47 lines
No EOL
1.3 KiB
Python
47 lines
No EOL
1.3 KiB
Python
## (c) Peter Norvig, 2010-16; See http://norvig.com/lispy.html
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program = "(begin (define r 10) (* pi (* r r)))"
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################ Types
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Symbol = str
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List = list
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Number = (int, float)
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Atom = (Symbol, Number)
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Exp = (Atom, list)
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Env = dict
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################ Parsing: parse, tokenize, and read_from_tokens
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def parse(program:str) -> Exp:
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"Read a Scheme expression from a string."
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return read_from_tokens(tokenize(program))
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def tokenize(chars: str) -> list:
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"Convert a string of characters into list of tokens."
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return chars.replace('(', ' ( ').replace(')', ' ) ').split()
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def read_from_tokens(tokens: list) -> Exp:
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"Read an expression from a sequence of tokens."
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if len(tokens) == 0:
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raise SyntaxError('unexpected EOF while reading')
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token = tokens.pop(0)
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if token == '(':
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L = []
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while tokens[0] != ')':
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L.append(read_from_tokens(tokens))
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tokens.pop(0)
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return L
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elif token == ')':
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raise SyntaxError('unexpected )')
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else:
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return atom(token)
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def atom(token: str) -> Atom:
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"Numbers become numbers; every other token is a symbol."
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try: return int(token)
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except ValueError:
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try: return float(token)
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except ValueError:
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return Symbol(token)
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print(parse(program)) |