:title: mal Step 2
:date: 2026-09-04
:tags: mal, python
:identifier: 20260904T183038
:signature: 10=4

mal: Step 2
===========

:project: mal-py
:filename: step2_eval.py


**read**

.. code-block:: python

   from reader import S, read_str

   def read(ins):
       return read_str(ins)

**eval**

.. code-block:: python

   def evaluate(form, env):
       match form:
           case S(name):
               return env[name]
           case []:
               return []
           case [*fs]:
               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


   env = {
     '+': lambda a,b: a+b,
     '-': lambda a,b: a-b,
     '*': lambda a,b: a*b,
     '/': lambda a,b: int(a/b)
   }


   def rep(ins: str):
       try:
           return printit(evaluate(read(ins), env))
       except EOFError:
           return f"EOF"
       except KeyError as e:
           return f"undefined symbol {e.args[0]!r}"


**repl**

.. code-block:: python

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


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