diff --git a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionCatalog.java b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionCatalog.java
index c15706715..1b11d9115 100644
--- a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionCatalog.java
+++ b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionCatalog.java
@@ -17,9 +17,17 @@
 package org.springframework.cloud.function.context;
 
 import java.util.Set;
+import java.util.function.Function;
 
+import org.springframework.messaging.Message;
+import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.util.MimeType;
 
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.util.function.Tuple2;
+import reactor.util.function.Tuple3;
+
 
 /**
  * @author Dave Syer
@@ -29,42 +37,70 @@ public interface FunctionCatalog {
 
 
 
-	default <T> T lookup(String name, MimeType... acceptedOutputTypes) {
-		throw new UnsupportedOperationException(
-				"This instance of FunctionCatalog does not support this operation");
-	}
+    /**
+     * Will look up the instance of the functional interface. The 'acceptedOutputTypes'
+     * will be used by the available {@link MessageConverter}s to convert the values of output streams.
+     * This means that regardless of the actual function signature the return types of the function
+     * will always be {@link Message}.
+     * <br>
+     * For example,
+     * <br>
+     * Assume user function is:
+     * <br>
+     * {@code
+     * Function<Flux<String>, Tuple2<Flux<Foo>>, Mono<String>>>
+     * }
+     * <br>
+     * While you can provide input of any type (the internal conversion routing will kick in),
+     * the output type will always be assumed as Message<T> where 'T' is the type returned by
+     * the corresponding {@link MessageConverter#toMessage(Object, org.springframework.messaging.MessageHeaders)}
+     * method. For example, providing that the existing {@link MessageConverter}s convert to Message<byte[]>, the
+     * above user function could be invoked as:
+     * <br>
+     * {@code
+     * Function<Flux<String>, Tuple2<Flux<Message<byte[]>>, Mono<Message<byte[]>>>>
+     * }
+     * @param functionDefinition the definition of the functional interface. Must not be null;
+     * @param acceptedOutputTypes ordered array of accepted mime types to be used to
+     * generate output streams. The size of the array must match the count of function output.
+     * @return instance of the functional interface registered with this catalog
+     */
+    default <T> T lookup(String functionDefinition, MimeType... acceptedOutputTypes) {
+        throw new UnsupportedOperationException(
+                "This instance of FunctionCatalog does not support this operation");
+    }
 
 
-	/**
-	 * Will look up the instance of the functional interface by name only.
-	 * @param <T> instance type
-	 * @param name the name of the functional interface. Must not be null;
-	 * @return instance of the functional interface registered with this catalog
-	 */
-	default <T> T lookup(String name) {
-		return this.lookup(null, name);
-	}
+    /**
+     * Will look up the instance of the functional interface by name only.
+     * @param <T> instance type
+     * @param functionDefinition the definition of the functional interface. Must not be null;
+     * @return instance of the functional interface registered with this catalog
+     */
+    default <T> T lookup(String functionDefinition) {
+        return this.lookup(null, functionDefinition);
+    }
 
-	/**
-	 * Will look up the instance of the functional interface by name and type which can
-	 * only be Supplier, Consumer or Function. If type is not provided, the lookup will be
-	 * made based on name only.
-	 * @param <T> instance type
-	 * @param type the type of functional interface. Can be null
-	 * @param name the name of the functional interface. Must not be null;
-	 * @return instance of the functional interface registered with this catalog
-	 */
-	<T> T lookup(Class<?> type, String name);
+    /**
+     * Will look up the instance of the functional interface by name and type which can
+     * only be Supplier, Consumer or Function. If type is not provided, the lookup will be
+     * made based on name only.
+     * @param <T> instance type
+     * @param type the type of functional interface. Can be null
+     * @param functionDefinition the definition of the functional interface. Must not be null;
+     * @return instance of the functional interface registered with this catalog
+     */
+    <T> T lookup(Class<?> type, String functionDefinition);
 
-	Set<String> getNames(Class<?> type);
+    Set<String> getNames(Class<?> type);
 
-	/**
-	 * Return the count of functions registered in this catalog.
-	 * @return the count of functions registered in this catalog
-	 */
-	default int size() {
-		throw new UnsupportedOperationException(
-				"This instance of FunctionCatalog does not support this operation");
-	}
+    /**
+     * Return the count of functions registered in this catalog.
+     * @return the count of functions registered in this catalog
+     */
+    default int size() {
+        throw new UnsupportedOperationException(
+                "This instance of FunctionCatalog does not support this operation");
+    }
 
 }
diff --git a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/LazyFunctionRegistry.java b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/LazyFunctionRegistry.java
index 5c664fece..076c3d922 100644
--- a/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/LazyFunctionRegistry.java
+++ b/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/LazyFunctionRegistry.java
@@ -42,6 +42,7 @@ import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.converter.CompositeMessageConverter;
+import org.springframework.util.Assert;
 import org.springframework.util.MimeType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
@@ -83,6 +84,7 @@ public class LazyFunctionRegistry implements FunctionRegistry, FunctionInspector
 
 	@SuppressWarnings("unchecked")
 	public <T> T lookup(String definition, MimeType... acceptedOutputTypes) {
+		Assert.notEmpty(acceptedOutputTypes, "'acceptedOutputTypes' must not be null or empty");
 		return (T) this.compose(null, definition, acceptedOutputTypes);
 	}