CSE 3302CSE@UTA

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CSE3302Programming Languages(notes continued)

• Dr. Carter Tiernan

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Generality and HierarchyAlgol-60

• Close as possible to standard mathematical
  notation and readable
• Usable for describing computing processes
• Mechanically translatable to machine language

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Algol Design

• Hierarchical structure / Nesting
• (unlike C C)
• Declarative and Imperative constructs
• Variable, procedure, and switch
• Computational and control-flow
• Compilation handled differently than Fortran
• Stack for run-time structures

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Algol Name Structures

• Names are NOT statically bound
• Block structure supports scoping
• Blocks are compound statements
  with declarations
• Scope - contour diagrams
• Context
• Inner blocks implicitly inherit access to
  variables in its immediately surrounding block

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Algol Blocks

• Allows shared data to be easily organized with
  the subprograms that need it (but does allow
  indiscriminate access)
• Scoping choices
• Static called in the environment of its
  definition (used exclusively in Algol)
• Dynamic called in the environment of its caller

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Dynamic Scoping

• Called in environment of caller
• Can write general procedures that make use of
  variables and procedures supplied by callers
  environment
• Vulnerable to being called from an environment
  without correct supporting procedures
• Not generally used in current languages

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Static Scoping

• Called in the environment of its definition
• Requires the user to pass parameters of local
  environment
• Cannot behave differently based on call location
  only
• Ensures that the static structure of the program
  more closely agrees with the dynamic behavior of
  the program.

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Algol Stacks for management

• Disjoint blocks allow conservation of memory on
  the stack
• Assumption a variable is retained only so long
  as the program is executing in the scope of that
  variable or in a block that will return to the
  scope of that variable
• Blocks follow LIFO execution ergo, a
  stack handles the activation records in
  a LIFO fashion

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Responsible Design

• Understand the programmers problem
• Provide a high quality solution
• Which is not necessarily the solution the
  programmers (or users) asked for
• Responsible Design Principle
• Caveat If the user wants it and you dont
  supply it (in their fashion), they may not accept
  your alternate (even if better) solution

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Algol Data Structures

• Primitives are scalars
• Boolean, integer, and real
• No double precision because its machine
  dependent
• No complex due to tradeoffs of usefulness vs.
  complexity
• Limited string type that required procedures
  coded in some other language in order to be
  output or used

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Zero-One-Infinity Principle

• Supports regularity - easier to learn, remember,
  and master
• Eliminates arbitrary numerical limits
• Applies to arrays in
• Number of dimensions
• Start and end values for indices

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Algol Arrays

• Generalized
• Dynamic due to stack allocation

Data Types

• Strong typing
• Coercions and conversions
• Type system as safety feature

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Algol Control structures

• Primitives
• Assignment
• No input/output - handled with library routines
• Structures
• Regularize the FORTRAN structures
• if condition then statement1 else statement2
• for initialization keyword test do statement

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Algol Control and nesting

• All control structures can handle arbitrary
  number of bracketed statements
• Compound statements formed with begin and end
• Algol structuring led to Dijkstras letter, GOTO
  Considered Harmful

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Algol More control

• Hierarchical structure
• Structured programming
• Recursion supported
• Instantiation of locals
• Every call creates a new activation record
• Conditional expression

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Algol Parameter passing

• Passing by value
• Local copy of parameter
• Requires keyword value
• Pass by name
• Default mode
• Based on substitution - Copy Rule
• Not actually substituted for formal params

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Algol Pass by name

• Compare to pass by reference
• Name parameters bound to string
• Jensens device
• Use pass by name with index variable to cycle
  through array elements
• Implementation - thunk
• Compile parameter
• Pass address of compiled parameter code
• Inconsistent behavior

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Language Design Issues

• GO TO controversy
• Conceptual models
• System image
• Feature interaction

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Algol Control (more)

• Out of block GO TOs
• Levels of activation must be popped
• For-loop
• For-list-elements reevaluated every pass
• Step, until
• While
• Enumerated list
• All of the above
• Baroque

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Algol Control etal.

• Switch declaration
• Array of statement labels
• Processes cases - not necessarily distinct
• Best use recombines after cases
• Difficult to correlate with dynamic structure
• Baroque

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Algol Syntactic Structures

• Machine independent / portable
• Free-format
• Programmers had to think about how it should look
• Reflect the structure of the program
• Independent of character sets
• Reference language
• Publication language
• Hardware representation

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Algol Lexics

• Lexical conventions
• Reserved words
• Keywords (Algol approach)
• Keywords in context
• Dangling else
• Style

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Backus-Naur Form (BNF)

• Precise programming language descriptions
• English difficult to read
• Examples incomplete
• Perceptual clarity with precision
• Backus - syntactic categories
• Naur added improvements to notation
• Metalanguage

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BNF

• Terminals
• Nonterminals
• Defined in terms of nonterminals and terminals
• Recursive definition allowed
• Related to Chomsky language hierarchy
• Context-free grammar
• Regular (non-recursive) grammar

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Design Criteria

• Efficiency - scientific issues
• Minimize resources
• Maximize safety
• Economy - social issues
• Maximize social benefit
• Minimize cost
• Elegance - aesthetic issues
• Function follows form
• Manifest interactions

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Second Generation Languages

• Elaborations and generalizations of first
  generation languages
• Stronger typing
• Hierarchical structure
• Blocks
• Control flow
• Recursion
• Parameter passing modes
• Free-format syntax
• Machine independent conventions
• Reserved word/ Keywords

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