[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.
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