Execution
Interpreting is a simple procedure. The basic idea is to take the tree and walk through it to and evaluate arithmetic operations hierarchically. This process is recursively called over and over again till the entire tree is evaluated and the answer is retrieved. Let’s say, for example, 5 + 7 + 4. This character stream is first tokenized to token stream in a lexer. The token stream is then parsed to form a parse tree. The parse tree essentially returns (‘add’, (‘add’, (‘num’, 5), (‘num’, 7)), (‘num’, 4)). (see image below)
The interpreter is going to add 5 and 7 first and then recursively call walkTree and add 4 to the result of addition of 5 and 7. Thus, we are going to get 16. The below code does the same process.
Python3
class BasicExecute: def __init__(self, tree, env): self.env = env result = self.walkTree(tree) if result is not None and isinstance(result, int): print(result) if isinstance(result, str) and result[0] == '"': print(result) def walkTree(self, node): if isinstance(node, int): return node if isinstance(node, str): return node if node is None: return None if node[0] == 'program': if node[1] == None: self.walkTree(node[2]) else: self.walkTree(node[1]) self.walkTree(node[2]) if node[0] == 'num': return node[1] if node[0] == 'str': return node[1] if node[0] == 'add': return self.walkTree(node[1]) + self.walkTree(node[2]) elif node[0] == 'sub': return self.walkTree(node[1]) - self.walkTree(node[2]) elif node[0] == 'mul': return self.walkTree(node[1]) * self.walkTree(node[2]) elif node[0] == 'div': return self.walkTree(node[1]) / self.walkTree(node[2]) if node[0] == 'var_assign': self.env[node[1]] = self.walkTree(node[2]) return node[1] if node[0] == 'var': try: return self.env[node[1]] except LookupError: print("Undefined variable '"+node[1]+"' found!") return 0 |
How to Create a Programming Language using Python?
In this article, we are going to learn how to create your own programming language using SLY(Sly Lex Yacc) and Python. Before we dig deeper into this topic, it is to be noted that this is not a beginner’s tutorial and you need to have some knowledge of the prerequisites given below.
Prerequisites
- Rough knowledge about compiler design.
- Basic understanding of lexical analysis, parsing and other compiler design aspects.
- Understanding of regular expressions.
- Familiarity with Python programming language.
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