Learn Datalog - 10 Code Examples & CST Typing Practice Test

Datalog is a declarative logic programming language based on first-order logic and Horn clauses. It is widely used for querying relational data, building rule-based systems, static analysis, and reasoning engines due to its logical purity, strong mathematical foundations, and deterministic evaluation model.

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Datalog Counter and Theme Toggle Datalog Simple Facts Query Datalog Even Numbers Datalog Factorial Datalog Fibonacci Datalog Prime Numbers Datalog List Membership Datalog Length of List Datalog Maximum Element Datalog Sum of List

Learn DATALOG with Real Code Examples

Updated Nov 20, 2025

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Code Sample Descriptions

Datalog Counter and Theme Toggle

% Facts
count(0).
isDark(false).

% Rules
updateUI(Count, Theme) :- count(Count), isDark(IsDark), (IsDark -> Theme = 'Dark'; Theme = 'Light').

increment(NewCount) :- count(Count), NewCount is Count + 1, retract(count(Count)), assert(count(NewCount)).
decrement(NewCount) :- count(Count), NewCount is Count - 1, retract(count(Count)), assert(count(NewCount)).
reset() :- retract(count(_)), assert(count(0)).
toggleTheme() :- isDark(IsDark), NewDark is not IsDark, retract(isDark(IsDark)), assert(isDark(NewDark)).

% Simulate actions
updateUI(Count, Theme).
increment(NewCount).
updateUI(NewCount, Theme).
increment(NewCount2).
updateUI(NewCount2, Theme).
toggleTheme().
updateUI(NewCount2, Theme).
decrement(NewCount3).
updateUI(NewCount3, Theme).
reset().
updateUI(0, Theme).

Demonstrates a simple counter with theme toggling using Datalog facts and rules.

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Datalog Simple Facts Query

parent(alice, bob).
parent(bob, carol).
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- parent(X,Z), ancestor(Z,Y).
? ancestor(alice, carol).

Queries simple facts from a Datalog database.

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Datalog Even Numbers

number(0).
number(N) :- number(M), N is M + 1, N =< 10.
even(N) :- number(N), N mod 2 = 0.
? even(N).

Generates even numbers up to 10.

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Datalog Factorial

fact(0,1).
fact(N,F) :- N>0, M is N-1, fact(M,F1), F is N*F1.
? fact(5,F).

Calculates factorial using recursive rules.

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Datalog Fibonacci

fib(0,0).
fib(1,1).
fib(N,F) :- N>1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1+F2.
? fib(7,F).

Computes Fibonacci numbers using recursion.

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Datalog Prime Numbers

number(2). number(3). number(4). number(5).
prime(N) :- number(N), not(divisor(X,N)), X<N, X>1.
divisor(X,N) :- N mod X = 0.
? prime(N).

Checks if a number is prime.

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Datalog List Membership

member(X,[X|_]).
member(X,[_|T]) :- member(X,T).
? member(b,[a,b,c]).

Checks if an element is in a list.

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Datalog Length of List

length([],0).
length([_|T],L) :- length(T,L1), L is L1+1.
? length([a,b,c,d],L).

Calculates the length of a list.

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Datalog Maximum Element

max([X],X).
max([H|T],M) :- max(T,M1), M is (H>M1 -> H ; M1).
? max([3,7,2,5],M).

Finds the maximum in a list.

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Datalog Sum of List

sum([],0).
sum([H|T],S) :- sum(T,S1), S is H+S1.
? sum([1,2,3,4,5],S).

Computes the sum of a list.

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Frequently Asked Questions about Datalog

What is Datalog?

What are the primary use cases for Datalog?

Database querying and rule-based inference. Static program analysis (Soufflé, Doop). Authorization and access control systems (e.g., Google Zanzibar variants). Knowledge graph reasoning. Graph algorithms (reachability, dependency tracking)

What are the strengths of Datalog?

