Document emit option in process outputs (#4437)

Signed-off-by: Marcel Ribeiro-Dantas <mribeirodantas@seqera.io>
Signed-off-by: Ben Sherman <bentshermann@gmail.com>
Co-authored-by: Paolo Di Tommaso <paolo.ditommaso@gmail.com>
Co-authored-by: Ben Sherman <bentshermann@gmail.com>

Author: 3 people

docs/process.md

@@ -1177,6 +1177,52 @@ output:
 
 In this example, the process is normally expected to produce an `output.txt` file, but in the cases where the file is legitimately missing, the process does not fail. The output channel will only contain values for those processes that produce `output.txt`.
 
+(process-multiple-outputs)=
+
+### Multiple outputs
+
+When a process declares multiple outputs, each output can be accessed by index. The following example prints the second process output (indexes start at zero):
+
+```groovy
+process FOO {
+    output:
+    path 'bye_file.txt'
+    path 'hi_file.txt'
+
+    """
+    echo "bye" > bye_file.txt
+    echo "hi" > hi_file.txt
+    """
+}
+
+workflow {
+    FOO()
+    FOO.out[1].view()
+}
+```
+
+You can also use the `emit` option to assign a name to each output and access them by name:
+
+```groovy
+process FOO {
+    output:
+    path 'bye_file.txt', emit: bye_file
+    path 'hi_file.txt',  emit: hi_file
+
+    """
+    echo "bye" > bye_file.txt
+    echo "hi" > hi_file.txt
+    """
+}
+
+workflow {
+    FOO()
+    FOO.out.hi_file.view()
+}
+```
+
+See {ref}`workflow-process-invocation` for more details.
+
 ## When
 
 The `when` block allows you to define a condition that must be satisfied in order to execute the process. The condition can be any expression that returns a boolean value.

docs/workflow.md

@@ -41,6 +41,8 @@ The `main:` label can be omitted if there are no `take:` or `emit:` blocks.
 Workflows were introduced in DSL2. If you are still using DSL1, see the {ref}`dsl1-page` page to learn how to migrate your Nextflow pipelines to DSL2.
 :::
 
+(workflow-process-invocation)=
+
 ## Process invocation
 
 A process can be invoked like a function in a workflow definition, passing the expected input channels like function arguments. For example:
@@ -142,6 +144,8 @@ workflow {
 }
 ```
 
+See {ref}`process-multiple-outputs` for more details.
+
 ### Process named stdout
 
 The `emit` option can also be used to name a `stdout` output: