docs/howto: conditionally include list elements

This adds a guide that demonstrates how to include elements in a list
conditionally.

Closes cue-lang/docs-and-content#117

Preview-Path: /docs/howto/conditionally-include-list-elements/
Signed-off-by: Jonathan Matthews <[email protected]>
Change-Id: I4eea7a9294a189f8a7c160eb16f06736f7afa74a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1197391
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>

content/docs/howto/conditionally-include-list-elements/en.md

@@ -0,0 +1,47 @@
+---
+title: Conditionally including elements in a list
+authors: [jpluscplusm]
+toc_hide: true
+tags: [commented cue]
+---
+
+This [Commented CUE]({{< relref "docs/howto#commented-cue-guides" >}})
+demonstrates how to include elements in a list based on some testable condition.
+
+{{{with code "en" "cc"}}}
+exec cue export
+cmp stdout out
+-- example.cue --
+package example
+
+#a: "yes"
+
+A: [
+	1,
+	2,
+
+	// Include a single element using this form:
+	if #a == "yes" {
+		3
+	},
+
+	// Include multiple elements using this form,
+	// which only tests the condition once:
+	if #a == "yes" for e in [
+		4,
+		5,
+		6,
+	] {e},
+]
+-- out --
+{
+    "A": [
+        1,
+        2,
+        3,
+        4,
+        5,
+        6
+    ]
+}
+{{{end}}}

content/docs/howto/conditionally-include-list-elements/gen_cache.cue

@@ -0,0 +1,18 @@
+package site
+{
+	content: {
+		docs: {
+			howto: {
+				"conditionally-include-list-elements": {
+					page: {
+						cache: {
+							code: {
+								cc: "7HIdQ5DAaafYiGotXzpD2y8Dvs1Xs3PlUfim+NB9dW4="
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}

content/docs/howto/conditionally-include-list-elements/page.cue

@@ -0,0 +1,3 @@
+package site
+
+content: docs: howto: "conditionally-include-list-elements": page: _

hugo/content/en/docs/howto/conditionally-include-list-elements/index.md

@@ -0,0 +1,48 @@
+---
+title: Conditionally including elements in a list
+authors: [jpluscplusm]
+toc_hide: true
+tags: [commented cue]
+---
+
+This [Commented CUE]({{< relref "docs/howto#commented-cue-guides" >}})
+demonstrates how to include elements in a list based on some testable condition.
+
+{{< code-tabs >}}
+{{< code-tab name="example.cue" language="cue" area="top-left" >}}
+package example
+
+#a: "yes"
+
+A: [
+	1,
+	2,
+
+	// Include a single element using this form:
+	if #a == "yes" {
+		3
+	},
+
+	// Include multiple elements using this form,
+	// which only tests the condition once:
+	if #a == "yes" for e in [
+		4,
+		5,
+		6,
+	] {e},
+]
+{{< /code-tab >}}
+{{< code-tab name="TERMINAL" language="" area="top-right" type="terminal" codetocopy="Y3VlIGV4cG9ydA==" >}}
+$ cue export
+{
+    "A": [
+        1,
+        2,
+        3,
+        4,
+        5,
+        6
+    ]
+}
+{{< /code-tab >}}
+{{< /code-tabs >}}