docs/howto: combine multiple if clauses

This adds a Commented CUE that shows several examples of combining the
same set of if clauses.

Unification at the top-level of the evaluation is used in order to keep
the scope of the examples as general as possible, so that the reader can
more easily extract and use their lessons in both field and list
comprehensions - without having to first extract them from a
comprehension type that they're /not/ placed inside.

Closes cue-lang/docs-and-content#185.

Preview-Path: /docs/howto/combine-multiple-if-clauses/
Signed-off-by: Jonathan Matthews <[email protected]>
Change-Id: I899019354b0b31c66dcff8a2458812011499f384
Dispatch-Trailer: {"type":"trybot","CL":1203075,"patchset":1,"ref":"refs/changes/75/1203075/1","targetBranch":"master"}

content/docs/howto/combine-multiple-if-clauses/en.md

@@ -0,0 +1,66 @@
+---
+title: 'Combining multiple "if" clauses'
+authors: [jpluscplusm]
+toc_hide: true
+tags: [commented cue]
+---
+
+This [Commented CUE]({{< relref "docs/howto/about-commented-cue-guides" >}})
+demonstrates how to combine more than one `if` clause. This can be useful when
+your decison to include data needs to be based on multiple conditions.
+
+{{{with code "en" "cc"}}}
+exec cue export
+cmp stdout out
+-- file.cue --
+package example
+
+// All these examples represent identical
+// combinations of the same set of "if" clauses.
+
+// All conditions on separate lines:
+if #A < 100
+if #B > 100
+if (#C & number) != _|_
+if #A < #C
+if #B < #C {
+	success: true
+}
+
+// Some conditions on the same line and some on
+// separate lines:
+if #A < 100 if #B > 100
+if (#C & number) != _|_
+if #A < #C if #B < #C {
+	success: true
+}
+
+// All conditions on the same line:
+if #A < 100 if #B > 100 if (#C & number) != _|_ if #A < #C if #B < #C {
+	success: true
+}
+
+#A: 42
+#B: 137
+#C: 10101
+-- out --
+{
+    "success": true
+}
+{{{end}}}
+
+{{<info>}}
+Because `if` clauses can be used in
+[list comprehensions]({{< relref "/docs/tour/expressions/listcomp" >}})
+and
+[field comprehensions]({{< relref "/docs/tour/expressions/fieldcomp" >}}),
+the
+[conditional field]({{< relref "/docs/tour/expressions/conditional" >}})
+examples demonstrated above are valid in both contexts.
+{{</info>}}
+
+## Related content
+
+- {{< linkto/related/tour "expressions/conditional" >}}
+- {{< linkto/related/tour "expressions/listcomp" >}}
+- {{< linkto/related/tour "expressions/fieldcomp" >}}

content/docs/howto/combine-multiple-if-clauses/gen_cache.cue

@@ -0,0 +1,18 @@
+package site
+{
+	content: {
+		docs: {
+			howto: {
+				"combine-multiple-if-clauses": {
+					page: {
+						cache: {
+							code: {
+								cc: "8/NaB+qCrVCbjIoj3uoriSzwKl8fmeyoJebohy2SLW4="
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}

content/docs/howto/combine-multiple-if-clauses/page.cue

@@ -0,0 +1,3 @@
+package site
+
+content: docs: howto: "combine-multiple-if-clauses": page: _

hugo/content/en/docs/howto/combine-multiple-if-clauses/index.md

@@ -0,0 +1,67 @@
+---
+title: 'Combining multiple "if" clauses'
+authors: [jpluscplusm]
+toc_hide: true
+tags: [commented cue]
+---
+
+This [Commented CUE]({{< relref "docs/howto/about-commented-cue-guides" >}})
+demonstrates how to combine more than one `if` clause. This can be useful when
+your decison to include data needs to be based on multiple conditions.
+
+{{< code-tabs >}}
+{{< code-tab name="file.cue" language="cue" area="top-left" >}}
+package example
+
+// All these examples represent identical
+// combinations of the same set of "if" clauses.
+
+// All conditions on separate lines:
+if #A < 100
+if #B > 100
+if (#C & number) != _|_
+if #A < #C
+if #B < #C {
+	success: true
+}
+
+// Some conditions on the same line and some on
+// separate lines:
+if #A < 100 if #B > 100
+if (#C & number) != _|_
+if #A < #C if #B < #C {
+	success: true
+}
+
+// All conditions on the same line:
+if #A < 100 if #B > 100 if (#C & number) != _|_ if #A < #C if #B < #C {
+	success: true
+}
+
+#A: 42
+#B: 137
+#C: 10101
+{{< /code-tab >}}
+{{< code-tab name="TERMINAL" language="" area="top-right" type="terminal" codetocopy="Y3VlIGV4cG9ydA==" >}}
+$ cue export
+{
+    "success": true
+}
+{{< /code-tab >}}
+{{< /code-tabs >}}
+
+{{<info>}}
+Because `if` clauses can be used in
+[list comprehensions]({{< relref "/docs/tour/expressions/listcomp" >}})
+and
+[field comprehensions]({{< relref "/docs/tour/expressions/fieldcomp" >}}),
+the
+[conditional field]({{< relref "/docs/tour/expressions/conditional" >}})
+examples demonstrated above are valid in both contexts.
+{{</info>}}
+
+## Related content
+
+- {{< linkto/related/tour "expressions/conditional" >}}
+- {{< linkto/related/tour "expressions/listcomp" >}}
+- {{< linkto/related/tour "expressions/fieldcomp" >}}