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Why is Java so popular?Floundered until adapted for internet.

90% of personal computers have JavaRun on client – so load to server is

drastically reducedPortable – cross platformObjected oriented – leads to

extensibilityBuilt in securityRemoves undesirable features of C++Hype

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Microsoft's C# is Java without the reliability, productivity or security - They had this problem in their design rules that they had to support C and C++, which means you have to have a memory model where you can access everything at all times. It's the existence of those loopholes that is the source of security, reliability and productivity problems for developers.

(Gosling, the inventor of Java, is also a creator of Netbeans)

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Why is C++ popular Criticisms (too high or too low):

– writing efficient code in C is easier than C++ because the level of information hiding hides inefficiencies

– its powerful features and complexities are often the source of many errors.

– C++ suffers under the burden of backwards compatibility Success

– dominant language for game dev– C++ adds layers of abstraction to C– multi-paradigm (generic, imperative, OO)– large audience of C programmers. Early C++ compilers

just emitted C code.– efficiency– willingness and readiness of compiler vendors to add

another language to their packages

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Chapter 3 - Language Design Principles

Programming Languages:

Principles and Practice, 2nd Ed.

Kenneth C. Louden

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Thought question

What characteristics should be present in your “perfect language”?

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The language design problem

Language design is difficult, and success is hard to predict:– Pascal a success, Modula-2 (successor

to Pascal – fixed syntactic ambiguity and added modules) a failure

– Algol60 a success, Algol68 a failure– FORTRAN a success, PL/I a failure

Conflicting advice

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Efficiency The “first” goal (FORTRAN): execution efficiency. Still an important goal in some settings (C++, C). Many other criteria can be interpreted from the point of

view of efficiency:– programming efficiency: writability or expressiveness

(ability to express complex processes and structures)– reliability (software doesn’t crash, is secure).– maintenance efficiency: readability.

(saw this as a goal for first time in Cobol)

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Reliability and Programming Language?

Hurts bad syntax source of bugs

(if a=b when mean if a==b) complex syntax distracts the

programmer (errors are more likely)

Name hiding - confusion machine dependencies (length

of Integer could cause different results when ported)

Helps rigorous type checking eliminate undefined

variables (always init) runtime checks (array

bounds, types) simple syntax less implicit computation

(force explicit casts) compiler helps (check for

declared but not used) correct return value types

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Other kinds of efficiency

efficiency of execution (optimizable)efficiency of translation. Are there

features which are extremely difficult to check at compile time (or even run time)? e.g. Algol68 – solved dangling else problem (case/esac if/fi, do/od closing)

Implementability (cost of writing translator)

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Features that aid efficiency of execution

Static data types allow efficient allocation and access.

Manual memory management avoids overhead of “garbage collection”.

Simple semantics allow for simple structure of running programs (class variables, run time inheritance type checking - expensive).

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Visit with you neighbor: can you see any conflicts with efficiency?

Writability, expressiveness: Software reliability, efficiency: Expressiveness, writability, readability:

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Some goals conflict with efficiency Writability, expressiveness:

– idea: no static data types (variables can hold anything, no need for type declarations). [harder to maintain]

Reliability, efficiency: – idea: automatic memory management (no need

for pointers). [runs slower] Expressiveness, writability, readability:

– idea: more complex semantics, allowing greater abstraction. [harder to translate]

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Internal consistency of a language design: Regularity Regularity is a measure of how well a language

integrates its features, so that there are no unusual restrictions, interactions, or behavior. Easy to remember.

Regularity issues can often be placed in subcategories:– Generality: combining closely related constructs into

a single more general one. Ex. Ruby collections/ranges/ arrays/files all have each

– Orthogonality (independence): are there strange interactions?

– Uniformity: Do similar things look the same, and do different things look different?

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Generality deficiencies

In pascal, procedures can be passed as parameters, but no procedure variable to store them (not a true object).

Pascal has no variable length arrays –length is defined as part of definition (even when parameter). This is a much bigger deal than you would think.

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Orthogonality: independence Not context sensitive

Seems similar to “generality” but more of an “odd” decision rather than a limitation.

For example, if I buy a top:– sweater, sweatshirt, shirt

– short sleeve, long sleeve, or sleeveless– small, medium, or large– red, green, or blue

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Orthogonal – independent decisions

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  Sweater Sweatshirt Shirt

Size S,M,L,XL S,M,L,XL S,M,L,XL

Sleeve Length Short/Long Short/Long Short/Long

Color RGB RGB RGB

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Non-Orthogonal – dependent decisions (if sweater, no choice on sleeve length)

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  Sweater Sweatshirt Shirt

Size S,M,L,XL M,L,XL (no S) S,M,L,XL,XXL

Sleeve Length Long (no Short) Long (no Short)Sleeveless/Short/Long

Color RGB Yellow, Purple RGB RGB

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Orthogonalitya relatively small set of primitive constructs can be combined

in a relatively small number of ways. Every possible combination is legal.

Is orthogonal if it can be used without consideration as to how its use will affect something else (no side effects)

Language features are orthogonal when they can be understood in isolation and freely combined with no surprises. Orthogonality thus allows a simple definition to cover lots of possibilities.

Orthogonality makes it easier to understand what happens when things combine.

