How Do You Spell UPVALUE?

1. An "upvalue" refers to a programming term commonly used in languages that support closures, such as Lua. It is essentially a variable that is accessible from an inner function or closure even after the parent function has finished executing. In other words, it is a local variable that remains in memory and retains its value for subsequent invocations of the parent function.

   When a function with an upvalue is created, the upvalue is allocated in memory along with the function itself. The inner function can access and modify the upvalue, and any changes made to it will persist across different invocations of the outer function.

   Upvalues are useful in situations where functions need to retain state or carry additional information beyond their arguments. They provide a way for inner functions to access and manipulate variables that are local to the outer function, enhancing flexibility and enabling more efficient code organization.

   Although similar in concept to a closure, an upvalue should be distinguished from an enclosing environment. An upvalue represents the actual value or reference being captured, while the enclosing environment encompasses the variables and their values that are accessible to a closure.

   In summary, an upvalue is a variable in a programming language that allows an inner function to access and maintain a value from an outer function, even after the outer function has finished executing. It simplifies coding by preserving variable values between function invocations and provides a powerful tool to handle state and maintain context.

The term "upvalue" was coined by Guy L. Steele Jr. in the programming language Lisp in the early 1970s. The word is a combination of "up" and "value".

In programming languages, a variable typically holds a value that can be changed or accessed. However, in Lisp, an "upvalue" is created when a function refers to a variable defined in an outer function that has already returned. The upvalue retains its value even after the outer function has completed.

The term "upvalue" reflects the concept that the variable's value is looked up in an "upward" direction, rather than being a traditional variable with a fixed value. It distinguishes this type of variable from a regular global or local variable. As Lisp heavily influenced the development of subsequent programming languages, "upvalue" became a commonly used term in several other languages.

Conjugate verb Upvalue