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CSE 3341: Principles of Programming Languages
PL OverviewJeremy Morris
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Why are we here?
Quick quiz: How many programming languages do you know? How many
programming languages do you think you will know
ten years from now? How many programming languages do you think
you will need to
know before you retire? Who thinks they will never have to? How
many programming languages do you think there are?
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Why are we here?
How many programming languages do you think there are? Hard to
say – new ones come along constantly The “99 Bottles of Beer”
website currently has implementations of
the famous song in over 1500 languages Many variants of the same
languages (BASIC, FORTRAN, etc.) but
many distinct languages as well Why do programming languages
exist?
Why are there so many of them?
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http://www.99-bottles-of-beer.net/p.html
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Why are we here?
Understanding language design and implementation can help you
learn new languages. See connections between languages in similar
families. Understand the common things that all languages share.
Understand why languages are designed in certain ways Be able to
have intelligent and informed opinions on
programming languages Be able to chose the right tool for the
right job
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Why are we here?
A few other reasons to study programming languages To understand
what the machine is doing when it compiles your
code. To be able to understand features in languages outside of
your
own experience. And possibly think of ways to simulate them in a
language that
doesn’t have them. To be able to make intelligent choices about
your
implementations. Some solutions may be better or worse in the
language of your
choice. To understand the algorithms used for programming
languages
and apply them to other tasks.
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How do we study a language?
Syntax What do legal programs written in the language look
like?
Semantics What do the various instructions in the language do
when
executed?
Methodology How are you supposed to use the language? What
features does it have? What kinds of problems is it supposed to
solve?
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Types of Programming Languages We often talk about two different
categories of
programming languages: Imperative Declarative
The author also talks about different subcategories of
programming languages: Functional Logic Object-oriented Scripting
von Neumann
Never heard of this one? Reasons for that. “Fish don’t have a
word for water…”
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Declarative vs. Imperative
Imperative Tell the computer how to perform an action Tied to
control flow – do this, then do this, then do this
Declarative Describe what the result should look like Abstracted
from control flow (to a degree anyway) A few different points of
view on what “declarative” means:
“Not imperative” – which is not helpful Sometimes described as
“rule based” or “function based” Often has direct ties to
mathematics – especially logic Lacking in “side effects” (or at
least minimizing side effects)
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Imperative example - Javapublic static int gcd(int a, int b)
{
while (a != b) {if (a > b) {
a = a – b;
}
else {b = b – a;
}
}
return a;}
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Declarative example - PROLOG
gcd(A,B,G) :- A = B, G = A.
gcd(A,B,G) :- A > B, C is A-B, gcd(C,B,G).
gcd(A,B,G) :- B > A, C is B-A, gcd(C,A,G).
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Declarative example - SQL
SELECT Name, Title, IdNum
FROM PEOPLE
WHERE DeptNo = 6 AND Salary > 40000;
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Functional example - Scheme
(define gcd
(lambda (a b)
(cond ( (= a b) a)
( (> a b) (gcd (- a b) b))
( else (gcd (- b a) a)))))))
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Compilation
Abstract overview of compilation
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SourceCode Compiler
Object Code
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Compilation (more formally)
𝑃𝑃𝐿𝐿 - Program in language L 𝑃𝑃𝑀𝑀 - Program in language M
𝐶𝐶𝐿𝐿→𝑀𝑀𝑀𝑀 - Compiler in language M that translates L to M
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𝑃𝑃𝐿𝐿 𝐶𝐶𝐿𝐿→𝑀𝑀𝑀𝑀 𝑃𝑃𝑀𝑀
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Execution (run time)
At runtime, the program written in machine languageexecutes
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𝑃𝑃𝑀𝑀Input Data Output Behavior
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A new language?
Suppose we now have a new language L’ We could write a new
compiler Or…
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𝑃𝑃𝐿𝐿 𝐶𝐶𝐿𝐿→𝑀𝑀𝑀𝑀 𝑃𝑃𝑀𝑀
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A new language?
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𝑃𝑃𝐿𝐿 𝐶𝐶𝐿𝐿→𝑀𝑀𝑀𝑀 𝑃𝑃𝑀𝑀
𝑃𝑃𝐿𝐿′ 𝐶𝐶𝐿𝐿′→𝐿𝐿𝑀𝑀 𝑃𝑃𝐿𝐿
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Intermediate Language
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𝑃𝑃𝐼𝐼 𝐶𝐶𝐼𝐼→𝑀𝑀𝑀𝑀 𝑃𝑃𝑀𝑀
𝑃𝑃𝐿𝐿 𝐶𝐶𝐿𝐿→𝐼𝐼𝑀𝑀 𝑃𝑃𝐼𝐼
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Interpretation
Abstract overview of interpretation
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Source Code Interpreter Output Behavior
Input Data
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Interpretation (more formally)
𝑃𝑃𝐿𝐿 - Program in language L 𝐼𝐼𝐿𝐿𝑀𝑀 - Interpreter in language M
that translates L outputs
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𝑃𝑃𝐿𝐿 𝐼𝐼𝐿𝐿𝑀𝑀 Output Behavior
Input Data
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Layers of Interpretation
𝑃𝑃𝐿𝐿 - Program in language L is an interpreter for language
L’
𝐼𝐼𝐿𝐿𝑀𝑀 - Interpreter in language M that translates L outputs
𝑃𝑃𝐿𝐿′ - Program for language L’ used as input to the
interpreted program 𝑃𝑃𝐿𝐿
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𝑃𝑃𝐿𝐿 𝐼𝐼𝐿𝐿𝑀𝑀 Output Behavior
𝑃𝑃𝐿𝐿′
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Virtual Machines?
Are languages that use VMs compiled or interpreted? Tricky
question actually Consider Java:
At compile-time, translated from source code into byte code At
run-time, a virtual machine loads the byte code and runs it
First part sounds like a compiler, second part like an
interpreter
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Java Compilation
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𝑃𝑃𝐽𝐽𝐽𝐽𝐽𝐽𝐽𝐽 𝐶𝐶𝐽𝐽𝐽𝐽𝐽𝐽𝐽𝐽→𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀 𝑃𝑃𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
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Java Runtime
Looks like interpretation! But wait, there’s more…
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𝐼𝐼𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀
(JVM)Output Behavior
Input Data
𝑃𝑃𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
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“Just in Time” compilation
The JVM is “smart” enough to do “just in time” compilation
Sometimes it compiles the bytecode into native machine code
and runs that instead of interpreting bytecode!
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𝐼𝐼𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀
(JVM)Output Behavior
Input Data
𝑃𝑃𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵
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Takeaways
Why study programming languages? What types of programming
languages are there? How do we translate from a programming
language to a
machine language? And what makes these styles of translation
different?
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Readings
Chapter 1 – Introduction For next time: Chapter 2 – Syntax
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CSE 3341: Principles of Programming Languages�PL OverviewWhy are
we here?Why are we here?Why are we here?Why are we here?How do we
study a language?Types of Programming LanguagesDeclarative vs.
ImperativeImperative example - JavaDeclarative example -
PROLOGDeclarative example - SQLFunctional example -
SchemeCompilationCompilation (more formally)Execution (run time)A
new language?A new language?Intermediate
LanguageInterpretationInterpretation (more formally)Layers of
InterpretationVirtual Machines?Java CompilationJava Runtime“Just in
Time” compilationTakeawaysReadings
