programming

Fun thought snack from SICP:

        Allowing quotation in a language wreaks havoc with the ability to reason about the language in simple terms, because it destroys the notion that equals can be substituted for equals. For example, three is one plus two, but the word “three” is not the phrase “one plus two.” Quotation is powerful because it gives us a way to build expressions that manipulate other expressions (as we will see when we write an interpreter in [[Chapter 4]]). But allowing statements in a language that talk about other statements in that language makes it very difficult to maintain any coherent principle of what “equals can be substituted for equals” should mean. For example, if we know that the evening star is the morning star, then from the statement “the evening star is Venus” we can deduce “the morning star is Venus.” However, given that “John knows that the evening star is Venus” we cannot infer that “John knows that the morning star is Venus.”

This highlights a fundamental challenge in programming languages and logic. Quotation introduces layers of meaning that complicate substitution principles. It emphasizes the need for careful semantics in language design to ensure clarity and coherence, especially when reasoning about knowledge and truth in expressions.