10/21/08 cs242 � Lisp � Algol 60 � Algol 68 � Pascal � Kathleen Fisher � ML � Modula � Haskell � Reading: “Concepts in Programming Languages” Chapter 5 except 5.4.5 � “Real World Haskell”, Chapter 0 and Chapter 1 � (http:/ /book.realworldhaskell.org/) � Many other languages: � Algol 58, Algol W, Euclid, EL1, Mesa (PARC), … � Modula-2, Oberon, Modula-3 (DEC) � Thanks to John Mitchell and Simon Peyton Jones for some of these slides. � Basic Language of 1960 � Simple imperative language + functions � real procedure average(A,n); Successful syntax, BNF -- used by many successors � No array bounds. � real array A; integer n; statement oriented � begin … end blocks (like C { … } ) � begin if … then … else � real sum; sum := 0; Recursive functions and stack storage allocation � for i = 1 step 1 until n Fewer ad hoc restrictions than Fortran � do General array references: A[ x + B[3] * y ] sum := sum + A[i]; Type discipline was improved by later languages � No “;” here. � average := sum/n Very influential but not widely used in US � end; Tony Hoare: “Here is a language so far ahead of its time that it was not only an improvement on its predecessors Set procedure return value by assignment. � but also on nearly all of its successors. ” � Question: � Holes in type discipline � Parameter types can be arrays, but � Is x := x equivalent to doing nothing? � No array bounds � Interesting answer in Algol: � Parameter types can be procedures, but � No argument or return types for procedure parameters � integer procedure p; Problems with parameter passing mechanisms � begin …. Pass-by-name “Copy rule” duplicates code, p := p interacting badly with side effects � …. Pass-by-value expensive for arrays � end; Some awkward control issues � goto out of block requires memory management � Assignment here is actually a recursive call! � 1
10/21/08 Substitute text of actual parameter Considered difficult to understand � Unpredictable with side effects! Idiosyncratic terminology � Types were called “modes” � Example Arrays were called “multiple values” � Used vW grammars instead of BNF � procedure inc2(i, j); Context-sensitive grammar invented by van Wijngaarden � integer i, j; Elaborate type system � begin begin Complicated type conversions � i := i+1; k := k+1; j := j+1 A[k] := A[k] +1 Fixed some problems of Algol 60 � end; end; Eliminated pass-by-name � inc2 (k, A[k]); Not widely adopted � Is this what you expected? � Adr Adriaan aan v van an Wijn jngaa aarden den � Primitive modes � Compound modes � Storage management � int � arrays � Local storage on stack � real � structures � char � Heap storage, explicit alloc, and garbage collection � procedures � bool � sets � Parameter passing � string � pointers � compl (complex) � Pass-by-value � bits � Use pointer types to obtain pass-by-reference � bytes � sema (semaphore) � Assignable procedure variables � Rich, structured, and format (I/O) � orthogonal type system is Follow “orthogonality” principle rigorously � file � a major contribution of Algol 68. A Tutorial on Algol 68 by Andrew S. Tanenbaum � Designed by Niklaus Wirth (Turing Award) � illegal � Array bounds part of type � Revised the type system of Algol � procedure p(a : array [1..10] of integer) Good data-structuring concepts � procedure p(n: integer, a : array [1..n] of integer) records, variants, subranges � Attempt at orthogonal design backfires � More restrictive than Algol 60/ 68 � – Parameter must be given a type � Procedure parameters cannot have procedure – Type cannot contain variables � parameters � How could this have happened? Emphasis on teaching? � Popular teaching language � Not successful for “industrial-strength” projects � Simple one-pass compiler � Kernighan: “Why Pascal is not my favorite language” � Left niche for C; niche has expanded!! � Niklaus Wirth � 2
10/21/08 ML ML Statically typed, general-purpose programming language � Type safe! � Designed by Dennis Ritchie, Turing Award winner, for writing Unix � Intended for interactive use � Combination of Lisp and Algol-like features � Evolved from B, which was based on BCPL � Expression-oriented � B was an untyped language; C adds some checking � Higher-order functions � Garbage collection � Relationship between arrays and pointers � Abstract data types � An array is treated as a pointer to first element � Module system � Exceptions � E1[E2] is equivalent to ptr dereference: *((E1)+(E2)) Pointer arithmetic is not common in other languages � Designed by Turing-Award winner Robin Milner for LCF Theorem Prover � Ritchie quote � Used in textbook as example language � “C is quirky, flawed, and a tremendous success. ” � Hask Ha skel ell Haskell is a programming language that is � Good vehicle for studying language concepts � Similar to ML: general-purpose, strongly typed, higher-order, functional, supports type inference, supports interactive and Types and type checking � compiled use � General issues in static and dynamic typing � Different from ML: lazy evaluation, purely functional, rapidly Type inference � evolving type system. � Parametric polymorphism � Designed by committee in 80’ s and 90’ s to unify Ad hoc polymorphism � research efforts in lazy languages. � Control � Haskell 1.0 in 1990, Haskell ‘98, Haskell’ ongoing. � Lazy vs. eager evaluation � “A History of Haskell: Being Lazy with Class” HOPL 3 � Tail recursion and continuations � Precise management of effects � Paul Hudak � Simon Peyton Jones � John Hughes � Phil Wadler � Practitioners � Functional programming will make you think 1,000,000 differently about programming. � Mainstream languages are all about state � 10,000 Functional programming is all about values � Ideas will make you a better programmer in 100 whatever language you regularly use. � Geeks � The quick death � 1 Haskell is “cutting edge. ” A lot of current research is done in the context of Haskell. � 1yr 5yr 10yr 15yr 3
10/21/08 Threshold of immortality � Practitioners � Practitioners � 1,000,000 1,000,000 10,000 10,000 The complete absence of death � 100 100 The slow death � Geeks � Geeks � 1 1 1yr 5yr 10yr 15yr 1yr 5yr 10yr 15yr “Learning Haskell is a great way of training yourself to think functionally so you are ready to take full advantage of “I'm already looking at coding In Haskell, f :: A → B means for every x ∈ A, � C# 3.0 when it comes out” � Practitioners � problems and my mental (blog Apr 2007) � perspective is now shifting 1,000,000 back and forth between purely OO and more FP styled f(x) = some element y = f(x) ∈ B � solutions” � (blog Mar 2007) � run forever � 10,000 100 The second life? � In words, “if f(x) terminates, then f(x) ∈ B. ” � Geeks � In ML, functions with type A → B can throw an 1 exception, but not in Haskell. � 1990 1995 2000 2005 2010 Functions that take other functions as arguments or Interactive Interpretor (ghci): read-eval-print � return as a result are higher-order functions. � ghci infers type before compiling or executing � Common Examples: � Type system does not allow casts or other loopholes! � Map: applies argument function to each element in a collection. � Examples � Reduce: takes a collection, an initial value, and a function, and combines the elements in the collection according to the function. � Prelude> (5+3)-2 6 list = [1,2,3] it :: Integer r = foldl (\accumulator i -> i + accumulator) 0 list Prelude> if 5>3 then “Harry” else “Hermione” “Harry” Google uses Map/Reduce to parallelize and distribute it :: [Char] -- String is equivalent to [Char] massive data processing tasks. Prelude> 5==4 False (Dean & Ghemawat, OSDI 2004) � it :: Bool 4
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