py2hy
py2hy is a compiler that compiles Python AST to Hy.
py2hy was used to create HyHy, the Hy language rewritten entirely in Hy.
Other Working demos are available in demo/.
Requirements
- Python >= 3.6
- Hy == 0.18.0
Currently, py2hy is based on Python 3.6's AST specs, and most tested in Python 3.6.
Usage
py2hy src.pyGenerating the Grammar Template
To parse the Python AST specs and generate the grammar template, do
make template # i.e. `python tools/parse_pygrammarspecs.py > tools/template.hy`How it works
The Transformation
The main idea is to treat the Python AST as if it was an S-expression, then
treat the Python AST keywords as if they were Hy macros, and let Hy
recursively macroexpand-1 the expression. For example, the Return Python
AST node has a field named value, so it would first be seen as the Hy code
(Return :value value). py2hy then treats this as if it was a
macroexpansion of a macro named Return, producing Hy code.
The original implementation actually used Hy's macro system and
macroexpand-1 for the transformation. The current implementation uses the
Python class system for an equivalent functionality and improved speed.
Generating the Grammar Template
Running the parser for the
Python AST specs creates a
template script to be filled in to create
py2hy.hy, a set of definitions of the transformations from Python AST to Hy.
This tool would be useful to create a py2hy for different versions of the
Python AST specs.
The return statement
At the time of writing, the return statement is not implemented in Hy. Here,
return is implemented in a similar manner mentioned in
the Hy issues.
For every def that contains a return statement, the entire function body is
wrapped in a try clause that catches a Py2HyReturnException. The return
statement is replaced by a raise statement that raises Py2HyReturnException
containing the value to be returned. Py2HyReturnException is defined in the
top of the entire script when the script contains any return statements.
Notes
Precompiling for the Travis CI Test
Since Travis CI is currently incompatible with Hy code, py2hy precompiles all of its Hy portions to Python when installing.
Contributing
py2hy currently does not have a full set of pytest tests. Contribution is
highly appreciated!
License
All of the code is licensed under the GNU Lesser General Public License version
3. See LICENSE for details.