Ideal for complex relational queries. Highly optimizable and parallelizable. Excellent for static analysis and graph reasoning. Simple, compact syntax. Predictable and analyzable execution model

What are the limitations of Datalog?

Not a general-purpose programming language. No complex terms or functions like in Prolog. Requires understanding of logic semantics. Can be difficult to debug recursion in large datasets. Limited tooling compared to mainstream languages

How can I practice Datalog typing speed?

CodeSpeedTest offers 10+ real Datalog code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.

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Learn Datalog - 10 Code Examples & CST Typing Practice Test

Code Sample Descriptions

Datalog Counter and Theme Toggle

% Facts
count(0).
isDark(false).

% Rules
updateUI(Count, Theme) :- count(Count), isDark(IsDark), (IsDark -> Theme = 'Dark'; Theme = 'Light').

increment(NewCount) :- count(Count), NewCount is Count + 1, retract(count(Count)), assert(count(NewCount)).
decrement(NewCount) :- count(Count), NewCount is Count - 1, retract(count(Count)), assert(count(NewCount)).
reset() :- retract(count(_)), assert(count(0)).
toggleTheme() :- isDark(IsDark), NewDark is not IsDark, retract(isDark(IsDark)), assert(isDark(NewDark)).

% Simulate actions
updateUI(Count, Theme).
increment(NewCount).
updateUI(NewCount, Theme).
increment(NewCount2).
updateUI(NewCount2, Theme).
toggleTheme().
updateUI(NewCount2, Theme).
decrement(NewCount3).
updateUI(NewCount3, Theme).
reset().
updateUI(0, Theme).

Demonstrates a simple counter with theme toggling using Datalog facts and rules.

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Datalog Simple Facts Query

parent(alice, bob).
parent(bob, carol).
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- parent(X,Z), ancestor(Z,Y).
? ancestor(alice, carol).

Queries simple facts from a Datalog database.

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Datalog Even Numbers

number(0).
number(N) :- number(M), N is M + 1, N =< 10.
even(N) :- number(N), N mod 2 = 0.
? even(N).

Generates even numbers up to 10.

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Datalog Factorial

fact(0,1).
fact(N,F) :- N>0, M is N-1, fact(M,F1), F is N*F1.
? fact(5,F).

Calculates factorial using recursive rules.

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Datalog Fibonacci

fib(0,0).
fib(1,1).
fib(N,F) :- N>1, N1 is N-1, N2 is N-2, fib(N1,F1), fib(N2,F2), F is F1+F2.
? fib(7,F).

Computes Fibonacci numbers using recursion.

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Datalog Prime Numbers

number(2). number(3). number(4). number(5).
prime(N) :- number(N), not(divisor(X,N)), X<N, X>1.
divisor(X,N) :- N mod X = 0.
? prime(N).

Checks if a number is prime.

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Datalog List Membership

member(X,[X|_]).
member(X,[_|T]) :- member(X,T).
? member(b,[a,b,c]).

Checks if an element is in a list.

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Datalog Length of List

length([],0).
length([_|T],L) :- length(T,L1), L is L1+1.
? length([a,b,c,d],L).

Calculates the length of a list.

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Datalog Maximum Element

max([X],X).
max([H|T],M) :- max(T,M1), M is (H>M1 -> H ; M1).
? max([3,7,2,5],M).

Finds the maximum in a list.

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Datalog Sum of List

sum([],0).
sum([H|T],S) :- sum(T,S1), S is H+S1.
? sum([1,2,3,4,5],S).

Computes the sum of a list.

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Frequently Asked Questions about Datalog

What is Datalog?

What are the primary use cases for Datalog?

What are the strengths of Datalog?

Ideal for complex relational queries. Highly optimizable and parallelizable. Excellent for static analysis and graph reasoning. Simple, compact syntax. Predictable and analyzable execution model

What are the limitations of Datalog?

How can I practice Datalog typing speed?

CodeSpeedTest offers 10+ real Datalog code examples for typing practice. You can measure your WPM, track accuracy, and improve your coding speed with guided exercises.