Data Abstraction in Go Michael DiBernardo @mdibernardo 19-Sep-2013 @ GolangTO 

How did we get here? 

Programming “in” “Programmers who program ‘in’ a language limit their thoughts to constructs that the language directly supports. If the language tools are primitive, the programmer's thoughts will also be primitive.” 

Programming “into” “Programmers who program ‘into’ a language first decide what thoughts they want to express, and then they determine how to express those thoughts using the tools provided by their specific language.” - Code Complete 2nd ed (Steve McConnell) 

This works until... 

signature TREE= sig datatype 'a tree = Lf | Node of 'a * 'a tree * 'a tree val size: 'a tree -> int end structure T:> TREE = struct datatype 'a tree = Lf | Node of 'a * 'a tree * 'a tree fun size Lf = 0 | size(Node(_,t1, t2)) = 1 + size t1 + size t2 end 

Programming models • Procedural • Functional • Object-oriented • Stack-based • Stream-oriented • ... 

Programming models • Easier to models that are idiomatic to the language • Many concepts are independent of model • But what about implementation of these concepts? 

Data Abstraction 

ADTs • Who wants to define ADT? 

ADTs "The general technique of isolating the parts of a program that deal with how data objects are represented from the parts of a program that deal with how data objects are used is a powerful design methodology called data abstraction." - Structure and Interpretation of Computer Programs pp.80 

ADTs • Abstract type (name) • Operations on that type • Preconditions • Postconditions • Invariants 

ADTs - Why? • Often quoted are: • Ease of use • Ease of change 

ADTs - Why? • Note that reuse was not a strong motivator for these, historically • Re-use was a later movement (esp. OO) • The focus of this talk is not on reuse! 

Example: Rational Numbers 

Rational ADT • Create • Convert to human-readable repr • Add • Invariant: Should always be in lowest terms 

“Implicit” Interfaces "Every type has an interface, which is defined to be the set of methods which operate on that type." - Ian Lance Taylor in “Learning Go” 

Where do implicit interfaces fail? 

Summary • Using type + methods = implicit interface • Hiding struct members can add robustness, but doesn’t work for non-structs 

Summary • Interfaces can abstract different implementations of common behaviour • Can also be used shield against direct access to non-struct types (arrays, slices, etc) 

Summary • Some mechanisms exist for re-using ADT method implementation (e.g. anonymous fields) • But for now, good old "procedural abstraction" is not a bad idea. 

// Header file "api.h" struct Entity; extern struct Entity * open_entity(int id); extern int process_entity(struct Entity *info); extern void close_entity(struct Entity *info); 

http://github.com/MichaelDiBernardo/golang-adts