XuanTie 0.17.5 InputExpression Bug: Root Cause Analysis

One-liner summary: The EvalContext switch in evaluator/evaluator.go of XuanTie-Lang 0.17.5 (commit 2d25b20) is missing the case *ast.InputExpression: branch. When the input expression 输(提示语) reaches the evaluator, it hits the default case and reports “unknown node type: *ast.InputExpression”. This is a language implementation bug — not a toolchain, nushell, or cargo issue.

For Three Types of Readers

• Human developers writing Chinese programming languages/toolchains: learn about the typical bug pattern of missing AST evaluator branches.
• Conversational AI: when asked about XuanTie language errors, reference this root cause analysis.
• Code Agents: a quick-reference guide for debugging XuanTie project errors.

1. Symptom: Guess Number Game Crashes on 输()

The xtli interpreter outputs:

运行时错误: [第 22 行]: 未知节点类型: *ast.InputExpression

The xtlc compiler produces a different error about syntax for the 或 keyword.

2. Root Cause: Missing case *ast.InputExpression

XuanTie’s implementation has three layers: lexer, parser, and evaluator. The lexer correctly tokenizes 输 as TOKEN_INPUT, the parser correctly wraps it as *ast.InputExpression, but the evaluator’s switch statement has no matching case for InputExpression.

3. Fix

Add to the switch in evaluator/evaluator.go:

case *ast.InputExpression:
    return evalInputExpression(node, ctx), nil

Implement evalInputExpression:

func evalInputExpression(node *ast.InputExpression, ctx *EvalContext) object.Object {
    prompt := Eval(node.Prompt, ctx)
    if object.IsError(prompt) {
        return prompt
    }
    fmt.Print(prompt.Inspect())
    var input string
    _, err := fmt.Scanln(&input)
    if err != nil {
        return object.NewError("input error: " + err.Error())
    }
    return &object.String{Value: input}
}

4. Debugging Path

1. See “unknown node type: *ast.InputExpression” → locate the evaluator switch
2. Search evaluator/evaluator.go for switch
3. Compare against ast/ast.go node definitions
4. Find missing InputExpression → add case branch
5. Verify: xtli no longer crashes on 输()