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Some more Semigroup instances #246

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Merged
jbgi merged 4 commits into from
May 13, 2016
Merged

Some more Semigroup instances #246

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6892c93
Add a Semigroup for IO
puffnfresh May 13, 2016
f89381d
Add "first" and "last" semigroups
puffnfresh May 13, 2016
637c18c
Add a Monoid for Unit
puffnfresh May 13, 2016
77f45b6
Add a Monoid for IO
puffnfresh May 13, 2016
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14 changes: 14 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 @@ -4,6 +4,8 @@
import static fj.Function.flip;
import fj.data.Array;
import fj.data.List;
import fj.data.IO;
import fj.data.IOFunctions;
import fj.data.Natural;
import fj.data.Option;
import fj.data.Set;
Expand Down Expand Up @@ -384,6 +386,18 @@ public static <A> Monoid<Array<A>> arrayMonoid() {
return monoid(Semigroup.<A>arraySemigroup(), Array.<A>empty());
}

/**
* A monoid for IO values.
*/
public static <A> Monoid<IO<A>> ioMonoid(final Monoid <A> ma) {
return monoid(Semigroup.ioSemigroup(ma.semigroup()), IOFunctions.unit(ma.zero()));
}

/**
* A monoid for the Unit value.
*/
public static final Monoid<Unit> unitMonoid = monoid(Semigroup.unitSemigroup, Unit.unit());

/**
* A monoid for sets.
*
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23 changes: 23 additions & 0 deletions core/src/main/java/fj/Semigroup.java
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,8 @@

import fj.data.Array;
import fj.data.List;
import fj.data.IO;
import fj.data.IOFunctions;
import fj.data.Natural;
import fj.data.NonEmptyList;
import fj.data.Option;
Expand Down Expand Up @@ -262,6 +264,20 @@ public static <A> Semigroup<A> semigroup(final F2<A, A, A> sum) {
public static final Semigroup<StringBuilder> stringBuilderSemigroup =
semigroup((s1, s2) -> new StringBuilder(s1).append(s2));

/**
* A semigroup which always uses the "first" (left-hand side) value.
*/
public static <A> Semigroup<A> firstSemigroup() {
return semigroup((a1, a2) -> a1);
}

/**
* A semigroup which always uses the "last" (right-hand side) value.
*/
public static <A> Semigroup<A> lastSemigroup() {
return semigroup((a1, a2) -> a2);
}

/**
* A semigroup for functions.
*
Expand Down Expand Up @@ -355,6 +371,13 @@ public static <A, B> Semigroup<P2<A, B>> p2Semigroup(final Semigroup<A> sa, fina
return semigroup((a1, a2) -> P.lazy(() -> sa.sum(a1._1(), a2._1()), () -> sb.sum(a1._2(), a2._2())));
}

/**
* A semigroup for IO values.
*/
public static <A> Semigroup<IO<A>> ioSemigroup(final Semigroup <A> sa) {
return semigroup((a1, a2) -> IOFunctions.liftM2(a1, a2, sa::sum));
}

/**
* A semigroup for the Unit value.
*/
Expand Down