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//! The [`guard!`] macro.
//!
//! The [`guard!`] macro implements a control-flow sugar that occurs very often in common Rust code:
//!
//! ```
//! fn foo(cond: bool) -> Option<i32> {
//!   if !cond {
//!     return None;
//!   }
//!
//!   // do something useful
//!
//!   Some(42)
//! }
//! ```
//!
//! This pattern of testing arguments and early-returning with an error is very typical.
//! Unfortunately, the [`?`] operator doesn’t help us here because we want to early-return on a
//! boolean value, not an error value.
//!
//! A not very idiomatic and weird way to rewrite that:
//!
//! ```
//! fn foo(cond: bool) -> Option<i32> {
//!   if cond { Some(()) } else { None }?;
//!   Some(42)
//! }
//! ```
//!
//! This crate provides the [`guard!`] macro — analoguous to the [`guard`] Haskell `Alternative`
//! function — that helps early-return from a function if a predicate is `false`:
//!
//! ```
//! #![feature(try_trait)]
//!
//! use try_guard::guard;
//!
//! fn foo(cond: bool) -> Option<i32> {
//!   guard!(cond);
//!   Some(42)
//! }
//! ```
//!
//! ## Custom guard types
//!
//! This crate also allows you to _guard_ to any thing that implements [`Try<Error = NoneError>`].
//! For instance, the following works:
//!
//! ```
//! #![feature(try_trait)]
//! # #[cfg(feature = "try-trait")] mod lol {
//!
//! use std::ops::Try;
//! use std::option::NoneError;
//! use try_guard::guard;
//!
//! enum MyGuard<T> {
//!   Just(T),
//!   Nothing
//! }
//!
//! impl<T> MyGuard<T> {
//!   fn new(x: T) -> Self {
//!     MyGuard::Just(x)
//!   }
//!
//!   fn none() -> Self {
//!     MyGuard::Nothing
//!   }
//! }
//!
//! impl<T> Try for MyGuard<T> {
//!   type Ok = T;
//!
//!   type Error = NoneError;
//!
//!   fn from_error(_: Self::Error) -> Self {
//!     MyGuard::none()
//!   }
//!
//!   fn from_ok(x: Self::Ok) -> Self {
//!     MyGuard::new(x)
//!   }
//!
//!   fn into_result(self) -> Result<Self::Ok, Self::Error> {
//!     match self {
//!       MyGuard::Just(x) => Ok(x),
//!       MyGuard::Nothing => Err(NoneError)
//!     }
//!   }
//! }
//!
//! fn foo(cond: bool) -> MyGuard<i32> {
//!   guard!(cond);
//!   MyGuard::new(42)
//! }
//!
//! # }
//! ```
//!
//! # Feature flags
//!
//!   - The `"try-trait"` flag allows to use `guard!` with any type that implements
//!     [`Try<Error = NoneError>`]. Disabling this will make `guard!` work only with
//!     [`Option`]. **Enabled by default.**
//!     - **This feature currently requires a nightly build.**
//!
//! [`guard!`]: guard
//! [`guard`]: http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Monad.html#v:guard
//! [`?`]: https://doc.rust-lang.org/std/ops/trait.Try.html
//! [`Try<Error = NoneError>`]: https://doc.rust-lang.org/std/ops/trait.Try.html

#![cfg_attr(feature = "try-trait", feature(try_trait))]

/// The [`guard!`] macro.
///
/// [`guard!`]: guard
#[macro_export]
macro_rules! guard {
  ($e:expr) => {
    if !$e {
      #[cfg(feature = "try-trait")]
      {
        return std::ops::Try::from_error(std::option::NoneError);
      }
      #[cfg(not(feature = "try-trait"))]
      {
        return None
      }
    }
  };
}

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

  #[test]
  fn success() {
    fn foo() -> Option<i32> {
      guard!(1 < 2);
      Some(10)
    }

    assert_eq!(foo(), Some(10));
  }

  #[test]
  fn failure() {
    fn foo() -> Option<i32> {
      guard!(1 > 2);
      Some(10)
    }

    assert_eq!(foo(), None);
  }
}