Sunday, February 7, 2021

Configuring Cabal Build Flags

*markdown-output*

Configuring Cabal build flags

It’s always an emergency when I go looking for this information!

Suppose you are building HLint.

mkdir ~/tmp && cd ~/tmp
curl -o hlint-3.2.7.tar.gz https://hackage.haskell.org/package/hlint-3.2.7/hlint-3.2.7.tar.gz
gunzip  hlint-3.2.7.tar.gz && tar xvf  hlint-3.2.7.tar

Left to their own devices, HLint and ghc-lib-parser-ex will default to auto mode meaning, they will decide for themselves if they should depend on ghc-lib-parser or native ghc libraries.

Sometimes it’s desirable to force the situation though and explicitly make them do one or the other. How you do that? The answer is of course package Cabal flags. There are two scenarios: building with stack or building with cabal.

  • stack.yaml

    • Force link with ghc-lib-parser

        flags:
          hlint:
            ghc-lib: true
          ghc-lib-parser-ex:
            auto: false
            no-ghc-lib: false
    • Force link with native ghc

        flags:
          hlint:
            ghc-lib: false
          ghc-lib-parser-ex:
            auto: false
            no-ghc-lib: true
  • cabal.project

    • Force link with ghc-lib-parser

        packages: hlint-3.2.7
        package hlint
          flags: +ghc-lib
        package ghc-lib-parser-ex
          flags: -auto -no-ghc-lib
    • Force link with native ghc

        packages: hlint-3.2.7
        package hlint
          flags: -ghc-lib
        package ghc-lib-parser-ex
          flags: -auto +no-ghc-lib

Monday, January 18, 2021

Two things in Rust

two_things_rust.html

Two things in Rust

Two things I needed to learn before Rust made sense to me.

1 Pattern binding modes

I don’t remember reading about this in the book. Default binding modes come into play when non-reference patterns are encountered.

Example


let x = Some(3);
let y: &Option<i32> = &x;
match y {
    Some(a) -> {
    // `y` is deref'd and `a` is bound as `ref a`
    }
    None => {}
}

The default binding mode starts as move. Each time a reference is matched using a non-reference pattern; it will automatically derefence the vaue and update the default binding mode - If the reference is &val, set the default binding mode to ref - If the reference is &mut val: if the current default binding is ref, it should remain ref. Otherwise, set the current binding mode to ref mut.

Full details are given in the 2005-match-ergonomics rustlang RFC.

Example

  • Example
match (&Some(3)) {
    Some(p) =>
      // This pattern is a "non-reference pattern".
      // Dereference the `&` and shift the default binding
      // mode to `ref`. `p` is read as `ref p` and given type `i32`.
   x => {
     // In this arm, we are still in `move` mode by default, so `x`
     // has type `&Option<32>`
   }
}
  • Desugared
    match(&Some(3)) {
      &Some(ref p) => {
         ...
      },
      x => {
         ...
      },
    }

2. Implict Deref Coercisons with Functions and Methods

This is another ergonomics feature that saves on explicit &s and *s when writing function and method calls. When we pass a reference to a function or method call deref implicitly as needed to coerce to the parameter target type.

  • Example:
fn hello(name: &str) {
    println!("Hello, {}", name);
}

fn main() {
    let m = MyBox::new(String::from("Rust"));
    hello(&m);
}
  • Example:
pub struct Point {
    x: Vec<i32>,
    y: Vec<i32>,
}

impl Point {
    pub fn x(&self) -> &Vec<i32> {
        match self {
            &Point { ref x, .. } => x,
        }
    }
}

fn use_i32(_: &i32) -> () {}

fn use_vi32(_: &Vec<i32>) -> () {}

fn use_str(_: &str) -> () {}

fn use_strr(_: &&String) -> () {}

fn main() {
    let p: Point = Point {
        x: vec![],
        y: vec![],
    };

    let rp: &Point = &p;
    let rrp: &&Point = &rp;

    println!("p.x = {:?}", rrp.x());

    let _s: &str = "foo";
    let s: String = String::from(_s);
    let bs: Box<String> = Box::new(s.clone());
    let bsr: Box<&String> = Box::new(&s);
    let bi32: Box<Vec<i32>> = Box::new(vec![]);

    use_i32(&&&&1i32);

    use_str(bs.deref());
    use_str(&s);
    use_strr(bsr.deref());
    let r: &&String = &bsr;
    let r2: &String = r.deref();

    use_str(r2);
    use_str(&bsr);
    use_vi32(&bi32);

    let p = Point {
        x: vec![1, 2, 3],
        y: vec![4, 5, 6],
    };
    match &p {
        Point { x, y } => {
            use_vi32(x);
            use_vi32(y);
            println!("{:?}, {:?}", x, y);
        }
    }
    println!("{:?}, {:?}", p.x, p.y);
}