Add mypy to Python REPL

I tried to add it to Lint.hs but I can't because mypy needs the target's
external libraries in its environment to load and check the types for that
dependency. So instead, I just added a function to the REPL environment that
runs the type checker. I already had mypy in the REPL environment so I must have
started down this path before and just didn't add the REPL tooling.

The automatic typechecking on load feels like Haskell. A previous version of
this patch had just provided a `typecheck()` function to the REPL, but it felt
awkward to type all that out just to check my code after loading it.

I would like to bind a key like ctrl-r or alt-r to the `reload()` function, but
I'm not sure how to do that. I think Python uses GNU readline so there should be
some docs, but a first pass search didn't find what I needed.

2 files changed, 31 insertions, 9 deletions

diff --git a/Biz/Ide/repl.sh b/Biz/Ide/repl.sh
index 8b28dcd..1aca862 100755
--- a/Biz/Ide/repl.sh
+++ b/Biz/Ide/repl.sh
@@ -69,7 +69,6 @@ fi
       ;;
     Py)
       langdeps+=("mypy")
-      flags+=(--packages ruff)
       flags+=(--packages "$BILD.bild.python.pythonWith (p: with p; [${langdeps[*]}])")
       PYTHONPATH=$CODEROOT:$PYTHONPATH
       pycommand="python -i $CODEROOT/Biz/Repl.py $module ${targets[*]}"
diff --git a/Biz/Repl.py b/Biz/Repl.py
index 9cc0c35..cb9dfe7 100644
--- a/Biz/Repl.py
+++ b/Biz/Repl.py
@@ -2,14 +2,26 @@
 Improve the standard Python REPL.
 
 This module attempts to emulate the workflow of ghci or lisp repls. It uses
-importlib to load a namespace from the given path. It then binds 'r()' to a
-function that reloads the same namespace.
+importlib to load a namespace from the provided path, typechecks it with mypy,
+and provides some tools for improving repl-driven development.
+
+This module is called in Biz/Ide/repl.sh like so:
+
+    python -i Biz/Repl.py NS PATH
+
+where NS is the dot-partitioned namespace of the main module, and PATH is the
+path to the same file. In the future this could be expanded to be a list of
+additional files to load.
 """
 
 import importlib
+import importlib.util
 import logging
+import os
 import sys
 
+import mypy.api
+
 from Biz import Log
 
 
@@ -34,20 +46,31 @@ def use(ns: str, path: str) -> None:
     globals().update({k: getattr(module, k) for k in names})
 
 
+def typecheck(path: str) -> None:
+    """Typecheck this namespace."""
+    # this envvar is undocumented, but it works
+    # https://github.com/python/mypy/issues/13815
+    os.environ["MYPY_FORCE_COLOR"] = "1"
+    logging.info("typechecking %s", path)
+    stdout, stderr, _ = mypy.api.run([path])
+    sys.stdout.write(stdout)
+    sys.stdout.flush()
+    sys.stderr.write(stderr)
+    sys.stderr.flush()
+
+
 if __name__ == "__main__":
     Log.setup()
     NS = sys.argv[1]
     PATH = sys.argv[2]
     use(NS, PATH)
+    typecheck(PATH)
 
-    logging.info("use reload() or _r() after making changes")
+    logging.info("use reload() after making changes")
     sys.ps1 = f"{NS}> "
     sys.ps2 = f"{NS}| "
 
     def reload() -> None:
         """Reload the namespace."""
-        return use(NS, PATH)
-
-    def _r() -> None:
-        """Shorthand: Reload the namespace."""
-        return use(NS, PATH)
+        use(NS, PATH)
+        typecheck(PATH)