The Hy REPL¶
Hy’s read-eval-print loop (REPL) is implemented in
hy.REPL. The REPL can be started interactively
from the command line or programmatically with the instance method
Two environment variables useful for the REPL are
HY_HISTORY, which specifies where the REPL input history is saved, and
HYSTARTUP, which specifies a file to run when the REPL starts.
- class hy.REPL(spy=False, output_fn=None, locals=None, filename='<stdin>', allow_incomplete=True)¶
A subclass of
A convenient way to use this class to interactively debug code is to insert the following in the code you want to debug:
(.run (hy.REPL :locals (locals)))
Note that as with
code.interact(), changes to
(locals)inside the REPL are not propagated back to the original scope.
By default, the return value of each REPL input is printed with
hy.repr. To change this, you can set the REPL output function with
e.g. the command-line argument
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.
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.
Any macros or Python objects defined in the REPL startup file will be brought into the REPL’s namespace. Two variables are special in the startup file:
If true, print equivalent Python code before executing each piece of Hy code.
The output function, as a unary callable object.
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.pypos * hyrule [pp pformat]) (require hyrule [unless]) (setv repl-spy True repl-output-fn pformat ;; Make the REPL prompt `=>` green. sys.ps1 "\x01\x1b[0;32m\x02=> \x01\x1b[0m\x02" ;; Make the REPL prompt `...` red. sys.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))