:title: mal Step 3
:date: 2026-09-04
:tags: mal, python
:identifier: 20260904T191754
:signature: 10=6

mal: Step 3
===========

:project: mal-py
:filename: step3_env.py


**read**

.. code-block:: python

   import itertools

   from env import Env
   from reader import S, read_str

   def read(ins):
       return read_str(ins)

**eval**

.. code-block:: python

   def evaluate(form, env: Env):
       match form:
           case S(name):
               return env.get(name)
           case []:
               return []
           case [*fs]:
               match fs[0]:
                   case S('def!'):
                       env.set(fs[1].name, v := evaluate(fs[2], env))
                       return v
                   case S('let*'):
                       letenv = Env(env)
                       for binding, expr in itertools.batched(fs[1], n=2):
                           letenv.set(binding.name, evaluate(expr, letenv))
                       return evaluate(fs[2], letenv)

                   case _:
                       f, *args = [evaluate(f, env) for f in fs]
                       return f(*args)
           case _:
               return form

**print**

.. code-block:: python

   from printer import print_form

   def printit(form):
       return print_form(form)

**rep**

.. code-block:: python

   def rep(ins: str, env: Env):
       try:
           return printit(evaluate(read(ins), env))
       except EOFError:
           return f"EOF"
       except KeyError as e:
           return f"{e.args[0]!r} not found"
       except Exception as e:
           print(f"RuntimeError: {e}")
           # import pdb; pdb.post_mortem()


**repl**

.. code-block:: python



   def repl(ins: io.TextIO):
       env = Env()
       env.set('+', lambda a,b: a+b)
       env.set('-', lambda a,b: a-b)
       env.set('*', lambda a,b: a*b)
       env.set('/', lambda a,b: int(a/b))

       print("user> ", end='', flush=True)
       while (line := ins.readline()) != "":
           print(rep(line, env))
           print("user> ", end='', flush=True)


   if __name__ == "__main__":
       import sys
       repl(sys.stdin)