Bratko, Chapter 8: Programming Style and Technique

8.1 General principles of good programming
- correctness (1)
- efficiency (3)
- readability (2)
- modifiability (4)
- robustness (5)
- documentation (6)

8.2 How to think about Prolog programs
- recursion
- generalization
- pictures

8.3 Programming style
- style
  - clauses about same procedure clustered together
  - each goal on a separate line
  - use cut sparingly
    - try using only green cuts, i.e., cuts that can be removed without
      altering the declarative meaning of the program
    - red cuts only when selecting among alternatives
      - not(P) :- P, !, fail ; true.
      - if Condition then Goal1 else Goal2
      - ( Condition, !, Goal1 ; Goal2 )
  - avoid program modification by assert and retract
  - prefer multiple clauses to semicolons
  - example
    - merge(L1,L2,L3) (p194) (HW2, Spr95)
- commenting
  - /* */ or %

8.4 Debugging
- Prolog is interpretive, so any procedure in a program can be called
  directly
- debugging through tracing goals
  - outputs entry information
    - predicate name
    - values of arguments
  - outputs exit information
    - on success, values of arguments
    - on failure, Fail
  - 'trace' - turn on tracing
  - 'notrace' - turn off tracing
  - spy(P) - trace only after encountering predicate P
    - P is the predicate name; all such predicates spied
    - can be executed multiple times
    - can use arity representation: man/1 or man/2
  - nospy(P) - turn off tracing for predicates named P

8.5 Efficiency
- compile your program using 'qpc'
  - ignore error about 'qld'
- avoid unnecessary backtracking
- stop execution of useless alternatives as soon as possible
- caching
- examples
  - reordering queens [1,2,3,4,5,6,7,8] -> [1,5,2,6,3,7,4,8] (p198)
  - reordering countries for map colouring (p199)
    - start with country having most neighbours
  - representing lists as differences
    - prevents backtracking on first list to find tail
    - concat(A1-Z1, Z1-Z2, A1-Z2)
      - concat([a,b,c | T1] - T1, [d,e | T2] - T2, L).
        T1 = [d,e|T2],
        T2 = _8970,
        L = [a,b,c,d,e|T2]-T2 ;
  - caching Fibonacci numbers (p203)
  - computing Fibonacci numbers bottom up (p205)