CSE-321 Programming Languages Continuation

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CSE-321 Programming LanguagesContinuation
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• POSTECH
• May 9, 2007

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Commonalities?
goto

• Too powerful
• Too dangerous

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"Precious"

• Too powerful
• turns you invisible!
• gives you an unusually long and active life!
• Too dangerous
• turns you into an evil creature!
• will eventually enslave you!

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goto

• Too powerful
• can simulate all control constructs
• for, while, repeat/until
• switch
• Too dangerous
• damages code readability

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Continuation goto of SML

• Too powerful
• code optimization
• simple, elegant implementation of concurrency
• compilation technique
• CPS transformation used in SML/NJ
• Too dangerous
• damages code readability

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Outline

• Introduction V
• Motivating example
• Evaluation contexts
• Continuations

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Multiplying a List of Integers
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An Improvement
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Non-local Jump
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Outline

• Introduction V
• Motivating example V
• Evaluation contexts
• Continuations

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Evaluation Contexts

• An evaluation context
• an expression with a hole
• "Give me an expression of type A"
• "Then I will return an expression of type B"

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Typing Evaluation Contexts
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What are Continuations?

• ?
• Give me an expression of type A.
• Then I will return an expression of type C.

? is a continuation!
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Where is a continuation?
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Outline

• Introduction V
• Motivating example V
• Evaluation contexts V
• Continuations

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Simply Typed ?-Calculus
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Reduction Rules
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First-class Continuations
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Exercises
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Makes Sense?
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Typing Continuations
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(No Transcript)
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See the Course Notes for an answer.
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(No Transcript)
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How do we return x?
How can we pass x to the outside of throw?
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Answer Throw!
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Big Announcement!
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Special Assignment

• Credit
• unknown
• Deadline midnight May 29 (Tuesday)
• no extension
• Where
• handin directory cs321/handin/hw_photo/
• What
• JPEG picture of YOU with a big smile -)
• size 200 by 260
• name me.jpg
• case-sensitive