Is closely related to simplicity - the more orthogonal, the fewer rules to remember.

Can make implementation more complicated as implement things that might not really need.

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Orthogonal: Alternative definition Cleanly integrated features: All features must be cleanly and elegantly

integrated into the language.

Composability of features: It must be possible to use features in combination to achieve solutions that would otherwise have required extra separate features. Ex. Arrays of records

Avoid special purpose features: There should be as few spurious and "special purpose" features as possible.

Performance independence: A feature should be such that its implementation does not impose significant overheads on programs that do not require it. Ex. array element deletion

Understatement independence: A user need only know about the subset of the language explicitly used to write the program. Ex. reserve word

Understatement is a form of speech which contains an expression of less strength than what would be expected.

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OperatorsBad: For example - in IBM assembly language there are

different instructions for adding memory to register or register to register (non-orthogonal).

Good: In Vax, a single add instruction can have arbitrary operands.

Orthogonality seems to be in opposition with security, since it allows potential errors to go unnoticed

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Most Languages

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contains array records files

arrays yes yes no 

records yes yes no 

files yes res  no

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Pascal

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Functions Integers Float Arrays Strings

input parameters Yes yes yes yes

return values Yes yes no yes

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If not allow user to overload operators

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Operator Use integer float complexstrings

User defined types

  + yes yes yes yes no

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Most Languages

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Switch integer Boolean Float string

Case Labels yes yes no no

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Java – non orthogonalities

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Declarations Type = intType = SomeClass

Type ii is variable of type integer

i is a pointer to SomeClass type object

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Thought question

Where does non-orthogonality come from? If it is “bad design”, why not eliminate it?

How would you rate Ruby in terms of regularity?

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For examples of non-orthogonality consider C++:

– We can convert from integer to float by simply assigning a float to an integer, but not vice versa. (not a question of ability to do – generality, but of the way it is done)

– Arrays are pass by “what appears to be” reference while integers are pass by value.

– A switch statement works with integers, characters, or enumerated types, but not doubles or Strings.

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Regularity examples from C++ Declarations are not uniform: data

declarations must be followed by a semicolon, function declarations must not.

Lots of ways to increment – lack of uniformity (++i, i++, i = i+1)

i=j and i==j look the same, but are different. Lack of uniformity

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Other design principles

Simplicity: make things as simple as possible, but not simpler (Einstein). (Pascal, C)

We can make things so simple that it doesn’t work well – no string handling, no reasonable I/0 (some languages left out of documentation)

Can be cumbersome to use or inefficient.

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Other design principles

Expressiveness: make it possible to express conceptual abstractions directly and simply. (Scheme)

Helps you to think about the problem. Ruby, for example, allows you to return

multiple arguments:

a,b= swap(a,b)

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Other design principles Extensibility: allow the programmer to

extend the language in various ways.

Types, operators, create libraries, extend built in classes

Security: programs cannot do unexpected damage. (Java)– discourages errors– allows errors to be discovered– type checking

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Other design principles (cont.) Preciseness: having a language definition

that can answer programmers and implementers questions.

If it isn’t clear, there will be differences.Example: Declaration in local scope (for loop)

unknown/known after exitExample: implementation of switch statementExample: constants – expressions or not?Example: how much accuracy of float?Example: order of operations

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Other design principles (cont.) Machine-independence: should run the same on any

machine. (Java- big effort) Consistent with accepted notations – easy to learn and

understand for experienced programmers (Most languages today, but not Smalltalk & Ruby)– 42 factorial (sends 42 to function factorial – Smalltalk)– Symbols, yield, leave off parens in method calls, use

of $1

Restrictability: a programmer can program effectively in a subset of the full language. (C++: avoids runtime penalties)

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Wikipedia moment: Syntactic sugar is a term coined by Peter J.

Landin for additions to the syntax of a computer language that do not affect its expressiveness but make it "sweeter" for humans to use.

Syntactic sugar gives the programmer (designer, in the case of specification computer languages) an alternative way of coding (specifying) that is more practical, either by being more succinct or more like some familiar notation.

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C++ case study

Thanks to Bjarne Stroustrup, C++ is not only a great success story, but also the best-documented language development effort in history:– 1997: The C++ Programming Language,

3rd Edition (Addison-Wesley).– 1994: The Design and Evolution of C++

(Addison-Wesley).– 1993: A History of C++ 1979-1991,

SIGPLAN Notices 28(3).

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Major C++ design goals

OO features: class, inheritanceStrong type checking for better

compile-time debuggingEfficient executionPortableEasy to implementGood interfaces with other tools

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Supplemental C++ design goals C compatibility (but not an absolute goal:

no gratuitous incompatibilities) Incremental development based on

experience. No runtime penalty for unused features. Multiparadigm Stronger type checking than C Learnable in stages Compatibility with other languages and

systems

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C++ design errorsToo big?

– C++ programs can be hard to understand and debug

– Not easy to implement– Defended by Stroustrup: multiparadigm

features are worthwhileNo standard library until late (and

even then lacking major features)– Stroustrup agrees this has been a major

problem

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Review: Language Design Features Efficiency

-execution

-translation

-implementation Reliability Readability Writability Expressiveness

Regularity

-generality,

-orthogonality

-uniformity Simplicity Extensibility Security Preciseness

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