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Intro to metaprogramming

Many languages have facilities for metaprogramming: writing code that generates or modifies code. Python has facilities for dynamic metaprogramming: features like decorators, metaclasses, and many more. These features make Python very flexible and productive, but since they're dynamic, they come with run-time overhead. Other languages have static or compile-time metaprogramming features, like C preprocessor macros and C++ templates. These can be limiting and hard to use.

Mojo's compile-time metaprogramming system uses the same language as run-time programs, so you don't have to learn a new language—just a few new features. The primary features you'll need to learn are:

  • Compile-time statements and expressions
  • Parameters
  • Traits

Compile-time statements and expressions​

The comptime keyword identifies a statement or expression that needs to be evaluated at compile time. For example, the comptime keyword is used to declare compile-time constant values and to introduce compile-time conditionals and loops. For information on compile-time assignments and control flow, see Compile-time evaluation.

Parameters​

Functions and structs can be parameterized with compile-time parameters, allowing you to define a container that holds different data types, or a matrix multiplication algorithm that's parameterized by the matrix dimensions. Compile-time parameters are similar to C++ template parameters or Rust generic parameters. At compile time, Mojo specializes parameterized code to make concrete versions—that is, it replaces parameters with constant values.

For example, a matrix multiplication function parameterized on its matrix dimensions can be specialized at compile time to select the most efficient algorithm based on those dimensions. For information on parameterization, see Parameters.

Traits​

Type-parameterized functions and structs work across many types. For example, a list might hold Int, Float32, or String values. Type-parameterized code needs to know what operations those types support.

A trait defines a set of behaviors that types provide. Instead of pre-selecting specific types, a parameterized sort function can just require Comparable. For more information, see traits and parameterized declarations.

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