let target_of args =
+  List.find_map
+    (fun a ->
+       if String.starts_with ~prefix:target_prefix a then
+         Some (String.sub a (String.length target_prefix)
+                 (String.length a - String.length target_prefix))
+       else None)
+    args
+
 let () =
   match Array.to_list Sys.argv with
   | _ :: "read" :: files when files <> [] ->
@@ -82,10 +100,10 @@ let () =
                   (Flan.Types.to_string f.ret) (Array.length f.slots))
              p.fns))
       files
-  | _ :: "emit" :: args when List.exists (fun a -> not (List.mem a flags)) args ->
+  | _ :: "emit" :: args when List.exists (fun a -> not (is_flag a)) args ->
     let checks = not (List.mem no_checks_flag args) in
     let dev = List.mem dev_flag args in
-    let files = List.filter (fun a -> not (List.mem a flags)) args in
+    let files = List.filter (fun a -> not (is_flag a)) args in
     List.iter
       (fun path ->
          with_errors path (fun () ->
@@ -94,20 +112,28 @@ let () =
   | _ :: "build" :: path :: rest ->
     let checks = not (List.mem no_checks_flag rest) in
     let dev = List.mem dev_flag rest in
+    let target = target_of rest in
     let out =
-      match List.filter (fun a -> not (List.mem a flags)) rest with
+      match List.filter (fun a -> not (is_flag a)) rest with
       | [ "-o"; o ] -> o
-      | [] -> Filename.remove_extension (Filename.basename path)
+      | [] ->
+        let base = Filename.remove_extension (Filename.basename path) in
+        (* A wasm module is not an executable and must not be named like one:
+           the extension is what tells a runtime, and a reader, what it is. *)
+        (match target with
+         | Some t when String.starts_with ~prefix:"wasm32" t -> base ^ ".wasm"
+         | _ -> base)
       | _ ->
         prerr_endline
-          "usage: flan build <file.flan> [-o out] [--no-bounds-checks] [--dev]";
+          "usage: flan build <file.flan> [-o out] [--no-bounds-checks] \
+           [--dev] [--target=wasm32-wasi]";
         exit 2
     in
     with_errors path (fun () ->
       let l = load path in
       let p = Flan.Check.program l.decls in
       ignore (Flan.Build.executable
-                ~opts:{ Flan.Build.default with checks; dev }
+                ~opts:{ Flan.Build.default with checks; dev; target }
                 ~csrcs:l.csrcs ~lflags:l.lflags p ~out))
   (* The daemon an editor talks to: one session, the program it belongs to
      running beside it, and a socket. Unlike [flan reload] the session persists,
@@ -146,6 +172,14 @@ let () =
       Printf.eprintf "%s  %s  llc %.1fms  ld %.1fms\n" out
         (String.concat " " c.Flan.Session.fns) timing.Flan.Build.llc_ms
         timing.Flan.Build.link_ms)
+  (* [run] builds and execs. A .wasm is not executable, and picking a runtime
+     for it is a decision this command has no business making, so a cross
+     target is refused here by name rather than half-supported. *)
+  | _ :: "run" :: _ :: args when target_of args <> None ->
+    prerr_endline
+      "flan run: --target is refused — a cross-built module is not something \
+       this host can exec. Use flan build --target=... and a wasm runtime.";
+    exit 2
   | _ :: "run" :: path :: args ->
     with_errors path (fun () ->
       let exe =
@@ -163,7 +197,8 @@ let () =
   | _ ->
     prerr_endline
       "usage: flan (read|parse|check|emit) <file.flan>...\n\
-      \       flan build <file.flan> [-o out] [--no-bounds-checks] [--dev]\n\
+      \       flan build <file.flan> [-o out] [--no-bounds-checks] [--dev] \
+       [--target=wasm32-wasi]\n\
       \       flan run <file.flan> [args...]\n\
       \       flan reload <program.flan> <forms.flan> [-o out.so]\n\
       \       flan dev <program.flan> [-s socket]";