[dannyvassallo]

Issue #, if available: None

Description of changes:

Uses an enum instead of hardcoded strings for function names and allows for function invocation without arguments.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[joshfried-aws]

What do you think about adding an impl for the Function enum like this

pub fn get_expected_number_of_args(&self) -> usize {
        match self {
            Function::Join => 2,
            Function::Substring | Function::RegexReplace => 3,
            Function::Count
            | Function::JsonParse
            | Function::ToUpper
            | Function::ToLower
            | Function::URLDecode
            | Function::ParseString
            | Function::ParseBoolean
            | Function::ParseFloat
            | Function::ParseInt
            | Function::ParseChar => 1,
        }
    }

This would allow us to move up the check for the num of params to the time of parsing like so

fn function_expr(input: Span) -> IResult<Span, FunctionExpr> {
    let location = FileLocation {
        file_name: input.extra,
        line: input.location_line(),
        column: input.get_column() as u32,
    };
    let (input, (name, parameters)) = call_expr(input)?;

    let name = Function::try_from(name.as_str()).map_err(|e| {
        nom::Err::Error(ParserError {
            context: e.to_string(),
            span: input,
            kind: ErrorKind::AlphaNumeric,
        })
    })?;

    if parameters.len() != name.get_expected_number_of_args() {
        return Err(nom::Err::Error(ParserError {
            context: format!(
                "function: {name} requires: {} parameters to be passed, but received: {}",
                name.get_expected_number_of_args(),
                parameters.len()
            ),
            span: input,
            kind: ErrorKind::AlphaNumeric,
        }));
    }

    Ok((
        input,
        FunctionExpr {
            location,
            name,
            parameters,
        },
    ))
}

[joshfried-aws]

I left a few minor comments, imo nothing is major / blocking. Great work on this Dan, this is a much cleaner approach than the initial implementation 😎