CS代考计算机代写 Haskell c++ scheme flex Java Semantics
Semantics
Aditya Thakur
ECS 140A Programming Languages – Winter 2019
Scope in Go
• https://play.golang.org/p/TW23ukHm6ew • https://play.golang.org/p/2yCBYxxBlk_k
• https://play.golang.org/p/Uigll4QhfNd
• https://play.golang.org/p/MMTqTqnv0Mz
Types
A type is a set of values, together with a collection of operations on those values
int n;
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Types (cont’d)
• Simple types/basic types/primitive types
– From which all other types are constructed
– Specified either using keywords or predefined identifiers
• Constructed types
– Any type that a program can define for itself using the primitive
types
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Type Constructors
A type constructor is a standard set operation applied to a set of types to construct a new type
• Examples:
Cartesian product type constructor int X char X double
struct IntCharReal {
int i;
char c;
double r;
};
struct IntCharReal x;
x.i = 42;
Union type constructor
union IntOrReal {
int i;
double r;
};
union IntOrReal u;
int num = u.i;
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Types in C
14
Types in Java
15
Types in Go
• Basic types
• https://tour.golang.org/basics/11
• Struct types
• https://tour.golang.org/moretypes/2
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Benefits of Type Information
• Program organization and documentation
• Separate types for separate concepts
• Represent domain-specific concepts
• Indicate intended use of declared identifiers • Types can be checked, unlike program comments
• Detect and prevent errors • 3 + true – “Bill”
• Support Optimization
• Example: Short integers require fewer bits
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Type Equivalence
When are two types the same? • Structural equivalence
• Types are the same if they have the same structure
typedef struct {int a; char b;} X, Y;
typedef struct {char b; int a;} Z;
typedef struct {int first; char second;} W;
Are types X, Y, Z, and W equivalent?
• Name equivalence
• https://play.golang.org/p/GDmL5VqNUce
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Type Checking
• Compile time or static
• C, C++, Java, Go, Haskell, ML, etc.
• Example: C/Java: f(x) • f:A->Bandx:A
• Run time or dynamic
• Lisp, Scheme, etc.
• Example: Lisp: (car x)
• Makes sure x is list a before taking car
• Both static and dynamic prevent type errors
• There are tradeoffs
• Dynamic: slower
• Static: restrict program flexibility
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•
Type Annotations vs.Type Inference
Many languages require, or at least allow type
annotations
• Programmer supplies explicit type information to the
language system • Examples:
• Java requires a definition for each variable
• ML allows to attach type annotations to variables,
functions and expressions
Most languages systems also collect information
from other sources
• Constants have types
• Expressions have types (depending on its operands)
• Types can be inferred!
•
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•
Standard type checking
int f(int x) { return x+1; };
int g(int y) { return f(y+1)*2;};
• Look at body of each function and use declared types of identifiers to check agreement
Type inference
int f(int x) { return x+1; };
int g(int y) { return f(y+1)*2;};
• Look at code without type information and figure out what types could have been declared
•
Type Checking and Type Inference
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Basic Types in Go
• bool
• string
• int,int8,in16,int32,int64
• uint,uint8,uint16,uint32,uint64,uintptr
• byte(aliasforuint8)
• rune(aliasforint32,representsaunicode) • float32,float64
• complex64,complex128
https://tour.golang.org/basics/11
More Types
• Pointers https://play.golang.org/p/tgSutr3fGmr
• Arrays and slices https://tour.golang.org/moretypes/6 • Maps https://play.golang.org/p/U1B_gxkv7vL
User-defined Types
• Based on basic types
• https://play.golang.org/p/P8kE3sJWx1r
• Structs
• https://play.golang.org/p/3wOX38kj-FT
• Structs inside structs https://play.golang.org/p/5wv9utsqXZM
• All types can have operations associated with them
Type errors
• Types help catch common mistakes
• https://play.golang.org/p/cJDAvlwkVFf • https://play.golang.org/p/K0OKFsiFm_r
Type Conversion
• https://tour.golang.org/basics/13
• https://play.golang.org/p/bMIbGkzZhLF
Interfaces
• An interface is an abstract type: it only defines a set of methods; nothing about the internal representation.
• A type satisfies an interface if it possesses all the methods the interface requires.
• The ability to replace one type for another that satisfies the same interface is called substitutability.
• https://tour.golang.org/methods/9
• https://play.golang.org/p/iEmPmAX5FMN
• https://play.golang.org/p/J-0zOGsKzxT
Type assertions
• Provides access to an interface value’s underlying concrete value. • https://play.golang.org/p/CWPHr0VcZvz
• Similar to dynamic_cast in C++
Empty interface
• The interface{} type is the interface that has no methods. • All types satisfy the empty interface.
• https://play.golang.org/p/LbJvtn-NdWl
Empty interface and Type assertions
• https://tour.golang.org/methods/15
Empty interface and type switches
• https://play.golang.org/p/sDOlRsHcaJF
• https://play.golang.org/p/xHT-WXZ-Kei
• Also see expr/print.go in hw2 for an example of type switches
Function values
• Functions are first-class values in Go
• Function values have types, may be assigned to variables, passed to and
returned from functions
• A function value may be called like any other function • https://play.golang.org/p/bC1VTNdTxai
Closures
• A closure is a function value that references variables from outside its body.
• Note that the lifetime of a variable is not determined by its scope
• https://play.golang.org/p/N_mHpPrS0wr
• https://play.golang.org/p/odkhcRsAlif
• https://play.golang.org/p/jcGg3dndii2
• https://play.golang.org/p/FO9J0EB5nm4 • https://play.golang.org/p/a4pSFX0QzNe
Type Embedding
• Take existing types and both extend and change their behavior
• https://play.golang.org/p/U3tBEcWmDyU
• Compare the above code to https://play.golang.org/p/5wv9utsqXZM
Exporting and unexporting identifiers
• Package-level visibility of identifiers
• Identifier is unexported or unknown to code outside the package iff
the identifier starts with a lowercase letter.
• Note that identifiers are (un)exported, not values.
Object-Oriented Programming
• Encapsulation and information hiding • Package-level exports in Go
• Public and private keywords in C++
• Composition and inheritance
• “has-a” vs “is-a”
• Type embedding in Go vs inheritance in C++
• Polymorphism
• Interfaces in Go similar to abstract base classes in C++ • Templates in C++
• Operator and function overloading in C++
