Java does not (yet) have pattern matching although writing a legible DSL for it is not too hard. I'm certainly not the first to take a hand at it.
An example DSL in action. Lambdas and method references sure help a lot:
public static void main(final String... args) {
asList(0, 1, 2, 3, 13, 14, null, -1).stream().
peek(n -> out.print(format("%d -> ", n))).
map(matching(Integer.class, Object.class).
when(Objects::isNull).then(n -> "!").
when(is(0)).then(nil()).
when(is(1)).then("one").
when(is(13)).then(() -> "unlucky").
when(is(14)).then(printIt()).
when(even()).then(scaleBy(3)).
when(gt(2)).then(dec()).
none().then("no match")).
map(MatchingTest::toString).
forEach(out::println);
out.flush(); // Avoid mixing sout and serr
matching(Integer.class, Void.class).
none().thenThrow(RuntimeException::new).
apply(0);
} Implementation:
/**
* {@code Matching} represents <a href="https://en.wikipedia.org/wiki/Pattern_matching">Pattern
* Matching</a> in Java as a function production an optional. Example: <pre>
* asList(0, 1, 2, 3, 13, 14, null, -1).stream().
* peek(n -> out.print(format("%d -> ", n))).
* map(matching(Integer.class, Object.class).
* when(Objects::isNull).then(n -> "!").
* when(is(0)).then(nil()).
* when(is(1)).then("one").
* when(is(13)).then(() -> "unlucky").
* when(is(14)).then(printIt()).
* when(even()).then(scaleBy(3)).
* when(gt(2)).then(dec()).
* none().then("no match")).
* map(MatchingTest::toString).
* forEach(out::println);</pre>
* <p>
* <i>NB</i> — There is no way to distinguish from an empty optional if
* there was no match, or if a match mapped the input to {@code null}, without
* use of a {@link When#then(Object) sentinel value} or {@link
* When#thenThrow(Supplier) thrown exception}.
* <p>
* <strong>NB</strong> — There is no formal destructuring, but this can
* be simulated in the {@code Predicate} to {@link #when(Predicate) when}.
*
* @param <T> the input type to match against
* @param <U> the output type of a matched pattern
*
* @author <a href="mailto:binkley@alumni.rice.edu">B. K. Oxley (binkley)</a>
*/
@NoArgsConstructor(access = PRIVATE)
public final class Matching<T, U>
implements Function<T, Optional<U>> {
private final Collection<Case> cases = new ArrayList<>();
/**
* Begins pattern matching with a new pattern matcher.
*
* @param inType the input type token, never {@code null}
* @param outType the output type token, never {@code null}
* @param <T> the input type to match against
* @param <U> the output type of a matched pattern
*
* @return the pattern matcher, never {@code null}
*
* @todo Avoid the type tokens
*/
public static <T, U> Matching<T, U> matching(final Class<T> inType,
final Class<U> outType) {
return new Matching<>();
}
/**
* Begins a when/then pair.
*
* @param when the pattern matching test, never {@code null}
*
* @return the pattern continuance, never {@code null}
*/
public When when(final Predicate<? super T> when) {
return new When(when);
}
/**
* Begins a default when/then pair, always placed <strong>last</strong> in
* the list of cases (evaluates no cases after this one).
*
* @return then pattern continuance, never {@code null}
*/
public When none() {
return when(o -> true);
}
/**
* Evaluates the pattern matching.
*
* @param in the input to match against, possibly {@code null}
*
* @return the match result (empty if no match), never {@code null}
*/
@Override
public Optional<U> apply(final T in) {
return cases.stream().
filter(c -> c.p.test(in)).
findFirst().
map(c -> c.q.apply(in));
}
@RequiredArgsConstructor(access = PRIVATE)
public final class When {
/**
* Number of frames to discard when creating an exception for a match.
* Very sensitive to implementation. This aids in understanding stack
* traces from matching, discarding internal machinery and leaving the
* actual throwing call at the top of the stack.
*/
private static final int N = 7;
private final Predicate<? super T> when;
/**
* Ends a when/then pair, evaluating <var>then</var> against the input
* if matched.
*
* @param then the pattern matching function, never {@code null}
*
* @return the pattern matcher, never {@code null}
*/
public Matching<T, U> then(
final Function<? super T, ? extends U> then) {
cases.add(new Case(when, then));
return Matching.this;
}
/**
* Ends a when/then pair, returning <var>then</var> if matched.
*
* @param then the pattern matching value, possibly {@code null}
*
* @return the pattern matcher, never {@code null}
*/
public Matching<T, U> then(final U then) {
cases.add(new Case(when, x -> then));
return Matching.this;
}
/**
* Ends a when/then pair, evaluating <var>then</var> independent of
* supplier if matched.
*
* @param then the pattern matching supplier, never {@code null}
*
* @return the pattern matcher, never {@code null}
*/
public Matching<T, U> then(final Supplier<? extends U> then) {
cases.add(new Case(when, x -> then.get()));
return Matching.this;
}
/**
* Ends a when/then pair, evaluating <var>then</var> to {@code null}
* if matched.
*
* @param then the input consumer, never {@code null}
*
* @return the pattern matcher, never {@code null}
*/
public Matching<T, U> then(final Consumer<? super T> then) {
cases.add(new Case(when, o -> {
then.accept(o);
return null;
}));
return Matching.this;
}
/**
* Ends a when/then pair, evaluating <var>then</var> independent of
* supplier and throwing the new exception if matched.
*
* @param then the pattern matching exception supplier, never {@code
* null}
*
* @return the pattern matcher, never {@code null}
*/
public Matching<T, U> thenThrow(
final Supplier<RuntimeException> then) {
cases.add(new Case(when, x -> {
final RuntimeException e = then.get();
final List<StackTraceElement> stack = asList(
e.getStackTrace());
e.setStackTrace(stack.subList(N, stack.size()).
toArray(new StackTraceElement[stack.size() - N]));
throw e;
}));
return Matching.this;
}
}
@RequiredArgsConstructor(access = PRIVATE)
private final class Case {
private final Predicate<? super T> p;
private final Function<? super T, ? extends U> q;
}
} UPDATE: I recently found the very nice Derive4J annotation processor library which provides similar functionality in the form of Algebraic Data Types (ADTs), aka, Sum Types in functional languages.
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