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@ -46,7 +46,7 @@ date: November 21, 2018 |
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- Model "essence" of real languages |
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## 2. Formalize what programs "do" |
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- Usually: run it on a machine and find out! |
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- Run it on a machine and find out? |
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- Not very useful for proofs... |
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- Formalize behavior mathematically, on paper |
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- Discard "unimportant" details |
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@ -115,7 +115,7 @@ date: November 21, 2018 |
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# Operational Semantics |
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## Programs execute by "stepping" |
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## Execute by "stepping" |
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- Start with program |
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- Imperative: plus variable setting |
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- In each step, perform update: |
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@ -137,8 +137,8 @@ date: November 21, 2018 |
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- A type $\tau$ describes a class of programs |
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- Usually: well-behaved in some way |
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- Can automatically check if program has type $\tau$ |
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- Type of program depends on types of its pieces |
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- Scales to large programs |
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- Type of program depends on types of components |
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- Analysis scales to large programs |
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## Example |
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@ -146,18 +146,18 @@ date: November 21, 2018 |
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- Lightweight |
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- Checking types is simple, automatic |
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- Don't need to run program |
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- Intuitive meaning |
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- Natural and intuitive |
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- Can't add a String to a Boolean |
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- Programmers often think in terms of types |
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- Identify correct programs |
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## Weaknesses |
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- Programmer may need to add some type hints |
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- Extra annotations in program |
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- Programmer may need to add annotations |
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- Extra hints for compiler |
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- Common for more complex types |
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- Compiler sometimes rejects correct programs |
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- Figuring out why can be very frustrating |
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- May need to write program in more awkward form |
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- May need to write program in less natural form |
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## Types can be complex |
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- Simpler types |
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@ -170,9 +170,9 @@ date: November 21, 2018 |
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- ... |
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## Prove: soundness |
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- "Soundness theorem" |
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- Relate type system to operational behavior |
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- If program has a type, it should be well-behaved |
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- Relate type system to operational behavior |
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- "Soundness theorem" |
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- Many possible notions of "well-behaved" |
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- Don't add Strings to Bools |
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- Don't mix public and private data |
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