Passbyneed

Passbyneed is a programming concept that describes a method of parameter passing where a function receives a copy of an argument's value. However, the evaluation of that argument is delayed until its value is actually needed within the function's execution. This is in contrast to eager evaluation, where arguments are evaluated before the function is called.

The primary benefit of passbyneed is its potential for efficiency, especially when dealing with large or computationally expensive arguments. If an argument's value is never used within the function, it is never computed, saving processing time and resources. This is particularly useful in scenarios involving lazy evaluation or when dealing with potentially infinite data structures.

Passbyneed is often implemented using a mechanism called a thunk. A thunk is essentially a piece of code that, when executed, computes the value of an argument. When an argument is passed by need, it is passed as a thunk. The thunk is only evaluated the first time the argument's value is accessed. Subsequent accesses will retrieve the already computed value, avoiding redundant computation.

This parameter passing mechanism is not as common in mainstream imperative programming languages as pass by value or pass by reference. However, it is a fundamental feature in some functional programming languages, such as Haskell, where it contributes to the language's expressive power and performance characteristics. Understanding passbyneed is key to comprehending how certain lazy evaluation strategies are implemented and utilized effectively.