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Fixes #417. Add fj.data.State.bind(F<A, State<S, B>>). #418

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Merged
tonymorris merged 1 commit into from
Aug 6, 2020
Merged

Fixes #417. Add fj.data.State.bind(F<A, State<S, B>>). #418

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18 changes: 18 additions & 0 deletions core/src/main/java/fj/data/State.java
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Original file line number Diff line number Diff line change
Expand Up @@ -111,6 +111,24 @@ public State<S, A> withs(F<S, S> f) {
return suspended(s -> runF.f(f.f(s)));
}

/**
* Bind the given function across this state.
*
* @param f the given function
* @param <B> the type of the output value
* @return the state
*/
public <B> State<S, B> bind(F<A, State<S, B>> f) {
return flatMap(f);
}

/**
* Bind the given function across this state.
*
* @param f the given function
* @param <B> the type of the output value
* @return the state
*/
public <B> State<S, B> flatMap(F<A, State<S, B>> f) {
return suspended(s -> runF.f(s).bind(result -> Trampoline.pure(f.f(result._2()).run(result._1()))));
}
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39 changes: 36 additions & 3 deletions core/src/test/java/fj/data/StateTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -2,14 +2,47 @@

import org.junit.Test;

import static fj.P.p;
import static org.junit.Assert.assertEquals;

/**
* Created by MarkPerry on 18/12/2014.
*/
public class StateTest {

@Test
public void map() {
@Test
public void testBind() {
assertEquals(p(2, "one"), state().run(1));
assertEquals(p(3, "two"), state().run(2));
assertEquals(p(4, "three"), state().run(3));
assertEquals(p(2, "?"), state().bind(state -> State.constant("?")).run(1));
assertEquals(p(3, "?"), state().bind(state -> State.constant("?")).run(2));
assertEquals(p(4, "?"), state().bind(state -> State.constant("?")).run(3));
}

@Test
public void testFlatMap() {
assertEquals(p(2, "one"), state().run(1));
assertEquals(p(3, "two"), state().run(2));
assertEquals(p(4, "three"), state().run(3));
assertEquals(p(2, "?"), state().flatMap(state -> State.constant("?")).run(1));
assertEquals(p(3, "?"), state().flatMap(state -> State.constant("?")).run(2));
assertEquals(p(4, "?"), state().flatMap(state -> State.constant("?")).run(3));
}

}
private static final State<Integer, String> state() {
return State.<Integer, String>unit(i -> p(i + 1, toLapine(i)));
}

private static String toLapine(
final int i) {
return i == 1 ?
"one" :
i == 2 ?
"two" :
i == 3 ?
"three" :
i == 4 ?
"four" : "hrair";
}
}