Add Result.fromMaybe(Maybe, Supplier<Exception>)

src/main/java/net/kemitix/mon/maybe/Just.java

@@ -36,7 +36,6 @@ import java.util.stream.Stream;
  * A Maybe where a value is present.
  *
  * @param <T> the type of the content
- *
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 @RequiredArgsConstructor(access = AccessLevel.PROTECTED)
@@ -45,7 +44,7 @@ final class Just<T> implements Maybe<T> {
     private final T value;
 
     @Override
-    public <R> Maybe<?> map(final Function<T, R> f) {
+    public <R> Maybe<R> map(final Function<T, R> f) {
         return new Just<>(f.apply(value));
     }
 

src/main/java/net/kemitix/mon/maybe/Maybe.java

@@ -25,46 +25,21 @@ import lombok.NonNull;
 import net.kemitix.mon.Functor;
 
 import java.util.Optional;
+import java.util.function.Function;
 
 /**
  * A value that may or may not be present.
  *
  * @param <T> the type of the content of the Just
- *
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 public interface Maybe<T> extends Functor<T, Maybe<?>>, MaybeStream<T>, MaybeOptional<T> {
 
-    /**
-     * Create a Maybe for the value that is present.
-     *
-     * @param value the value, not null
-     * @param <T>   the type of the value
-     *
-     * @return a Maybe of the value
-     */
-    static <T> Maybe<T> just(@NonNull final T value) {
-        return new Just<>(value);
-    }
-
-    /**
-     * Create a Maybe for a lack of a value.
-     *
-     * @param <T> the type of the missing value
-     *
-     * @return an empty Maybe
-     */
-    @SuppressWarnings("unchecked")
-    static <T> Maybe<T> nothing() {
-        return (Maybe<T>) Nothing.INSTANCE;
-    }
-
     /**
      * Create a Maybe for the value that may or may not be present.
      *
      * @param value the value, may be null
      * @param <T>   the type of the value
-     *
      * @return a Maybe, either a Just, or Nothing if value is null
      */
     static <T> Maybe<T> maybe(final T value) {
@@ -74,12 +49,33 @@ public interface Maybe<T> extends Functor<T, Maybe<?>>, MaybeStream<T>, MaybeOpt
         return just(value);
     }
 
+    /**
+     * Create a Maybe for a lack of a value.
+     *
+     * @param <T> the type of the missing value
+     * @return an empty Maybe
+     */
+    @SuppressWarnings("unchecked")
+    static <T> Maybe<T> nothing() {
+        return (Maybe<T>) Nothing.INSTANCE;
+    }
+
+    /**
+     * Create a Maybe for the value that is present.
+     *
+     * @param value the value, not null
+     * @param <T>   the type of the value
+     * @return a Maybe of the value
+     */
+    static <T> Maybe<T> just(@NonNull final T value) {
+        return new Just<>(value);
+    }
+
     /**
      * Create a Maybe from an {@link Optional}.
      *
      * @param optional the Optional
      * @param <T>      the type of the Optional
-     *
      * @return a Maybe
      * @deprecated need to find a better way of converting an Optional to a Maybe, but
      * without having to pass the Optional as a parameter
@@ -91,4 +87,6 @@ public interface Maybe<T> extends Functor<T, Maybe<?>>, MaybeStream<T>, MaybeOpt
                 .orElse(nothing());
     }
 
+    @Override
+    <R> Maybe<R> map(Function<T, R> f);
 }

src/main/java/net/kemitix/mon/maybe/Nothing.java

@@ -32,7 +32,6 @@ import java.util.stream.Stream;
  * A Maybe where no value is present.
  *
  * @param <T> the type of the missing content
- *
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 final class Nothing<T> implements Maybe<T> {
@@ -40,8 +39,8 @@ final class Nothing<T> implements Maybe<T> {
     static final Maybe<?> INSTANCE = new Nothing<>();
 
     @Override
-    public <R> Maybe<?> map(final Function<T, R> f) {
+    public <R> Maybe<R> map(final Function<T, R> f) {
-        return this;
+        return (Maybe<R>) INSTANCE;
     }
 
     @Override

src/main/java/net/kemitix/mon/result/Result.java

@@ -26,25 +26,27 @@ import net.kemitix.mon.maybe.Maybe;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Predicate;
+import java.util.function.Supplier;
 
 /**
  * An Either type for holding a result or an error (exception).
  *
  * @param <T> the type of the result when a success
- *
  * @author Paul Campbell (pcampbell@kemitix.net)
  */
 public interface Result<T> {
 
