Strange Prolog Coder Hacks

Prolog programmers often unconventional techniques to handle scenarios like undefined predicates, conditional execution, or prototyping code. This page explores some of those tricks.

Hack 1: The nop/1 Predicate

Example:

nop(_).

do_stuff:- 
  nop(step1),
  step2.

Explanation:

The nop/1 predicate always succeeds without doing anything, regardless of the argument passed to it. This trick is useful for temporarily disabling parts of code while maintaining readability and structure.

Why Use It?

• Placeholders for Disabled or Future Code: Use nop/1 as a quick way to "comment out" code without removing it.
• Debugging and Prototyping: Selectively deactivate steps in a process without breaking the flow of the program.
• Readable and Modular Code: Even when a step is inactive, it remains visible in the code, preserving the overall structure.

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Hack 2: Delayed Code Definition with call(call, ...)

Example:

do_something:- 
  call(call, some_pred(stuff)).

Explanation:

In SWI-Prolog, when you use call(some_pred(stuff)) and some_pred/1 is not yet defined, you’ll receive a warning like "Procedure some_pred/1 does not exist." However, by wrapping the call in an extra call, like call(call, some_pred(stuff)), you can reference a predicate that hasn’t been defined yet without triggering this warning.

Why Use It?

• Deferring Definition: This trick is handy when you want to reference a predicate that you’ll define later. It allows you to build the structure of your code without needing every part implemented upfront.
• Modular Development: You can design complex systems where certain predicates are loaded or defined dynamically, while still referencing them early in the code.
• Avoiding Compilation Warnings: SWI-Prolog is smart enough to issue warnings about undefined predicates when you use call(some_pred(stuff)). Using call(call, some_pred(stuff)) suppresses these warnings until the predicate is defined.

How It Works:

• The first call effectively delays the evaluation of the inner call(some_pred(stuff)), allowing Prolog to treat it as a more dynamic or indirect reference.
• Once some_pred/1 is eventually defined, the code will run as expected.

When to Use It:

• For scenarios where predicates are loaded dynamically, such as in modular or plugin-based systems.
• When you’re building code that evolves over time, and you want to define certain predicates later, use nop/1 instead.