Syntax Lookup placeholder

misc_docs/syntax/language_placeholder.mdx

@@ -0,0 +1,57 @@
+---
+id: "placeholder"
+keywords: ["placeholder"]
+name: "_"
+summary: "This is a `placeholder`."
+category: "languageconstructs"
+---
+
+Placeholders may be used for [ignoring parts of values](/docs/manual/latest/pattern-matching-destructuring#ignore-part-of-a-value) (including serving as a catch-all in `switch` statements), and [specifying the position of an argument](/docs/manual/latest/pipe#pipe-placeholders).
+
+### Example: Ignoring values 
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+switch person1 {
+| Teacher(_) => Js.log("Hi teacher")
+| Student(_) => Js.log("Hey student")
+}
+```
+
+```js
+if (person1.TAG === /* Teacher */ 0) {
+  console.log("Hi teacher");
+} else {
+  console.log("Hey student");
+}
+```
+
+</CodeTab>
+
+### Example: Specifying an argument position
+
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let greet = (greeting, name) => greeting ++ " " ++ name
+
+// Pipe the value "John" into the second argument of the greet() function.
+let greetJohn = "John"->greet("Hello", _)
+```
+
+```js
+function greet(greeting, name) {
+  return greeting + " " + name;
+}
+
+var greetJohn = "Hello John";
+```
+
+</CodeTab>
+
+### References
+
+* [Ignore part of a value](/docs/manual/latest/pattern-matching-destructuring#ignore-part-of-a-value)
+* [Pipe placeholders](/docs/manual/latest/pipe#pipe-placeholders)

src/SyntaxLookup.res

@@ -31,12 +31,13 @@ let requireSyntaxFile: string => MdxComp.t = %raw(`
 `)
 
 module Category = {
-  type t = Decorators | ControlFlow | Operators | SpecialValues | Other
+  type t = Decorators | ControlFlow | Operators | LanguageConstructs | SpecialValues | Other
 
   let toString = t =>
     switch t {
     | Decorators => "Decorators"
     | Operators => "Operators"
+    | LanguageConstructs => "Language Constructs"
     | SpecialValues => "Special Values"
     | ControlFlow => "Control Flow"
     | Other => "Other"
@@ -48,6 +49,7 @@ module Category = {
     | "controlflow" => ControlFlow
     | "specialvalues" => SpecialValues
     | "operators" => Operators
+    | "languageconstructs" => LanguageConstructs
     | _ => Other
     }
   }
@@ -241,6 +243,7 @@ let make = () => {
     let initial = [
       Decorators,
       Operators,
+      LanguageConstructs,
       ControlFlow,
       SpecialValues,
       Other,