43.1 Folder module

source bash-setup.sh

Let us create a project with a binary crate:

cargo new --bin foo

    Creating binary (application) `foo` package

note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

pushd foo
cat >src/math.rs <<EOF
pub fn add(a: i16, b: i16) -> i16 {
  a + b
}
EOF

/tmp/tmp.qF3xiE5d1v/foo /tmp/tmp.qF3xiE5d1v

A module can also be written as a folder, in which case a file mod.rs must be created inside the folder. For instance if we want to define our math module as a folder, we need to create a file src/math/mod.rs.

Here is the content of our file:

rm src/math.rs
mkdir src/math
cat >src/math/mod.rs <<EOF
pub fn add(a: i16, b: i16) -> i16 {
  a + b
}
EOF

bat src/math/mod.rs

pub fn add(a: i16, b: i16) -> i16 {

  a + b

}

The main code file is written to use the math module:

cat >src/main.rs <<EOF
mod math;

fn main () {
  println!("{}", math::add(2, 6));
}
EOF

bat src/main.rs

mod math;

fn main () {

  println!("{}", math::add(2, 6));

}

The program still compiles:

cargo build

   Compiling foo v0.1.0 (/tmp/tmp.qF3xiE5d1v/foo)

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s

And we can run it:

cargo run

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s

     Running `target/debug/foo`

8

The advantage of using a folder module is that we can define file sub-modules.

popd

/tmp/tmp.qF3xiE5d1v