Mon
===
TypeAlias, Maybe and Result for Java
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## Maven
```xml
net.kemitix
mon
RELEASE
```
The latest version should be shown above with the nexus and maven-central badges.
## Usage
### TypeAlias
More of a type-wrapper really. It's as close as I could get to a Haskell type alias in Java.
```java
class Goal extends TypeAlias {
private Goal(final String goal) {
super(goal);
}
public static Goal of(final String goal) {
return new Goal(goal);
}
}
```
```java
class Example {
Goal goal = Goal.of("goal");
void foo(final Goal goal) {
System.out.println("The goal is " + goal.getValue());
}
}
```
### Maybe
A Monad.
A non-final substitute for Optional with `peek()` and `stream()` methods.
```java
class Test {
@Test
public void maybeTests() {
// Constructors: maybe(T), just(T) and nothing()
assertThat(Maybe.maybe(null)).isEqualTo(Maybe.nothing());
assertThat(Maybe.maybe(1)).isEqualTo(Maybe.just(1));
// .orElseGet(Supplier)
assertThat(Maybe.nothing().orElseGet(() -> 1)).isEqualTo(1);
assertThat(Maybe.just(1).orElseGet(() -> 2)).isEqualTo(1);
// .orElse(Supplier)
assertThat(Maybe.nothing().orElse(1)).isEqualTo(1);
assertThat(Maybe.just(1).orElse(2)).isEqualTo(1);
// .filter(Predicate)
assertThat(Maybe.just(1).filter(v -> v > 2)).isEqualTo(Maybe.nothing());
assertThat(Maybe.just(3).filter(v -> v > 2)).isEqualTo(Maybe.just(3));
assertThat(Maybe.just(1).toOptional()).isEqualTo(Optional.of(1));
assertThat(Maybe.nothing().toOptional()).isEqualTo(Optional.empty());
// .fromOptional(Optional) is deprecated
assertThat(Maybe.fromOptional(Optional.of(1))).isEqualTo(Maybe.just(1));
assertThat(Maybe.fromOptional(Optional.empty())).isEqualTo(Maybe.nothing());
// An alternative to using .fromOptional(Optional)
assertThat(Optional.of(1).map(Maybe::just).orElse(Maybe::nothing)).isEqualTo(Maybe.just(1));
assertThat(Optional.empty().map(Maybe::just).orElse(Maybe::nothing)).isEqualTo(Maybe.nothing());
// .peek(Consumer)
final AtomicInteger reference = new AtomicInteger(0);
assertThat(Maybe.just(1).peek(reference::set)).isEqualTo(Maybe.just(1));
assertThat(reference).hasValue(1);
assertThat(Maybe.nothing().peek(v -> reference.incrementAndGet())).isEqualTo(Maybe.nothing());
assertThat(reference).hasValue(1);
// .orElseThrow(Supplier)
assertThatCode(() -> Maybe.just(1).orElseThrow(IllegalStateException::new)).doesNotThrowAnyException();
assertThatThrownBy(() -> Maybe.nothing().orElseThrow(IllegalStateException::new)).isInstanceOf(IllegalStateException.class);
// .stream()
assertThat(Maybe.just(1).stream()).containsExactly(1);
assertThat(Maybe.nothing().stream()).isEmpty();
}
}
```
### Result
A container for method return values that may raise an Exception. Useful for when a checked exceptions can't be added
to the method signature.
```java
package net.kemitix.mon;
import net.kemitix.mon.result.Result;
import java.io.IOException;
class ResultExample implements Runnable {
public static void main(String[] args) {
new ResultExample().run();
}
@Override
public void run() {
System.out.println("run");
final Result goodResult = goodMethod();
if (goodResult.isOkay()) {
doGoodThings();
}
if (goodResult.isError()) {
notCalled(0);
}
goodResult.flatMap(number -> convertToString(number))
.flatMap(str -> stringLength(str))
.match(
success -> System.out.format("Length is %s%n", success),
error -> System.out.println("Count not determine length")
);
final Result badResult = badMethod();
badResult.match(
success -> notCalled(success),
error -> handleError(error)
);
}
private Result goodMethod() {
System.out.println("goodMethod");
return Result.ok(1);
}
private void doGoodThings() {
System.out.println("doGoodThings");
}
private void notCalled(final Integer success) {
System.out.println("notCalled");
}
private Result convertToString(final Integer number) {
System.out.println("convertToString");
return Result.ok(String.valueOf(number));
}
private Result stringLength(final String value) {
System.out.println("stringLength");
if (value == null) {
return Result.error(new NullPointerException("value is null"));
}
return Result.ok(value.length());
}
// doesn't need to declare "throws IOException"
private Result badMethod() {
System.out.println("badMethod");
return Result.error(new IOException("error"));
}
private void handleError(final Throwable error) {
System.out.println("handleError");
throw new RuntimeException("Handled exception", error);
}
}
```
Will output:
```text
run
goodMethod
doGoodThings
convertToString
stringLength
Length is 1
badMethod
handleError
Exception in thread "main" java.lang.RuntimeException: Handled exception
at net.kemitix.mon.ResultExample.handleError(ResultExample.java:72)
at net.kemitix.mon.ResultExample.lambda$run$5(ResultExample.java:34)
at net.kemitix.mon.result.Err.match(Err.java:56)
at net.kemitix.mon.ResultExample.run(ResultExample.java:32)
at net.kemitix.mon.ResultExample.main(ResultExample.java:10)
Caused by: java.io.IOException: error
at net.kemitix.mon.ResultExample.badMethod(ResultExample.java:67)
at net.kemitix.mon.ResultExample.run(ResultExample.java:31)
... 1 more
```