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//! The concrete syntax tree.
//!
//! Note that `Decl` and `Expr` both impl `FromStr`; hence they can be parsed
//! with a simple `.parse()` call:
//!
//! ```
//! # use evaltrees::ast::{Literal, Op, Pattern};
//! # use evaltrees::cst::{Decl, Expr};
//! # fn main() {
//! let expr: Expr = "(1 + 2) :: 3 :: []".parse().unwrap();
//! assert_eq!(expr, Expr::Op(Op::Cons,
//!     Box::new(Expr::Op(Op::Add,
//!         Box::new(Expr::Literal(Literal::Int(1))),
//!         Box::new(Expr::Literal(Literal::Int(2))),
//!     )),
//!     Box::new(Expr::Op(Op::Cons,
//!         Box::new(Expr::Literal(Literal::Int(3))),
//!         Box::new(Expr::Literal(Literal::Nil)),
//!     )),
//! ));
//!
//! let decl: Decl = "id x = x".parse().unwrap();
//! assert_eq!(decl, Decl {
//!     name: "id".into(),
//!     args: vec![
//!         Pattern::Binding("x".into(), ()),
//!     ],
//!     body: Expr::Variable("x".into()),
//! });
//! # }
//! ```

mod display;
mod parser;

use symbol::Symbol;

use ast::{Literal, Op, Pattern};
pub use cst::parser::parse_decls;

/// A function or value declaration.
#[derive(Clone, Debug, PartialEq)]
pub struct Decl {
    /// The name of the function or value.
    pub name: Symbol,

    /// The arguments to the function. If empty, the decl is for a value.
    pub args: Vec<Pattern<()>>,

    /// The body of the function, or the expression assigned to the value.
    pub body: Expr,
}

/// An expression.
#[derive(Clone, Debug, PartialEq)]
pub enum Expr {
    /// A conditional expression.
    If(Box<Expr>, Box<Expr>, Box<Expr>),

    /// A list.
    ///
    /// In theory, this can't be empty (since that'd be the nil literal).
    /// Either way, one should handle all cases.
    List(Vec<Expr>),

    /// A literal value.
    Literal(Literal),

    /// A binary operation.
    Op(Op, Box<Expr>, Box<Expr>),

    /// A variable.
    Variable(Symbol),
}