Text Templates in Prolog

Prolog offers string manipulation capabilities that can be used to create dynamic content or show customized output to the user. While Prolog doesn’t have built-in template support like some other languages, we can implement similar functionality using Prolog’s string operations and list processing.

:- use_module(library(format)).

main :-
    % We can create a simple template by using a string with placeholders
    Template1 = "Value is ~w~n",
    
    % We can "execute" the template by using the format predicate
    format(Template1, ["some text"]),
    format(Template1, [5]),
    format(Template1, [["Prolog", "Erlang", "Haskell", "Lisp"]]),
    
    % For structured data, we can use compound terms
    Template2 = "Name: ~w~n",
    format(Template2, ["Jane Doe"]),
    
    % We can use a key-value list to simulate a map
    format(Template2, ["Mickey Mouse"]),
    
    % Conditional execution can be achieved using Prolog's if-then-else construct
    Template3 = "~w~n",
    (   "" \= []
    ->  format(Template3, ["yes"])
    ;   format(Template3, ["no"])
    ),
    (   [] \= []
    ->  format(Template3, ["yes"])
    ;   format(Template3, ["no"])
    ),
    
    % We can use recursion to process lists, similar to a range block
    Template4 = "Range: ",
    print_list(["Prolog", "Erlang", "Haskell", "Lisp"]).

% Helper predicate to print list elements
print_list([]) :- nl.
print_list([H|T]) :-
    format("~w ", [H]),
    print_list(T).

In this Prolog implementation:

1. We use the format/2 predicate to simulate template execution. The first argument is the template string, and the second is a list of values to be inserted into the template.

2. Placeholders in the template are represented by ~w, which will be replaced by the corresponding value from the list.

3. For structured data, we can use Prolog’s compound terms or key-value lists to represent objects or maps.

4. Conditional execution is achieved using Prolog’s if-then-else construct (Condition -> ThenClause ; ElseClause).

5. To process lists (similar to a range block), we use recursion with the print_list/1 predicate.

To run the program, save it as templates.pl and use the Prolog interpreter:

$ swipl -s templates.pl -g main -t halt
Value is some text
Value is 5
Value is [Prolog,Erlang,Haskell,Lisp]
Name: Jane Doe
Name: Mickey Mouse
yes
no
Range: Prolog Erlang Haskell Lisp 

This example demonstrates how to achieve similar functionality to text templates in Prolog, even though the language doesn’t have a built-in template system. The approach uses Prolog’s string formatting and list processing capabilities to create dynamic content.