chapter 5

Dr. Muhammed Al-Mulhem

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Chapter 5

The Algol Family and Haskell


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Algol Family

The Algol family developed in parallel with Lisp languages to the late development of Haskell and Modula.

The main characteristics of the Algol family are: Colon-separated sequence of statements. Block structure Functions and procedures Static typing.


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Algol 60

Algol 68

ML Modula

Lisp

Many other languages:Algol 58, Algol W, Euclid, EL1, Mesa, Modula-2, Oberon, Modula-3

Pascal

Haskell

Language Sequence


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Designed between 1958 and 1963 by a committee that include many computer pioners such as:John McCarthy (designer of Lisp)

John Backus (designer of Fortran)

Intended to be a general purpose language.

Algol 60


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Important features of Algol 60 Simple imperative language + functions Successful syntax, BNF -- used by many successors

statement oriented begin … end blocks (like C { … } ) if … then … else

Recursive functions and stack storage allocation Fewer ad hoc restrictions than Fortran

General array references: A[ x + B[3]*y] Procedures with procedure parameters.

A primitive static type system, improved by later languages (Algol 68, and Pascal)

Very influential but not widely used in US

Tony Hoare: “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 of its successors.”

Algol 60


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real procedure average(A,n);

real array A; integer n;

begin

real sum; sum := 0;

for i = 1 step 1 until n do

sum := sum + A[i];

average := sum/n

end;

no ; here

no array bounds

set procedure return value by assignment

Algol 60 Sample


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Question: Is x := x equivalent to doing nothing?

Interesting answer in Algolinteger procedure p;begin

….p := p

….end;

Assignment here is actually a recursive call

Algol 60 Oddity


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Holes in type discipline Parameter types can be arrays, but

No array bounds Parameter types can be procedures, but

No argument or return types for procedure parameters

Problems with parameter passing mechanisms Pass-by-name “Copy rule” duplicates code,

interacting badly with side effects (How?). Pass-by-value expensive for arrays

Some awkward control issues goto out of block requires memory management

(Why?).

Some trouble spots in Algol 60


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Substitute text of actual parameterUnpredictable with side effects!

Exampleprocedure inc2(i, j);

integer i, j;begin

i := i+1; j := j+1

end;inc2 (k, A[k]);

begin

k := k+1; A[k] := A[k]

+1end;

Is this what you expected?

Algol 60 Pass-by-name


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Considered difficult to understandnew terminology

Types were called “modes” Arrays were called “multiple values”

One contribution of Algol68 is its type system Type construction can be combined without

restriction (array of pointers to procedures).Eliminated pass-by-name

Not widely adopted

Algol 68


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Primitive modes int real char bool string compl (complex) bits bytes sema (semaphore) format ( for I/O) file

Compound modes arrays structures procedures sets pointers

Rich, structured, and type system is a major contribution of Algol 68.

Algol 68 Modes


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Storage management Stack for Local variables Heap storage for data intended to live beyond the

current function call. As in C, data on the heap is explicitly allocated. But unlike C, heap data are recalimed by garbage

collection. This combination of explicit allocation and garbage

collection carried over to Pascal

Parameter passing Pass-by-value Use pointer types to obtain pass-by-reference (like C)

Source: Tanenbaum, Computing Surveys

Advances in Algol 68


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Designed by Niklaus Wirth (Turing Award) in the 1970s

Revised the type system of AlgolGood data-structuring concepts

records, variants, subrangesMore restrictive than Algol 60/68

Procedure parameters cannot have procedure parameters

Popular teaching language

Simple one-pass compiler

Niklaus Wirth

Pascal


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Array bounds part of type procedure p(a : array [1..10] of integer) procedure p(n: integer, a : array [1..n] of integer)

Example: A procedure of the form:

procedure p(a : array [1..10] of integer)

May be called only with an array of length 10 as an actual parameter.

Later versions of Pascal removed this limitation.

illegal

Limitations of Pascal


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Designed by Dennis Ritchie, Turing Award winner, for writing Unix

Evolved from B, which was based on BCPL B was an untyped language; C adds some checking

Relationship between arrays and pointers An array is treated as a pointer to first element E1[E2] is equivalent to ptr dereference: *((E1)+(E2)) Pointer arithmetic is not common in other languages

Ritchie quote “C is quirky, flawed, and a tremendous success.”


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Statically typed, general-purpose programming language

Type safe!

Intended for interactive use

Combination of Lisp and Algol-like features Expression-oriented Higher-order functions Garbage collection Abstract data types Module system Exceptions

Designed by Turing-Award winner Robin Milner for LCF theorem prover

Used in textbook as example language


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Haskell is a programming language that is Similar to ML: general-purpose, strongly typed, higher-

order, functional, supports type inference, supports interactive and compiled use

Different from ML: lazy evaluation, purely functional, rapidly evolving type system.

Designed by committee in 80’s and 90’s to unify research efforts in lazy languages. Haskell 1.0 in 1990, Haskell ‘98, Haskell’ ongoing. “A history of Haskell: Being lazy with class” HOPL 3

Paul Hudak

John HughesSimon Peyton Jones

Phil Wadler


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Good vehicle for studying language conceptsTypes and type checking

General issues in static and dynamic typing Type inference Polymorphism and generic programming

Control Lazy vs. eager evaluation Tail recursion and continuations Precise management of effects


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Functional programming will make you think differently about programming.Mainstream languages are all about stateFunctional programming is all about values

Ideas will make you a better programmer in whatever language you regularly use.

Haskell is “cutting edge.” A lot of current research is done in the context of Haskell.

chapter 5

Documents

algol w

algol 60ics535

later languages algol

major contribution of

important features of

type disciplineparameter

procedure parametersproblems

eliminated pass