Printing a document twice with a different renderer function

[zbyrn]

The Issue

I found this issue while porting this great library to Standard ML. The following snippet reproduces it:

let print_doc_choice (w : int) =
  let cf = Printer.default_cost_factory ~page_width:w () in
  let module P = Printer.Make (val cf) in
  let open P in

  let d = text "while (true) {" ^^
          nest 4
            (nl ^^ text "f();" ^^ nl ^^ text "if (done())" ^^
             (let exit_d = text "exit();" in
              (space ^^ exit_d) <|> nest 4 (nl ^^ exit_d))) ^^
          nl ^^ text "}"
  in
  print_string (pretty_format d);
  print_string "\n====================\n";
  print_string (pretty_format d)

Output:

utop # print_doc_choice 80;;
while (true) {
    f();
    if (done()) exit();
}
====================
while (true) {
    
}

Although rare in practice, I believe it should be safe for a document to be rendered more than once.

The Cause

In each measure set in the memoization table, the function/thunk layout : unit -> unit captures the provided renderer. However, when one tries to print an already resolved document with a new renderer, the printer retrieves the layout function from the memo table and calls the old renderer.

In the above example, the second call to pretty_format writes to the buffer created for the first call.

Potential Fix

A potential fix is to change to the type of layout to unit -> string or unit -> string list/treeof, which returns the rendered string(s) instead of calling the renderer. However, this method does buffer the entire rendered document before sending it to renderer.

[sorawee]

That is an embarrassing mistake! A fix that I have in mind is to change the type of layout to Signature.renderer -> unit. I'll do that later today. Thanks so much for reporting the issue!