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Mini Parser

Pybites

Level: intro (score: 1)

🚀 These intro exercises are prep for our Rust Intro Cohort Program.

🎯 Mini Parser Challenge: Parse a Primitive

You’ll parse a single JSON‑style primitive from a string:

true, false, null
integer (base‑10, optional leading + or -, at least one digit)

Ignore leading/trailing ASCII whitespace. Input must contain exactly one primitive (no extra trailing chars after trimming).

✅ Your task

Define:

#[derive(Debug, PartialEq, Eq)]
pub enum JsonValue {
    Bool(bool),
    Null,
    Number(i64),
}

#[derive(Debug, PartialEq, Eq)]
pub enum ParseError {
    Empty,
    Invalid,
    Overflow,
}

Implement:

pub fn parse_primitive(input: &str) -> Result<JsonValue, ParseError>

Rules:

Trim ASCII whitespace first; if empty → Err(ParseError::Empty)
Accept exactly "true", "false", "null"
Else parse an integer with optional +/- prefix; on parse failure → Invalid
If the integer doesn’t fit in i64 → Overflow
Reject any extra non‑whitespace after the token (e.g. "123abc") → Invalid

💡 Hints

Use trim() to strip ws; then decide which branch to take.
For integers, manually scan the sign and digits; don’t accept _ or empty digits.
Use i128 for accumulation and check bounds against i64::MIN..=i64::MAX before casting.

Example tests:

use JsonValue::*; use ParseError::*;

assert_eq!(parse_primitive("true"), Ok(Bool(true)));
assert_eq!(parse_primitive(" false "), Ok(Bool(false)));
assert_eq!(parse_primitive("null"), Ok(Null));

assert_eq!(parse_primitive("42"), Ok(Number(42)));
assert_eq!(parse_primitive("-7"), Ok(Number(-7)));
assert_eq!(parse_primitive("+9"), Ok(Number(9)));

assert_eq!(parse_primitive(""), Err(Empty));
assert_eq!(parse_primitive("  "), Err(Empty));
assert_eq!(parse_primitive("12x"), Err(Invalid));

enums parser json

Code

#[derive(Debug, PartialEq, Eq)]
pub enum JsonValue {
    Bool(bool),
    Null,
    Number(i64),
}

#[derive(Debug, PartialEq, Eq)]
pub enum ParseError {
    Empty,
    Invalid,
    Overflow,
}

pub fn parse_primitive(input: &str) -> Result<JsonValue, ParseError> {
    // TODO:
    // - trim
    // - match true/false/null
    // - else parse signed integer (at least one digit)
    // - check i64 bounds
    // - return precise errors
    unimplemented!()
}

#[cfg(test)]
mod tests {
    use super::*;
    use JsonValue::*;
    use ParseError::*;

#[test]
    fn test_bools_null() {
        assert_eq!(parse_primitive("true"), Ok(Bool(true)));
        assert_eq!(parse_primitive(" false "), Ok(Bool(false)));
        assert_eq!(parse_primitive("null"), Ok(Null));
    }

#[test]
    fn test_integers() {
        assert_eq!(parse_primitive("42"), Ok(Number(42)));
        assert_eq!(parse_primitive("-7"), Ok(Number(-7)));
        assert_eq!(parse_primitive("+9"), Ok(Number(9)));
    }

#[test]
    fn test_errors() {
        assert_eq!(parse_primitive(""), Err(Empty));
        assert_eq!(parse_primitive("  "), Err(Empty));
        assert_eq!(parse_primitive("12x"), Err(Invalid));
        assert_eq!(parse_primitive("+"), Err(Invalid));
        assert_eq!(parse_primitive("-"), Err(Invalid));
    }
}