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.. _repl:
===========
The Hy REPL
===========
Hy's `read-eval-print loop
<https://en.wikipedia.org/wiki/Read-eval-print_loop>`_ (REPL) is implemented in
the class :class:`hy.REPL`. The REPL can be started interactively
:doc:`from the command line <cli>` or programmatically with the instance method
:meth:`hy.REPL.run`.
Two :doc:`environment variables <env_var>` useful for the REPL are
``HY_HISTORY``, which specifies where the REPL input history is saved, and
``HYSTARTUP``, which specifies :ref:`a file to run when the REPL starts
<startup-file>`.
Due to Python limitations, a Python :class:`code.InteractiveConsole`
launched inside the Hy REPL, or a Hy REPL inside another Hy REPL, may
malfunction.
.. autoclass:: hy.REPL
:members: run
.. _repl-output-function:
Output functions
----------------
By default, the return value of each REPL input is printed with
:hy:func:`hy.repr`. To change this, you can set the REPL output function with
e.g. the command-line argument ``--repl-output-fn``. Use :func:`repr` to get
Python representations, like Python's own REPL.
Regardless of the output function, no output is produced when the value is
``None``, as in Python.
.. _repl-specials:
Special variables
-----------------
The REPL maintains a few special convenience variables. ``*1`` holds the result
of the most recent input, like ``_`` in the Python REPL. ``*2`` holds the
result of the input before that, and ``*3`` holds the result of the input
before that. Finally, ``*e`` holds the most recent uncaught exception.
.. _startup-file:
Startup files
-------------
Any macros or Python objects defined in the REPL startup file will be brought
into the REPL's namespace. A few variables are special in the startup file:
``repl-spy``
If true, print equivalent Python code before executing each piece of Hy code.
``repl-output-fn``
The :ref:`output function <repl-output-function>`, as a unary callable
object.
``repl-ps1``, ``repl-ps2``
Strings to use as the prompt strings :data:`sys.ps1` and
:data:`sys.ps2` for the Hy REPL.
Hy startup files can do a number of other things like set banner messages or
change the prompts. The following example shows a number of possibilities::
;; Wrapping in an `eval-and-compile` ensures these Python packages
;; are available in macros defined in this file as well.
(eval-and-compile
(import sys os)
(sys.path.append "~/<path-to-global-libs>"))
(import
re
json
pathlib [Path]
hy.pyops *
hyrule [pp pformat])
(require
hyrule [unless])
(setv
repl-spy True
repl-output-fn pformat
;; Make the REPL prompt `=>` green.
repl-ps1 "\x01\x1b[0;32m\x02=> \x01\x1b[0m\x02"
;; Make the REPL prompt `...` red.
repl-ps2 "\x01\x1b[0;31m\x02... \x01\x1b[0m\x02")
(defn slurp [path]
(setv path (Path path))
(when (path.exists)
(path.read-text)))
(defmacro greet [person]
`(print ~person))
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