     /**
-     * Create a Result for a success.
+     * Creates a Result from the Maybe, where the Result will be an error if the Maybe is Nothing.
      *
-     * @param value the value
+     * @param maybe the Maybe the might contain the value of the Result
-     * @param <T> the type of the value
+     * @param error the error that will be the Result if maybe is Nothing
-     * @return a successful Result
+     * @param <T>   the type of the Maybe and the Result
+     * @return a Result containing the value of the Maybe when it is a Just, or the error when it is Nothing
      */
-    static <T> Result<T> ok(final T value) {
+    static <T> Result<T> fromMaybe(final Maybe<T> maybe, final Supplier<Exception> error) {
-        return new Success<>(value);
+        return maybe.map(Result::ok)
+                .orElseGet(() -> Result.error(error.get()));
     }
 
     /**
@@ -58,6 +60,17 @@ public interface Result<T> {
         return new Err<>(error);
     }
 
+    /**
+     * Create a Result for a success.
+     *
+     * @param value the value
+     * @param <T>   the type of the value
+     * @return a successful Result
+     */
+    static <T> Result<T> ok(final T value) {
+        return new Success<>(value);
+    }
+
     /**
      * Checks of the Result is an error.
      *
@@ -77,7 +90,6 @@ public interface Result<T> {
      *
      * @param f   the mapping function the produces a Result
      * @param <R> the type of the value withing the Result of the mapping function
-     *
      * @return a Result
      */
     <R> Result<R> flatMap(Function<T, Result<R>> f);
@@ -87,7 +99,6 @@ public interface Result<T> {
      *
      * @param f   the mapping function to produce the new value
      * @param <R> the type of the result of the mapping function
-     *
      * @return a Result
      */
     <R> Result<R> map(Function<T, R> f);
@@ -107,4 +118,5 @@ public interface Result<T> {
      * @return a Result containing a Maybe that may or may not contain a value
      */
     Result<Maybe<T>> maybe(Predicate<T> predicate);
+
 }

src/test/java/net/kemitix/mon/ResultTest.java

@@ -6,8 +6,6 @@ import net.kemitix.mon.result.Result;
 import org.assertj.core.api.WithAssertions;
 import org.junit.Test;
 
-import java.util.function.Predicate;
-
 public class ResultTest implements WithAssertions {
 
     @Test
@@ -195,6 +193,35 @@ public class ResultTest implements WithAssertions {
         );
     }
 
+    @Test
+    public void justMaybe_isSuccess() {
+        //given
+        final Maybe<Integer> just = Maybe.just(1);
+        //when
+        final Result<Integer> result = Result.fromMaybe(just, () -> new RuntimeException());
+        //then
+        assertThat(result.isOkay()).isTrue();
+        result.match(
+                success -> assertThat(success).isEqualTo(1),
+                error -> fail("not an error")
+        );
+    }
+
+    @Test
+    public void nothingMaybe_isError() {
+        //given
+        final Maybe<Object> nothing = Maybe.nothing();
+        final RuntimeException exception = new RuntimeException();
+        //when
+        final Result<Object> result = Result.fromMaybe(nothing, () -> exception);
+        //then
+        assertThat(result.isError()).isTrue();
+        result.match(
+                success -> fail("not a success"),
+                error -> assertThat(error).isSameAs(exception)
+        );
+    }
+
     @Test
     public void useCase_whenOkay_thenReturnSuccess() {
         //given
@@ -265,20 +292,20 @@ public class ResultTest implements WithAssertions {
                                     calculateAverage(adjustedIntFromFile1, intFromFile2))));
         }
 
-        private Result<Double> calculateAverage(final Integer val1, final Integer val2) {
+        private Result<Integer> readIntFromFile(final String fileName) {
-            return Result.ok((double) (val1 + val2) / 2);
-        }
-
-        private Result<Integer> adjustValue(Integer value) {
-            return Result.ok(value + 2);
-        }
-
-        private Result<Integer> readIntFromFile(String fileName) {
             if (okay) {
                 return Result.ok(fileName.length());
             }
             return Result.error(new RuntimeException(fileName));
         }
 
+        private Result<Integer> adjustValue(final Integer value) {
+            return Result.ok(value + 2);
+        }
+
+        private Result<Double> calculateAverage(final Integer val1, final Integer val2) {
+            return Result.ok((double) (val1 + val2) / 2);
+        }
+
     }
 }