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Fix regression in lifted semigroup sum. Fix #365 #366

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Merged
jbgi merged 1 commit into from
Oct 7, 2018
Merged

Fix regression in lifted semigroup sum. Fix #365 #366

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31 changes: 31 additions & 0 deletions core/src/main/java/fj/Monoid.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@
import static fj.data.List.nil;
import static fj.data.Natural.natural;
import static fj.data.Option.none;
import static fj.data.Option.some;
import static fj.data.Stream.iterableStream;

import java.math.BigInteger;
Expand Down Expand Up @@ -867,6 +868,36 @@ public List<A> sum(F0<Stream<List<A>>> as) {
});
}

/**
* Lift a {@code Semigroup<A>} for A to a {@code Monoid<Option<A>>}, using Option.none() as zero.
*
* @return A monoid for option.
*/
public static <A> Monoid<Option<A>> optionMonoid(Semigroup<A> aSemigroup) {
return monoidDef(new Monoid.Definition<Option<A>>() {
@Override
public Option<A> empty() {
return none();
}

@Override
public Option<A> append(Option<A> a1, Option<A> a2) {
return a1.liftM2(a2, aSemigroup::sum).orElse(a1).orElse(a2);
}

@Override
public Option<A> multiply(int n, Option<A> oa) {
return n > 0 ? oa.map(a -> aSemigroup.multiply1p(n - 1, a)) : none();
}

@Override
public Option<A> sum(F0<Stream<Option<A>>> oas) {
Stream<A> as = oas.f().bind(Option::toStream);
return as.uncons(none(), h -> tail -> some(aSemigroup.sumStream(h, tail::_1)));
}
});
}

/**
* A monoid for options.
* @deprecated since 4.7. Use {@link #firstOptionMonoid()}.
Expand Down
24 changes: 1 addition & 23 deletions core/src/main/java/fj/Semigroup.java
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Expand Up @@ -172,29 +172,7 @@ public Semigroup<A> dual() {
* Lifts the semigroup to obtain a trivial monoid.
*/
public Monoid<Option<A>> lift() {
Definition<A> def = this.def;
return monoidDef(new Monoid.Definition<Option<A>>() {
@Override
public Option<A> empty() {
return none();
}

@Override
public Option<A> append(Option<A> a1, Option<A> a2) {
return a1.liftM2(a1, def::append).orElse(a1).orElse(a2);
}

@Override
public Option<A> multiply(int n, Option<A> oa) {
return n > 0 ? oa.map(a -> def.multiply1p(n - 1, a)) : none();
}

@Override
public Option<A> sum(F0<Stream<Option<A>>> oas) {
Stream<A> as = oas.f().bind(Option::toStream);
return as.uncons(none(), h -> tail -> some(def.sum(h, tail::_1)));
}
});
return Monoid.optionMonoid(this);
}

/**
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19 changes: 19 additions & 0 deletions core/src/test/java/fj/MonoidTest.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
package fj;

import fj.data.Option;
import fj.data.Stream;
import org.junit.Test;

import static fj.data.Option.some;
import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat;

public class MonoidTest {

@Test
public void lifted_sum_of_two_numbers() {
Monoid<Option<Integer>> optionMonoid = Semigroup.intAdditionSemigroup.lift();
assertThat(optionMonoid.sum(some(3), some(5)), is(some(8)));
assertThat(optionMonoid.sumLeft(Stream.arrayStream(some(3), some(5))), is(some(8)));
}
}