From 83b7c726d05a748437bf43c35d7e8d9b03530559 Mon Sep 17 00:00:00 2001
From: Richard Eisenberg <reisenberg@janestreet.com>
Date: Wed, 21 Dec 2022 17:40:34 -0500
Subject: [PATCH] Document the debug_printers script

Thanks to Chris for explaining this to me!
---
 HACKING.jst.adoc | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/HACKING.jst.adoc b/HACKING.jst.adoc
index 76b8830efef9..1c9511198822 100644
--- a/HACKING.jst.adoc
+++ b/HACKING.jst.adoc
@@ -54,3 +54,19 @@ where the test file or test dir are specified with respect to the
     $ make -f Makefile.jst test-one TEST=typing-local/local.ml
     $ make -f Makefile.jst test-one DIR=typing-local
 
+## Debugging
+
+OCaml 4.14 makes `type_expr` abstract, and thus normal debug printing
+of types no longer works. However, there is now an installable printer
+for types, which we can use to see the types. Here are the instructions:
+
+1. Use the old `Makefile`, not the new `Makefile.jst`. This is an infelicity
+we hope to fix.
+
+2. In the `tools` directory, run `make debug_printers.cmo`.
+
+3. In the debugger, execute some instructions, with e.g. `run` or `step`. This forces
+the debugger to load the compiler code, required for the next
+step.
+
+4. Execute `source tools/debug_printers` to install the printers.