# Macro synom::peek [−] [src]

    macro_rules! peek {
        ($i:expr, $submac:ident!( $($args:tt)* )) => { ... };
        ($i:expr, $f:expr) => { ... };
    }

Parse a value without consuming it from the input data.

- Syntax: `peek!(THING)`
- Output: `THING`

    extern crate syn;
    #[macro_use] extern crate synom;

    use syn::Expr;
    use syn::parse::{ident, expr};
    use synom::IResult;

    // Parse an expression that begins with an identifier.
    named!(ident_expr -> Expr,
        preceded!(peek!(ident), expr)
    );

    fn main() {
        // begins with an identifier
        let input = "banana + 1";
        let parsed = ident_expr(input).expect("ident");
        println!("{:?}", parsed);

        // does not begin with an identifier
        let input = "1 + banana";
        let err = ident_expr(input);
        assert_eq!(err, IResult::Error);
    }

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