27_Parser Implementation

# Parser Implementation

A parser is a crucial component in a compiler or interpreter, taking raw data (like code) and transforming it into a format that a machine can understand and execute.

## Types of Parsers

There are two main types of parsers: *Top-down* and *Bottom-up* parsers.

**Top-down Parsers** start with the highest level of syntax and expect the input to conform to the grammar of the language. An example of a top-down parser is a Recursive Descent Parser.

**Bottom-up Parsers** start with the input and try to derive the syntax tree by continually replacing the input's subsets with their corresponding syntax rules. An example is a Shift-Reduce Parser.

## Recursive Descent Parsing

A Recursive Descent Parser comprises several small parsers, each corresponding to a rule in the grammar. Here's a simple implementation for a parser that accepts strings of 'a's and 'b's ending in a 'c':

```python
def parse_A():
    return match('a') and parse_A() or True

def parse_B():
    return match('b') and parse_B() or True

def parse_input(input):
    global lookahead
    lookahead = input
    return parse_A() and match('c')
```

In the code snippet above, `match()` is a function that matches and consumes the next character in the lookahead if it matches the expected character.

## Shift-Reduce Parsing

Shift-reduce parsing is a type of bottom-up parsing. It involves shifting input to a stack until a rule is matched, at which point a reduction happens. Here's a basic implementation:

```python
def shift_reduce(input, grammar):
    stack = []
    while input or stack:
        for rule in grammar:
            if ''.join(stack[-len(rule):]) == rule:
                stack = stack[:-len(rule)]
                stack.append(rule[0])
                break
        else:
            if not input: return False
            stack.append(input.pop(0))
    return True
```

In this example, `grammar` is a list of rules (strings), and `input` is a list of characters. The parser tries to match and replace the end of the stack with the rules' left-hand side.

Remember, the choice of parser depends on the specific requirements and constraints of your project. Both types have their strengths and weaknesses.