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Programming Language Theory

Programming Language Theory. Leif Grönqvist The national Graduate School of Language Technology (GSLT) MSI. Contents. Leif’s three parts of the course: Functional programming Logical programming Similar ways of thinking, but different from the imperative way Formal semantics.

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Programming Language Theory

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  1. Programming Language Theory Leif Grönqvist The national Graduate School of Language Technology (GSLT) MSI

  2. Contents Leif’s three parts of the course: • Functional programming • Logical programming Similar ways of thinking, but different from the imperative way • Formal semantics

  3. Your background • Quite good at programming(?) • Not familiar to functional/logical programming(?) • Which languages do you use?

  4. Functional programming • Why? • We want to make programming easier and more effective • Some important features/drawbacks compared to imperative programming • Programs are functions • Functions may be data • Modularity, using higher order functions • No side effects

  5. Functional pr. cont. • Features/drawbacks • Automatic memory management (Java has it too, but not C/C++) • Not widely used in the industry – still true but for example Erlang is used professionally at Ericsson. Look at Joe Armstrong’s webcasted lecture about Erlang • Inefficiency in execution. Les and less important: • Computers get more and more powerful

  6. Performance • The compilers are getting better:

  7. The languages • C - gcc 3.0, Imperative • C++ - g++ 3.0, Imperative and oo • Java - Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1-b24), Imperative and oo • Mercury - Mercury 0.10.1, Logic • SML - MLton 20010706, Functional • Ocaml - The Objective Caml, v 3.04, Functional and oo • GHC - The Glorious Glasgow Haskell Compilation System, version 5.00.1, Purely functional and lazy

  8. The tests • Acker - A call to the Ackermann function with arguments 3, 8 • Sieve - Calculate primes with Sieve of Eratosthene • Hash - Insert and retrieve some values using a hash table • Array - Do some array access • FileReverse - Reverse i file • HeapSort - Sort a list using heap sort • NestLoop - Nested loops • Rank - Average ranking for all tests

  9. History • The first, and most spread, functional language is Lisp (late 50’s) • List = LISt Processor • Not strongly typed • A popular newer dialect is Scheme • Many functional enthusiasts do not agree that Lisp is functional, due to things like assignment and other imperative constructions • We will not talk more about Lisp/Scheme • Take a look at it in the book if you want

  10. Lambda Calculus • Published by Alonzo Church 1930 • Mathematical formalism expressing computing by functions • Same power as a Turing Machine • The idea behind the functional languages

  11. Syntax expression -> constant | variable | (expression expression) application | (λ variable . expression) lambda abstraction (λx . + 1 x) 2 -> (+ 1 2) -> 3

  12. Problems (λ x . x x) (λ x . x x) -> (λ x . x x) (λ x . x x) -> … Will not terminate, but (λy.2) ((λ x . x x) (λ x . x x)) will terminate if we use so called ”normal order evaluation” (lazy evaluation) If we use ”applicative evaluation order” it will not terminate

  13. Basic principles • Functional programming style possible in many languages • In a purely functional language you think functional all the time: • A program is a black box, a function f: X->Y where X is the domain and Y the range • No side effects • Recursion is the only way to make iteration and loops • Strong typing: passing functions as argument is easier and more flexible than in C for example • Higher order functions give modularity • Many functional languages look similar, we will use Haskell from now

  14. Haskell • First version in the en of the 80’s at Yale and Glasgow • Standardized in 1998 • Some new things compared to the older Miranda and ML • Monads (solves I/O handling) • Function overloading • Purely functional • Fully Curried • Lazy (gives lazy infinite lists!)

  15. Some small examples fact 0 = 1 fact n = n * fact (n-1) square x = x * x gcd u 0 = u gcd u v = gcd v (u `mod` v) reverse [] = [] reverse (h:t) = reverse t ++ [h]

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