[builtin] Builtin as a word

TODO.md

@@ -1,4 +1,3 @@
-[builtin] `Builtin` as a word
 [builtin] `Builtin` -- syntax
 [builtin] add `@` as prefix to all builtins
 [builtin] remove `isPrivate` from Definition

src/lang/compose/compose.ts

@@ -41,6 +41,22 @@ export function compose(
         }
       }
 
+      case "Builtin": {
+        const definition = mod.builtins.get(word.name)
+        if (definition === undefined) {
+          throw new Error(
+            [
+              `[compose / Builtin] I meet undefined builtin.`,
+              ``,
+              `  name: ${word.name}`,
+            ].join("\n"),
+          )
+        }
+
+        composeDefinition(env, definition, options)
+        return
+      }
+
       case "Local": {
         const port = env.stack.pop()
         if (port === undefined) {

src/lang/word/Word.ts

@@ -2,6 +2,7 @@ import { Span } from "../span"
 
 export type Word =
   | Call
+  | Builtin
   | Local
   | PortPush
   | PortReconnect
@@ -16,6 +17,13 @@ export type Call = {
   span: Span
 }
 
+export type Builtin = {
+  "@type": "Word"
+  "@kind": "Builtin"
+  name: string
+  span: Span
+}
+
 export type Local = {
   "@type": "Word"
   "@kind": "Local"

src/lang/word/formatWord.ts

@@ -6,6 +6,10 @@ export function formatWord(word: Word): string {
       return word.name
     }
 
+    case "Builtin": {
+      return `@${word.name}`
+    }
+
     case "Local": {
       return `$${word.name}`
     }