56 lines
1.6 KiB
TypeScript
56 lines
1.6 KiB
TypeScript
// Lispts: Scheme Interpreter in TypeScript
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const program = "(begin (define r 10) (* pi (* r r)))"
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type LSymbol = string // A Lisp Symbol(alias TSymbol) is implemented as TypeScript string
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type LNumber = number // A Lisp Symbol(alias TSymbol) is implemented as TypeScript number
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type Atom = LSymbol | number // A Lisp Atom is a Symbol or Number impl
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type List = Array<any> // A Lisp List is implemented as a TypeScript array
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type Exp = Atom | List // A Lisp expression is an Atom or List
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interface Env {
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[key: string | number]: any
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} // A Lisp environment is a mapping of {variable: value}
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/**
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* Read a Scheme expression from a string.
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*/
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const parse = (program: string): Exp => read_from_tokens(tokenize(program))
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/**
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* Convert a string into a list of tokens.
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*/
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const tokenize = (char: string): string[] =>
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char
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.replaceAll('(', ' ( ')
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.replaceAll(')', ' ) ')
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.split(' ')
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.filter((s) => s)
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/**
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* Numbers become numbers; every other token is a symbol(LSymbol).
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*/
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const atom = (token: string): Atom =>
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Number.isNaN(token) ? Number(token) : (token as LSymbol)
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/**
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* Read an expression from a sequence of tokens
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*/
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const read_from_tokens = (tokens: string[]): Exp => {
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if (tokens.length === 0 || !tokens) {
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throw new Error('unexpected EOF while reading')
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}
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const token = tokens.shift()
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if (token === '(') {
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const l = []
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while (tokens[0] !== ')') {
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l.push(read_from_tokens(tokens))
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}
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tokens.shift()
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return l
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} else if (token === ')') {
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throw new Error('unexpected )')
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} else {
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return atom(token as string)
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}
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}
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console.log(parse(program))
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