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