Acknowledgements: These slides are adapted from

• my previous notes on modular development and data abstraction

• slide set 9 for the course Programming in Lua created by Fabio Mascarenhas, Federal University of Rio de Janeiro, Brazil. It was presented at Nankai University, P. R. China, in July 2013.

Advisory: The HTML version of this document may require use of a browser that supports the display of MathML. A good choice as of September 2016 is a recent version of Firefox from Mozilla.

Building Larger Programs

• So far using small scripts, mostly in REPL

• Also using built-in functions such as ipairs and table.concat

• What if program gets big, uses many new functions?

• Organize program into parts that work together -- into modules

• But how should we partition program?

Module

• "a work assignment given to a programmer or group of programmers" (Parnas, 1978) -- software engineering concept

• also may be program unit defined with construct or convention -- programming language concept

• ideally language feature supports software engineering method

Goals of Modularization

1. enable programmers to understand the system by focusing on one module at a time (comprehensibility)

2. shorten development time by minimizing required communication among groups (independent development)

3. make system flexible by limiting number of modules affected by significant changes (changeability)

Information Hiding Module

• Forms cohesive unit of functionality separate from other modules

• Hides a design decision (its secret) from other modules

• Encapsulates aspect of system likely to change (its secret)

• Aspects likely to change independently become secrets of separate modules

• Aspects unlikely to change become interactions (connections) among modules

Module Interface

Interface:

• "set of assumptions ... each programmer needs to make about the other program ... to demonstrate the correctness of his own program" (Britton, Parker, & Parnas, 1981)

• includes function signatures (name, arguments, return)

• includes constraints on environment and argument values (preconditions, postconditions, invariants)

Abstract Interface for Module

Abstract interface:

• Does not change when one module implementation substituted for another

• concentrates on module's essential aspects and obscures incidental aspects that vary among implementations

Information hiding with abstract interfaces supports software reuse

Criteria for Good Interfaces (1)

Cohesion:

Fit all operations together to support single, coherent purpose

Consistency:

Provide internally consistent set of operations (naming, arguments, return, behavior) -- avoid surprises!

Simplicity:

Avoid needless features

No redundancy:

Avoid offering same service in multiple ways

Criteria for Good Interfaces (2)

Atomicity:

Do not combine several operations if needed individually -- each primitive, not decomposable into others

Completeness:

Include all primitive operations that make sense for abstraction

Reusability:

Make general for use beyond initial context

Robustness:

Keep interface stable even if implementation changes

Criteria for Good Interfaces (3)

Convenience:

Provide additional operations for user convenience -- after careful study

• Tradeoff conflicts among criteria: completeness vs. simplicity, reusability vs. simplicity, convenience vs. { consistency, simplciity, no redundancy, atomicity}

• Tradeoff criteria against efficiency and functionality

Lua Module

• Represented by a Lua table -- isn't everything?!

• Creates and returns the table from executing chunk

• Normally defined in a separate file

• Populates the table with functions and data structures to be exported

• Carries private features only in exported function closures

Lua Module Design Principles

• Design should use information hiding and abstract interfaces

• All variable and function names are Lua local variables

• Module only exports names of features users need

• Module does explicit return of its table

• Function documents its assumptions (preconditions, postconditions, invariants); checks preconditions where feasible

Using Lua Modules

• Lua standard library modules: table, io, string, math, os, etc.

• Standard modules preloaded into global variables of same name

• Program loads other modules with built-in require function, assigning module's table to (local) variable

A Simple Module

Can develop stub for complex number module, store in file complex.lua

    local M = {}          -- create the module's table
    function M.new(r, i)  -- could be: M.new = function(r, i)
        return { real = r or 0, im = i or 0 } 
    end 
    
    M.i = M.new(0, 1) 
    
    function M.add(c1, c2) 
        return M.new(c1.real + c2.real, c1.im + c2.im) 
    end 

    function M.tostring(c) 
        return tostring(c.real) .. "+" .. tostring(c.im) .. "i" 
    end 

    return M -- table with closures for keys "new", "add", "tostring"
             -- also constant value for "i"

Another Style Module

• Performs better but duplicates code. (Instructor uses)

  local function new(r, i) 
      return { real = r or 0, im = i or 0 } 
  end 
  
  local i = new(0, 1) -- call local function
  
  local function add(c1, c2) 
      return new(c1.real + c2.real, c1.im + c2.im) 
  end 
  
  local function tos(c) 
      return tostring(c.real) .. "+" .. tostring(c.im) .. "i" 
  end 
  
  -- module table, "export" list, export name changed for tos
  return { new = new, i = i, add = add, tostring = tos }

Loading Complex Module

• Can load using REPL

  > complex = require "complex" 
  > print(complex) 
  table: 0000000000439820 

• Use cached module subsequently

  > print(require "complex") 
  table: 0000000000439820 

• Can force reload

  > package.loaded.complex = nil 
  > complex = require "complex" 
  > print(complex) 
  table: 000000000042F8F0 

Using Complex Module

• Load a module and assign to variable

• Can use anything exported by module

  > c1 = complex.new(1, 2) 
  > print(complex.tostring(c1)) 
  1+2i 
  > c2 = complex.add(c1, complex.new(10,2)) 
  > print(complex.tostring(c2)) 
  11+4i 
  > c3 = complex.add(c2, complex.i) 
  > print(complex.tostring(c3)) 
  11+5i 

• Could assign to module table's fields, but bad style!

Search Path

• Where does require get modules?

• Uses search path in package.path

• Will normally include current directory, which is acceptable for this course

• Check documentation if different approach needed