use source_span::Span;

use super::{expression::Precedence, *};
use crate::{Parser, Spanned};

/// A function application expression.
///
/// ```trilogy
/// f x
/// ```
#[derive(Clone, Debug)]
pub struct Application {
    /// An expression that evaluates to the function being applied.
    pub function: Expression,
    /// An expression that evalutes to the argument to the function.
    pub argument: Expression,
    pub span: Span,
}

impl Spanned for Application {
    fn span(&self) -> Span {
        self.span
    }
}

impl Application {
    pub(crate) fn parse(parser: &mut Parser, function: Expression) -> SyntaxResult<Self> {
        let argument = Expression::parse_precedence(parser, Precedence::Application)?;
        Ok(Self {
            span: function.span().union(argument.span()),
            function,
            argument,
        })
    }
}

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

    test_parse!(application_simple: "hello world" => Expression::parse => Expression::Application(Application { .. }));
    test_parse!(application_path: "a::hello (b::world)" => Expression::parse => Expression::Application (Application { .. }));
    test_parse!(application_parenthesized: "hello (a + world)" => Expression::parse => Expression::Application (Application { .. }));
    test_parse!(application_unary_not: "hello !b" => Expression::parse => Expression::Application(Application { argument: Expression::Unary(..), .. }));
    test_parse!(not_application_unary_minus: "hello - b" => Expression::parse => Expression::Binary(..));
    test_parse!(application_unary_negate: "hello ~b" => Expression::parse => Expression::Application(Application { argument: Expression::Unary(..), .. }));
    test_parse!(application_keyword: "hello if x then 3 else 4" => Expression::parse => Expression::Application(Application { argument: Expression::IfElse(..), .. }));
    test_parse!(application_of_number: "3 4 5" => Expression::parse => Expression::Application(Application { .. }));
    test_parse!(application_binop: "hello a + world" => Expression::parse => Expression::Binary(BinaryOperation { lhs: Expression::Application(Application { .. }), .. }));
}