Hy’s REPL (read-eval-print loop) [1] functionality is implemented in the hy.cmdline.HyREPL class. The HyREPL extends the Python Standard Library’s code.InteractiveConsole [2] class. For more information about starting the REPL from the command line, see Command Line Interface. A REPL can also be instantiated programatically, by calling hy.cmdline.run_repl - see Launching a Hy REPL from Python.

From a high level, a single cycle of the REPL consists of the following steps:

  1. tokenize and parse input with hy.lex.hy_parse, generating Hy AST [3];
  2. compile Hy AST to Python AST with hy.compiler.hy_compile [4];
  3. execute the Python code with eval; and
  4. print output, formatted with output_fn.

REPL Built-ins

Recent Evalution Results

The results of the three most recent evaluations can be obtained by entering *1 (most recent), *2, and *3. For example:

=> "first"
=> "second"
=> "third"
=> f"{*1},{*2},{*3}"


The result of evaluating *i itself becomes the next most recent result, pushing *1 to *2, *2 to *3, and *3 off the cache.

Most Recent Exception

Once an exception has been thrown in an interactive session, the most recent exception can be obtained by entering *e. For example:

=> *e
Traceback (most recent call last):
File "stdin-8d630e81640adf6e2670bb457a8234263247e875", line 1, in <module>
NameError: name 'hyx_XasteriskXe' is not defined
=> (/ 1 0)
Traceback (most recent call last):
  File "stdin-7b3ace8766f1e1cfb3ae7c01a1a61cebed24f482", line 1, in <module>
    (/ 1 0)
ZeroDivisionError: division by zero
=> *e
ZeroDivisionError('division by zero')
=> (type *e)
<class 'ZeroDivisionError'>

[3]Steps 1 and 2: Tokenizing and Parsing
[4]Step 3: Hy Compilation to Python AST