compiler/front_end/parser.py

# Copyright 2019 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Routines to generate a shift-reduce parser from the module_ir module."""

from compiler.front_end import lr1
from compiler.front_end import module_ir
from compiler.front_end import tokenizer
from compiler.util import resources
from compiler.util import simple_memoizer


class ParserGenerationError(Exception):
  """An error occurred during parser generation."""
  pass


def parse_error_examples(error_example_text):
  """Parses error examples from error_example_text.

  Arguments:
    error_example_text: The text of an error example file.

  Returns:
    A list of tuples, suitable for passing into generate_parser.

  Raises:
    ParserGenerationError: There is a problem parsing the error examples.
  """
  error_examples = error_example_text.split("\n" + "=" * 80 + "\n")
  result = []
  # Everything before the first "======" line is explanatory text: ignore it.
  for error_example in error_examples[1:]:
    message_and_examples = error_example.split("\n" + "-" * 80 + "\n")
    if len(message_and_examples) != 2:
      raise ParserGenerationError(
          "Expected one error message and one example section in:\n" +
          error_example)
    message, example_text = message_and_examples
    examples = example_text.split("\n---\n")
    for example in examples:
      # TODO(bolms): feed a line number into tokenize, so that tokenization
      # failures refer to the correct line within error_example_text.
      tokens, errors = tokenizer.tokenize(example, "")
      if errors:
        raise ParserGenerationError(str(errors))

      for i in range(len(tokens)):
        if tokens[i].symbol == "BadWord" and tokens[i].text == "$ANY":
          tokens[i] = lr1.ANY_TOKEN

      error_token = None
      for i in range(len(tokens)):
        if tokens[i].symbol == "BadWord" and tokens[i].text == "$ERR":
          error_token = tokens[i + 1]
          del tokens[i]
          break
      else:
        raise ParserGenerationError(
            "No error token marker '$ERR' in:\n" + error_example)

      result.append((tokens, error_token, message.strip(), example))
  return result


def generate_parser(start_symbol, productions, error_examples):
  """Generates a parser from grammar, and applies error_examples.

  Arguments:
      start_symbol: the start symbol of the grammar (a string)
      productions: a list of parser_types.Production in the grammar
      error_examples: A list of (source tokens, error message, source text)
          tuples.

  Returns:
      A parser.

  Raises:
      ParserGenerationError: There is a problem generating the parser.
  """
  parser = lr1.Grammar(start_symbol, productions).parser()
  if parser.conflicts:
    raise ParserGenerationError("\n".join([str(c) for c in parser.conflicts]))
  for example in error_examples:
    mark_result = parser.mark_error(example[0], example[1], example[2])
    if mark_result:
      raise ParserGenerationError(
          "error marking example: {}\nExample:\n{}".format(
              mark_result, example[3]))
  return parser


@simple_memoizer.memoize
def _load_module_parser():
  error_examples = parse_error_examples(
      resources.load("compiler.front_end", "error_examples"))
  return generate_parser(module_ir.START_SYMBOL, module_ir.PRODUCTIONS,
                         error_examples)


@simple_memoizer.memoize
def _load_expression_parser():
  return generate_parser(module_ir.EXPRESSION_START_SYMBOL,
                         module_ir.PRODUCTIONS, [])


def parse_module(tokens):
  """Parses the provided Emboss token list into an Emboss module parse tree."""
  return _load_module_parser().parse(tokens)


def parse_expression(tokens):
  """Parses the provided Emboss token list into an expression parse tree."""
  return _load_expression_parser().parse(tokens)