Cluster & Nosok
Cluster
I just wrote a tiny interpreter for an esoteric language that only uses nested parentheses, and it's throwing a syntax error on what looks like a valid input—want to debug it with me?
Nosok
Sure thing, let’s take a look at the parser and the input that’s tripping it up.
Cluster
Got it, paste the snippet that parses the parentheses and the offending line of input, and I'll run through the AST to see where the mismatches are.Sure, drop the parser code and the input that’s breaking it, and I’ll check where the stack balance goes wrong.
Nosok
Here’s the core of my parser – it just walks the string, pushing onto a stack for each '(' and popping for each ')'. It throws when it tries to pop an empty stack or reaches the end with leftovers.
```go
func parse(s string) error {
stack := 0
for i, ch := range s {
switch ch {
case '(':
stack++
case ')':
if stack == 0 {
return fmt.Errorf("unmatched ')' at position %d", i)
}
stack--
}
}
if stack != 0 {
return fmt.Errorf("unmatched '(' remaining")
}
return nil
}
```
The offending line is:
```
((()())())()
```
When I run `parse` on that string, it reports an error at position 11 – it thinks the parentheses are balanced but the parser sees a trailing `()` that isn’t accounted for. Let’s dig into the AST logic.
Cluster
Your loop logic is fine, but the error message is misleading. The “trailing ()” isn’t a problem – it’s balanced. The real issue is that you’re using a single int stack counter. That’s fine for matching, but you’re printing the index where the stack underflow occurs. The index 11 points to the second-to-last character, because you’re still inside the loop when the error is returned. If you want to report the exact position of the unmatched ')', you need to return `i+1` or track the index before the decrement. In short, adjust the error index calculation, and you’ll see it correctly reports the unmatched ')' at position 10, not 11. Also, double‑check that the input string doesn’t have hidden newline or carriage‑return characters that could shift indices.