[Rust] Macros vs. Functions

In Rust, the exclamation mark (!) after a name indicates that it is a macro rather than a function. Macros and functions in Rust are called differently and serve different purposes. Macros, like anyhow!, can do things that functions cannot, such as generating code based on the input parameters at compile time, which is why they have a different syntax for invocation.

 

Macros vs. Functions

• Macros (anyhow!): Macros perform operations at compile time before the final binary is produced. They can take a variable number of arguments, and their implementation can generate code based on those arguments. The ! is mandatory to invoke a macro and is part of its syntax. Without the !, Rust will not recognize it as a macro call and will instead look for a function or another item with the provided name, resulting in a compilation error if no such item exists.

• Functions: Functions are called without an exclamation mark and perform operations at runtime. They have a fixed signature, meaning they take a specified number of arguments with defined types and return a specific type.

 

What Happens Without the !

If you try to use anyhow without the !, like this:

let error = anyhow("An error occurred");

Rust will attempt to find a function or another item named anyhow, not a macro. Since no such function exists in the anyhow crate (the main feature is the macro for creating errors), this will result in a compilation error.

 

Conclusion

Yes, without the !, calling the anyhow macro won't work because Rust's syntax requires the ! to identify and invoke macros. This syntax rule helps maintain clarity and distinction between macros and functions within Rust's ecosystem.