58 lines
1.5 KiB
Markdown
58 lines
1.5 KiB
Markdown
# Semantic Analysis
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* after passing the lexical and syntax analysis, there are still errors. so correcting usage of variables, objects and function... are needed.
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**Semantic Analysis** ensures the program satisfies a set of rules regarding the usage of programming constructs:
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* identifiers declared before used
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* types
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* inheritance relationships
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* single definition
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## Categories of Semantic Analysis
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* Scopes
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* Types
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### Scope
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**Lexical Scope**: textual region in the program
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**Symbol Tables**
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Symantic checks refer to properties of identifier in the program; it need an environment to store identifier info: **symbol table**.
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| name | kind | type |
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| ---- | ---- | ------------ |
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| foo | func | `int -> int` |
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| m | arg | `int` |
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| n | arg | `int` |
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| tmp | var | char |
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Each scope has symbol tables.
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And program has hierachy of symbol tables(scope).
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if the identifier is used traverse the hierachy of symbol tables upward until finding the identifier with the same name to determine the declaration from the current scope.
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#### Build a Symbol Table
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there are five operations:
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* insert scope
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* exit scope
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* find symbol(x)
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* add symbol(x)
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* check scope(x)
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### Type
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* Type checking
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* Type inferencing
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Three Language Types:
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* Statically typed
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* Dynamically typed
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* Untyped(machine code)
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**Static Type Checking**
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Does not require additional type checking instructions at runtime.
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and guarantees that the executions are safe at compile time.
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modern languages require both static and dynamic type checking(union, void ptr) |