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< BayHac2013
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*The Next 700 Programming Languages*:
- By Peter Landin in 1966. Enormously important figure in improving our understanding of programming languages.
- Prime move with Algol, about which Tony Hoare said “Here is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all its successors.”
- Realized that lambda calculus can model PLs.
- Worked out how to mechanically evaluate lambda calculus.

- The seminal paper on DSELs
- Led to the lambda-calculus-based languages, including Scheme, ML, and Haskell.
- Recommends replacing the fuzzy terms “functional”, “declarative”, and “non-procedural” with the substantive “denotative”.
- “… gives us a test for whether the notation is genuinely functional or merely masquerading.”

- Denotative:
- Give meaning by mapping to a mathematical type
- Meaning of a composite is a function of the meanings of the components
- Applies to:
- Programming
*languages*
- Programming
*libraries*:
- Give your data type a
*meaning* (model). Examples: `Image`

means function of space; `Animation`

means function of time; `Sequence`

means list.
- Everything operation your data type must be explainable via the meaning.
- Guides API, and defines correctness.
- Reason on meanings, not on implementations.
- For instance:
- Is append on tricky-sequences associative?
- Is my type a monoid, functor, applicative, etc?

- Strong & simple (practical) foundation for (correct) reasoning

- What about Haskell?
- “Purely functional”? Ill-defined question.
- “Purely denotative”? If not, what parts are and what parts are not?