Is a Set of Rules Which Defines How an Expression is Evaluated?

Yes, a set of rules that defines how an expression is evaluated is called an evaluation strategy.

According to the provided reference:

In a programming language, an evaluation strategy is a set of rules for evaluating expressions.

This means that when you write code, the programming language needs a defined way to understand and execute the instructions you give it. This is where an evaluation strategy comes into play. It's like a recipe for how the language should interpret your code step-by-step to arrive at a final result.

Understanding Evaluation Strategies

Evaluation strategies dictate:

• When an expression should be evaluated.
• How an expression should be evaluated.
• The order in which parts of an expression are evaluated.

Different programming languages may employ different evaluation strategies. The choice of strategy can have a significant impact on:

• Performance: Some strategies might be faster in certain situations than others.
• Side effects: Some strategies might cause side effects differently than others.
• Code behavior: Ultimately, the evaluation strategy affects how code behaves.

Common Evaluation Strategies

Here are some common examples of evaluation strategies:

• Eager Evaluation (Strict Evaluation): This strategy evaluates an expression as soon as it is encountered. Most languages, like C++, Java, and Python, use eager evaluation.

  • Example: When you write x = 2 + 3, the expression 2 + 3 is calculated immediately, and the result 5 is assigned to x.

• Lazy Evaluation (Non-strict Evaluation): This strategy delays the evaluation of an expression until its value is actually needed. Haskell is a prominent example of a language that uses lazy evaluation.

  • Example: If you define a list with a complex computation involved my_list = [computation_a(), computation_b()] using a lazy evaluation strategy, computation_a() and computation_b() will not be computed until they're actually used.

Key Differences Summarized

Feature	Eager Evaluation	Lazy Evaluation
Timing	Evaluates expressions immediately upon encounter	Evaluates expressions only when the result is needed
Example Languages	C++, Java, Python	Haskell
Pros	Generally easier to understand and debug	Can lead to better performance in certain situations, works well with infinite data structures
Cons	May evaluate unnecessary expressions	Can be harder to reason about due to delayed evaluation

In essence, an evaluation strategy acts as a critical part of the compiler and interpreter, defining how the programming language reads and translates your code.