Rethinking Prolog:—Guess Lazily

—-## What Does “Guess Lazily” Mean in Prolog?In traditional Prolog, when solving a problem, the interpreter often **guesses** values for variables early (eagerly), then checks constraints. This can lead to inefficiency—guessing many values that later fail constraints.**Guess lazily** means **delaying choices** (guesses) until you have more information, typically by propagating constraints as much as possible first. This is inspired by ideas in constraint logic programming and lazy evaluation in functional programming.---## Why Rethink Prolog This Way?- **Efficiency:** By postponing guesses, you reduce the search space and avoid unnecessary backtracking.- **Declarativity:** Programs become closer to the problem specification, focusing on constraints rather than control.- **Compositionality:** Easier to combine constraints and build modular programs.---## How Does Lazy Guessing Work?### 1. Propagate Constraints FirstInstead of immediately choosing values for variables, propagate all available constraints to narrow down possibilities.**Example:**```prolog% Eager guessing (traditional)member(X, [1,2,3]), X 2.% Lazy guessingX 2, member(X, [1,2,3]).```In the second version, the constraint `X 2` is applied before guessing values for `X`, so only `X=3` is considered.### 2. Use Constraint Logic Programming (CLP)Prolog extensions like **CLP(FD)** (Constraint Logic Programming over Finite Domains) embody this idea:```prolog:- use_module(library(clpfd)).X in 1..3, X # 2.```Here, the domain of `X` is narrowed to only `3` before any value is guessed.### 3. Delaying ConstructsSome Prolog systems support **delays** or **coroutining** (e.g., `when/2`), so that predicates only execute when enough information is available.---## Example: N-Queens (Traditional vs. Lazy Guessing)**Traditional Prolog:**```prologqueens(N, Qs) :- length(Qs, N), permute([1..N], Qs), safe(Qs).```Here, all permutations are generated, many of which are invalid.**With Lazy Guessing (CLP(FD)):**```prolog:- use_module(library(clpfd)).queens(N, Qs) :- length(Qs, N), Qs ins 1..N, all_different(Qs), safe_diagonals(Qs), label(Qs).```Constraints are propagated first; only valid assignments are considered.---## Further Reading- [“The Art of Prolog”](https://mitpress.mit.edu/9780262512277/the-art-of-prolog/) (Sterling & Shapiro)- [SWI-Prolog CLP(FD) Manual](https://www.swi-prolog.org/man/clpfd.html)- [Constraint Logic Programming: A Survey](https://en.wikipedia.org/wiki/Constraint_logic_programming)---## Summary**Rethinking Prolog: Guess Lazily** is about shifting from eager guessing to **constraint propagation and delayed choice**. This leads to more efficient, declarative, and modular logic programs. The idea is central to modern Prolog extensions and a cornerstone of constraint logic programming.If you’d like to see more code examples or discuss implementation techniques, let me know!Sources

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