Nimvar

Nimvar is a term primarily used within the Nim programming language community. It refers to a mutable global variable, specifically one that is initialized at compile time with a constant value. This distinction is important because it allows Nim to perform certain optimizations on these variables, treating them almost like constants in some contexts while still allowing their values to be changed during runtime.

The term "Nimvar" is not a formally defined keyword or concept in the Nim language specification itself. Instead, it's a shorthand or informal term used to describe the specific type of mutable global variable described above.

The key characteristic of a Nimvar is that it's a var (mutable variable) declared at the module level (global scope) and initialized with a compile-time constant expression. Because the compiler knows the initial value at compile time, it can sometimes perform optimizations that wouldn't be possible with a general mutable global variable. However, unlike const which are immutable, the value of a Nimvar can be modified after the program starts executing.

While the compiler can perform optimizations based on the initial, compile-time known value, developers must be mindful of the potential impact of modifying Nimvars during program execution. Depending on how the variable is used, changing its value could lead to unexpected behavior or performance bottlenecks if the compiler's initial assumptions are invalidated.

In summary, a Nimvar represents a balance between the performance benefits of compile-time constants and the flexibility of mutable variables. It's a common pattern in Nim code to use Nimvars for configuration settings or global state that needs to be modified at runtime but also benefits from the compiler's knowledge of its initial value.