SmallTalk: pure OO

everything's an object

  • ''You could think of Smalltalk's object model as Java on steroids''
  • common superclass Object
  • block construct to treat enclosed code as if it was an object:   [n <- n+1]

but...

SmallTalk highly supports features of functional languages, such as
  • higher order functions
  • first-class types

communication via messages

  • parameters passed by reference
  • idealized: there is only one 2; every smallInteger with value 2 actually is a reference to that object
Jack pass: theBall to: Jill

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Language Properties

scoping, binding, types

scoping

static, one global name space
no private classes -- problematic in code reuse!

binding

dynamic (messages are bound to methods at run-time)
 →Polymorphism (especially overloading) is supported

does SmallTalk have types?

  • Variables are untyped, checking (and exception-raising in case of type errors) happens at run-time
  • everything is of type Object, possibly subclassed
  • they appear not to be necessary in the SmallTalk language concept...
Classes and Arrays of ... can be used to create new types.

memory management

stack

.. for procedure execution, as usual

dynamic for data

  • scavenging to reclaim objects with short lifetime
  • global garbage collection (mark-and-sweep) to reclaim objects with long lifespan

other features

compiled, then executed on a VM

assertion

is supported, but beware of side effects inside assertions!

concurrency

threads can be forked out, then run until they're done

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Example

Programming Language Pragmatics, p580:
gcd: other
   [self = other]
     ifTrue: [↑self].
   [self < other]
     ifTrue: [↑self gcd: (other-self)]
     ifFalse: [↑other gcd: (self-other)]


© Amelie Stein