calculator: add executable calculator

It can both evaluate and print the given expression.

It uses `click` to parse its CLI options.

calculator/run.py

@@ -0,0 +1,50 @@
+#! /usr/bin/env python
+import click
+
+from calculator.eval import Evaluator
+from calculator.parse import parse_infix, parse_postfix, parse_prefix
+from calculator.print import Printer
+
+STR_TO_PARSER = {
+    "infix": parse_infix,
+    "prefix": parse_prefix,
+    "postfix": parse_postfix,
+}
+
+
+@click.group(invoke_without_command=True, chain=True)
+@click.option(
+    "--expr-type",
+    default="infix",
+    type=click.Choice(["infix", "prefix", "postfix"], case_sensitive=False),
+)
+@click.pass_context
+def cli(ctx, expr_type):
+    ctx.ensure_object(dict)
+    ctx.obj["input"] = STR_TO_PARSER[expr_type](
+        input(f"Input your {expr_type} expression: ")
+    )
+    if ctx.invoked_subcommand is None:
+        ctx.invoke(eval_tree)
+
+
+@cli.command(name="print")
+@click.pass_context
+def print_tree(ctx):
+    """
+    Print the parsed AST.
+    """
+    Printer().print(ctx.obj["input"])
+
+
+@cli.command(name="eval")
+@click.pass_context
+def eval_tree(ctx):
+    """
+    Evaluate the parsed expression.
+    """
+    print(Evaluator().visit_and_get_value(ctx.obj["input"]))
+
+
+if __name__ == "__main__":
+    cli(obj={})