Basic evaluator functionality
Interpreter now works for simple arithmetic expressions
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1c23329656
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@ -4,27 +4,77 @@ struct Evaluator {
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ast: AST
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}
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impl Evaluator {
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pub fn run(&mut self) -> String {
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while self.ast.can_reduce() {
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self.ast.reduce();
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}
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pub type EvaluatorResult<T> = Result<T, EvaluatorError>;
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format!("{}", self.ast)
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struct EvaluatorError {
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err: String
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}
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impl Evaluator {
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pub fn run(self) -> String {
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match reduce_full(self.ast) {
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Err(e) => format!("{}", e.err),
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Ok(ast) => format!("{}", ast)
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}
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}
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}
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impl AST {
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fn can_reduce(&self) -> bool {
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false
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use parser::AST::*;
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match self {
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&Number(_) => false,
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&Name(_) => false,
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_ => true
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}
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}
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}
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fn reduce_full(ast: AST) -> EvaluatorResult<AST> {
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let mut ast = ast;
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while ast.can_reduce() {
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ast = try!(reduce_step(ast));
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}
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fn reduce(&mut self) {
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Ok(ast)
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}
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fn reduce_step(ast: AST) -> EvaluatorResult<AST> {
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use parser::AST::*;
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match ast {
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BinOp(left, op, right) => {
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let left = try!(reduce_full(*left));
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let op = try!(reduce_full(*op));
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let right = try!(reduce_full(*right));
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match (left, op, right) {
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(Number(l), Name(op), Number(r)) => {
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match &op[..] {
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"+" => Ok(Number(l + r)),
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"-" => Ok(Number(l - r)),
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"*" => Ok(Number(l * r)),
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"/" => {
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if r == 0.0 {
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Err(EvaluatorError { err: format!("Divide by zero") })
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} else {
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Ok(Number(l / r))
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}
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},
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_ => Err(EvaluatorError { err: format!("Bad BinOp operator") })
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}
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},
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_ => Err(EvaluatorError {
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err: format!("Bad arguments for BinOp")
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})
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}
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},
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Number(_) => Ok(ast),
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Name(_) => Ok(ast),
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}
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}
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pub fn evaluate(ast: AST) -> String {
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let mut ev = Evaluator { ast: ast };
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let ev = Evaluator { ast: ast };
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ev.run()
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}
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@ -13,7 +13,11 @@ pub enum AST {
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impl fmt::Display for AST {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "{}", "GENERIC DISPLAY")
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match self {
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&AST::Number(ref n) => write!(f, "{}", n),
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&AST::Name(ref s) => write!(f, "{}", s),
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astnode => write!(f, "UNEXPANDED AST NODE: {:?}", astnode)
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}
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}
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}
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