Refactor integrator API.

examples/src/main/kotlin/space/kscience/kmath/functions/integrate.kt

@@ -5,8 +5,6 @@
 
 package space.kscience.kmath.functions
 
-import space.kscience.kmath.integration.integrate
-import space.kscience.kmath.integration.value
 import space.kscience.kmath.operations.DoubleField
 import kotlin.math.pow
 
@@ -18,5 +16,5 @@ fun main() {
     val result = DoubleField.integrate(0.0..10.0, function = function)
 
     //the value is nullable because in some cases the integration could not succeed
-    println(result.value)
+    println(result.valueOrNull)
 }

examples/src/main/kotlin/space/kscience/kmath/functions/matrixIntegration.kt

@@ -5,8 +5,6 @@
 
 package space.kscience.kmath.functions
 
-import space.kscience.kmath.integration.integrate
-import space.kscience.kmath.integration.value
 import space.kscience.kmath.nd.StructureND
 import space.kscience.kmath.nd.nd
 import space.kscience.kmath.operations.DoubleField
@@ -27,6 +25,6 @@ fun main(): Unit = DoubleField {
         val result = integrate(0.0..10.0, function = function)
 
         //the value is nullable because in some cases the integration could not succeed
-        println(result.value)
+        println(result.valueOrNull)
     }
 }

kmath-commons/src/main/kotlin/space/kscience/kmath/commons/integration/GaussRuleIntegrator.kt

@@ -78,6 +78,6 @@ public class GaussRuleIntegrator(
             function: (Double) -> Double,
         ): Double = GaussRuleIntegrator(numPoints, type).integrate(
             UnivariateIntegrand(function, IntegrationRange(range))
-        ).value!!
+        ).valueOrNull!!
     }
 }

kmath-functions/src/commonMain/kotlin/space/kscience/kmath/integration/GaussIntegrator.kt

@@ -66,31 +66,25 @@ public class GaussIntegrator<T : Any>(
         return integrand + IntegrandValue(res) + IntegrandCallsPerformed(integrand.calls + points.size)
     }
 
-    public companion object {
+    public companion object
-
-    }
 }
 
 /**
- * Integrate [T]-valued univariate function using provided set of [IntegrandFeature]
+ * Create a Gauss-Legendre integrator for this field
- * Following features are evaluated:
+ * Following integrand features are accepted:
  * * [GaussIntegratorRuleFactory] - A factory for computing the Gauss integration rule. By default uses [GaussLegendreRuleFactory]
  * * [IntegrationRange] - the univariate range of integration. By default uses 0..1 interval.
  * * [IntegrandMaxCalls] - the maximum number of function calls during integration. For non-iterative rules, always uses the maximum number of points. By default uses 10 points.
  * * [UnivariateIntegrandRanges] - Set of ranges and number of points per range. Defaults to given [IntegrationRange] and [IntegrandMaxCalls]
  */
-@UnstableKMathAPI
+public val <T:Any> Field<T>.integrator: GaussIntegrator<T> get() = GaussIntegrator(this)
-public fun <T : Any> Field<T>.integrate(
-    vararg features: IntegrandFeature,
-    function: (Double) -> T,
-): UnivariateIntegrand<T> = GaussIntegrator(this).integrate(UnivariateIntegrand(function, *features))
 
 
 /**
- * Use [GaussIntegrator.Companion.integrate] to integrate the function in the current algebra with given [range] and [numPoints]
+ * Use [integrate] to integrate the function in the current algebra with given [range] and [numPoints]
  */
 @UnstableKMathAPI
-public fun <T : Any> Field<T>.integrate(
+public fun <T : Any> GaussIntegrator<T>.integrate(
     range: ClosedRange<Double>,
     order: Int = 10,
     intervals: Int = 10,
@@ -104,7 +98,7 @@ public fun <T : Any> Field<T>.integrate(
     val ranges = UnivariateIntegrandRanges(
         (0 until intervals).map { i -> (rangeSize * i)..(rangeSize * (i + 1)) to order }
     )
-    return GaussIntegrator(this).integrate(
+    return integrate(
         UnivariateIntegrand(
             function,
             IntegrationRange(range),

kmath-functions/src/commonMain/kotlin/space/kscience/kmath/integration/UnivariateIntegrand.kt

@@ -35,37 +35,41 @@ public typealias UnivariateIntegrator<T> = Integrator<UnivariateIntegrand<T>>
 @JvmInline
 public value class IntegrationRange(public val range: ClosedRange<Double>) : IntegrandFeature
 
-public val <T : Any> UnivariateIntegrand<T>.value: T? get() = getFeature<IntegrandValue<T>>()?.value
+/**
+ * Value of the integrand if it is present or null
+ */
+public val <T : Any> UnivariateIntegrand<T>.valueOrNull: T? get() = getFeature<IntegrandValue<T>>()?.value
+
+/**
+ * Value of the integrand or error
+ */
+public val <T : Any> UnivariateIntegrand<T>.value: T get() = valueOrNull ?: error("No value in the integrand")
 
 /**
  * A shortcut method to integrate a [function] in [range] with additional [features].
  * The [function] is placed in the end position to allow passing a lambda.
  */
 @UnstableKMathAPI
-public fun UnivariateIntegrator<Double>.integrate(
+public fun <T: Any> UnivariateIntegrator<T>.integrate(
     range: ClosedRange<Double>,
     vararg features: IntegrandFeature,
-    function: (Double) -> Double,
+    function: (Double) -> T,
-): Double = integrate(
+): UnivariateIntegrand<T> = integrate(UnivariateIntegrand(function, IntegrationRange(range), *features))
-    UnivariateIntegrand(function, IntegrationRange(range), *features)
-).value ?: error("Unexpected: no value after integration.")
 
 /**
  * A shortcut method to integrate a [function] in [range] with additional [features].
  * The [function] is placed in the end position to allow passing a lambda.
  */
 @UnstableKMathAPI
-public fun UnivariateIntegrator<Double>.integrate(
+public fun <T: Any> UnivariateIntegrator<T>.integrate(
     range: ClosedRange<Double>,
     featureBuilder: MutableList<IntegrandFeature>.() -> Unit = {},
-    function: (Double) -> Double,
+    function: (Double) -> T,
-): Double {
+): UnivariateIntegrand<T> {
     //TODO use dedicated feature builder class instead or add extensions to MutableList<IntegrandFeature>
     val features = buildList {
         featureBuilder()
         add(IntegrationRange(range))
     }
-    return integrate(
+    return integrate(UnivariateIntegrand(function, *features.toTypedArray()))
-        UnivariateIntegrand(function, *features.toTypedArray())
-    ).value ?: error("Unexpected: no value after integration.")
 }

kmath-functions/src/commonTest/kotlin/space/kscience/kmath/integration/GaussIntegralTest.kt

@@ -19,7 +19,7 @@ class GaussIntegralTest {
         val res = DoubleField.integrate(0.0..2 * PI) { x ->
             sin(x)
         }
-        assertEquals(0.0, res.value!!, 1e-2)
+        assertEquals(0.0, res.valueOrNull!!, 1e-2)
     }
 
     @Test
@@ -31,7 +31,7 @@ class GaussIntegralTest {
                 0.0
             }
         }
-        assertEquals(20.0, res.value!!, 0.5)
+        assertEquals(20.0, res.valueOrNull!!, 0.5)
     }