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Mojo functions can take two kinds of input: parameters known at compile time, and arguments known at runtime. Knowing the difference is the key.

An argument is an ordinary value you pass when the program runs. Its actual number can change every single time you call the function.

A parameter is a value the compiler already knows before your program runs. It is fixed and baked right into the generated code. ⚙️

Mojo keeps them visually separate: parameters go in square brackets, while arguments stay in the familiar parentheses.

In repeat[count: Int](msg: String), the count is set at compile time and msg is supplied later when you actually call it.

Think of compile time as the moment Mojo translates your code, long before it runs. Parameters are decided in that earlier phase.

Keeping compile-time and runtime values separate lets the compiler specialize and optimize code that the runtime simply cannot.

You supply a parameter in brackets at the call site, just like you supply arguments in parentheses right after it.

A parameter value must be a compile-time constant. You cannot pass a number you only computed while the program was running.

The built-in SIMD type takes its element type and width as parameters, then holds real numbers as runtime values inside it.

Picture two slots: the bracket slot for compile-time facts, the parenthesis slot for runtime data. Mojo treats them very differently.

Tell apart parameters and arguments in Mojo.

You learned the split: parameters are compile-time values in brackets, arguments are runtime values in parentheses. That divide powers Mojo's speed. 🎯