modifying-libraries.md

Editing other BS libraries

We sometimes have to edit other bs libraries in tandem with our codebase, which is a little challenging. Here are the steps to make it work:

In client/package.json

-    "bucklescript-tea": "darklang/bucklescript-tea#master",
+    "bucklescript-tea": "file:../../bucklescript-tea",

In scripts/builder:

+  MOUNTS="$MOUNTS --mount type=bind,src=$PWD/../bucklescript-tea,dst=/home/dark/bucklescript-tea"