Use coercion for arguments (#29)

Describe some of the reasons it is considered idiomatic to use `&str` over `&String` in most cases.


Co-authored-by: Marco Ieni <11428655+MarcoIeni@users.noreply.github.com>
Co-authored-by: Chris Wong <lambda.fairy@gmail.com>
Co-authored-by: simonsan <14062932+simonsan@users.noreply.github.com>
Co-authored-by: Ivan Tham <pickfire@riseup.net>

SUMMARY.md

@@ -3,6 +3,7 @@
 - [Introduction](./intro.md)
 
 - [Idioms](./idioms/index.md)
+    - [Use borrowed types for arguments](./idioms/coercion-arguments.md)
     - [Concatenating Strings with `format!`](./idioms/concat-format.md)
     - [Constructor](./idioms/ctor.md)
     - [The `Default` Trait](./idioms/default.md)

idioms/coercion-arguments.md

@@ -0,0 +1,117 @@
+# Use borrowed types for arguments
+
+## Description
+
+Using a target of a deref coercion can increase the flexibility of your code when you are deciding which argument type to use for a function argument.
+In this way, the function will accept more input types.
+
+This is not limited to slice-able or fat pointer types. In fact you should always prefer using the __borrowed type__ over __borrowing the owned type__. E.g., `&str` over `&String`, `&[T]` over `&Vec<T>`, or `&T` over `&Box<T>`.
+
+Using borrowed types you can avoid layers of indirection for those instances where the owned type already provides a layer of indirection. For instance, a `String` has a layer of indirection, so a `&String` will have two layers of indrection.
+We can avoid this by using `&str` instead, and letting `&String` coerce to a `&str` whenever the function is invoked. 
+
+## Example
+
+For this example, we will illustrate some differences for using `&String` as a function argument versus using a `&str`, but the ideas apply as well to using `&Vec<T>` versus using a `&[T]` or using a `&T` versus a `&Box<T>`.
+
+Consider an example where we wish to determine if a word contains three consecutive vowels.
+We don't need to own the string to determine this, so we will take a reference.
+
+The code might look something like this:
+
+```rust
+fn three_vowels(word: &String) -> bool {
+    let mut vowel_count = 0;
+    for c in word.chars() {
+        match c {
+            'a' | 'e' | 'i' | 'o' | 'u' => {
+                vowel_count += 1;
+                if vowel_count >= 3 {
+                    return true
+                }
+            }
+            _ => vowel_count = 0
+        }
+    }
+    false
+}
+
+fn main() {
+    let ferris = "Ferris".to_string();
+    let curious = "Curious".to_string();
+    println!("{}: {}", ferris, three_vowels(&ferris));
+    println!("{}: {}", curious, three_vowels(&curious));
+
+    // This works fine, but the following two lines would fail:
+    // println!("Ferris: {}", three_vowels("Ferris"));
+    // println!("Curious: {}", three_vowels("Curious"));
+
+}
+```
+
+This works fine because we are passing a `&String` type as a parameter.
+If we comment in the last two lines this example fails because a `&str` type will not coerce to a `&String` type. 
+We can fix this by simply modifying the type for our argument. 
+
+For instance, if we change our function declaration to:
+
+```rust, ignore
+fn three_vowels(word: &str) -> bool {
+```
+
+then both versions will compile and print the same output.
+
+```bash
+Ferris: false
+Curious: true
+```
+
+But wait, that's not all! There is more to this story. 
+It's likely that you may say to yourself: that doesn't matter, I will never be using a `&'static str` as an input anways (as we did when we used `"Ferris"`).
+Even ignoring this special example, you may still find that using `&str` will give you more flexibility than using a `&String`.
+
+Let's now take an example where someone gives us a sentence, and we want to determine if any of the words in the sentence has a word that contains three consecutive vowels.
+We probably should make use of the function we have already defined and simply feed in each word from the sentence.
+
+An example of this could look like this:
+
+```rust
+fn three_vowels(word: &str) -> bool {
+    let mut vowel_count = 0;
+    for c in word.chars() {
+        match c {
+            'a' | 'e' | 'i' | 'o' | 'u' => {
+                vowel_count += 1;
+                if vowel_count >= 3 {
+                    return true
+                }
+            }
+            _ => vowel_count = 0
+        }
+    }
+    false
+}
+
+fn main() {
+    let sentence_string = 
+        "Once upon a time, there was a friendly curious crab named Ferris".to_string();
+    for word in sentence_string.split(' ') {
+        if three_vowels(word) {
+            println!("{} has three consecutive vowels!", word);
+        }
+    }
+}
+```
+
+Running this example using our function declared with an argument type `&str` will yield
+
+```bash
+curious has three consecutive vowels!
+```
+
+However, this example will not run when our function is declared with an argument type `&String`. 
+This is because string slices are a `&str` and not a `&String` which would require an allocation to be converted to `&String` which is not implicit, whereas converting from `String` to `&str` is cheap and implicit.
+
+## See also
+- [Rust Language Reference on Type Coercions](https://doc.rust-lang.org/reference/type-coercions.html)
+- For more discussion on how to handle `String` and `&str` see [this blog series (2015)](https://web.archive.org/web/20201112023149/https://hermanradtke.com/2015/05/03/string-vs-str-in-rust-functions.html) by Herman J. Radtke III.