Module lexpr::value

The Value enum, a dynamically typed way of representing any valid S-expression value.

Constructing S-Expressions

Lexpr provides a sexp! macro to build lexpr::Value objects with very natural S-expression syntax.

use lexpr::sexp;

// The type of `john` is `lexpr::Value`
let john = sexp!((
    (name . "John Doe")
    (age . 43)
    (phones "+44 1234567" "+44 2345678")
));

println!("first phone number: {}", john["phones"][0]);

// Convert to a string of S-expression data and print it out
println!("{}", john.to_string());

The Value::to_string() function converts a lexpr::Value into a String of S-expression text.

One neat thing about the sexp! macro is that variables and expressions can be interpolated directly into the S-expression value as you are building it. The macro will check at compile time that the value you are interpolating is able to be represented as S-expression data.

To interpolate, use the comma (,, also known as "unqote" in Lisp). The interpolated expression must either be a single token, or surrounded by round or curly braces.

let full_name = "John Doe";
let age_last_year = 42;

// The type of `john` is `lexpr::Value`
let john = sexp!((
    (name . ,full_name)
    (age . ,(age_last_year + 1))
    (phones ,{ format!("+44 {}", random_phone()) })
));

A string of S-expression data can be parsed into a lexpr::Value by the lexpr::from_str function. There is also from_slice for parsing from a byte slice &[u8] and from_reader for parsing from any io::Read like a file or a TCP stream. For all these functions there also is a _custom variant which allows for specifying parser options, in case the input deviates from the lexpr default behavior.

use lexpr::{sexp, parse::Error, Value};

// Some S-expression input data as a &str. Maybe this comes from the user.
let data = r#"(
        (name . "John Doe")
        (age . 43)
        (phones . (
            "+44 1234567"
            "+44 2345678"
        ))
    )"#;

// Parse the string of data into lexpr::Value.
let v: Value = lexpr::from_str(data)?;

// Access parts of the data by indexing with square brackets.
println!("Please call {} at the number {}", v["name"], v["phones"][0]);

Enums

Value

Represents an S-expression value.

Traits

Index

A type that can be used to index into a lexpr::Value.