add a standard library, and introduce formed imports (import std "myfile.pork")

frontend/src/main/kotlin/gay/pizza/pork/frontend/ImportLocator.kt

@@ -0,0 +1,3 @@
+package gay.pizza.pork.frontend
+
+data class ImportLocator(val path: String, val form: String? = null)

frontend/src/main/kotlin/gay/pizza/pork/frontend/ImportSource.kt

@@ -0,0 +1,7 @@
+package gay.pizza.pork.frontend
+
+interface ImportSource {
+  val fileContentSource: ContentSource
+
+  fun provideContentSource(form: String): ContentSource
+}

frontend/src/main/kotlin/gay/pizza/pork/frontend/StandardImportSource.kt

@@ -0,0 +1,14 @@
+package gay.pizza.pork.frontend
+
+class StandardImportSource(override val fileContentSource: ContentSource) : ImportSource {
+  private val providers = mutableMapOf<String,ContentSource>()
+
+  override fun provideContentSource(form: String): ContentSource {
+    return providers[form] ?:
+      throw RuntimeException("Unknown import form: $form")
+  }
+
+  fun addContentSource(form: String, contentSource: ContentSource) {
+    providers[form] = contentSource
+  }
+}

frontend/src/main/kotlin/gay/pizza/pork/frontend/World.kt

@@ -7,37 +7,57 @@ import gay.pizza.pork.parser.Parser
 import gay.pizza.pork.parser.TokenStreamSource
 import gay.pizza.pork.parser.Tokenizer
 
-class World(val contentSource: ContentSource) {
+class World(val importSource: ImportSource) {
   private val internalUnits = mutableMapOf<String, CompilationUnit>()
 
   val units: List<CompilationUnit>
     get() = internalUnits.values.toList()
 
-  private fun loadOneUnit(path: String): CompilationUnit {
-    val stableIdentity = contentSource.stableContentIdentity(path)
-    val cached = internalUnits[stableIdentity]
+  private fun loadOneUnit(importLocator: ImportLocator): CompilationUnit {
+    val contentSource = pickContentSource(importLocator.form)
+    val stableKey = stableIdentity(importLocator, contentSource = contentSource)
+    val cached = internalUnits[stableKey]
     if (cached != null) {
       return cached
     }
-    val charSource = contentSource.loadAsCharSource(path)
+    val charSource = contentSource.loadAsCharSource(importLocator.path)
     val tokenizer = Tokenizer(charSource)
     val tokenStream = tokenizer.tokenize()
     val parser = Parser(TokenStreamSource(tokenStream), DiscardNodeAttribution)
-    return parser.readCompilationUnit()
+    val unit = parser.readCompilationUnit()
+    internalUnits[stableKey] = unit
+    return unit
   }
 
   private fun resolveAllImports(unit: CompilationUnit): Set<CompilationUnit> {
     val units = mutableSetOf<CompilationUnit>()
     for (declaration in unit.declarations.filterIsInstance<ImportDeclaration>()) {
-      val importedUnit = loadOneUnit(declaration.path.text)
+      val importLocator = ImportLocator(declaration.path.text, form = declaration.form?.id)
+      val importedUnit = loadOneUnit(importLocator)
       units.add(importedUnit)
     }
     return units
   }
 
-  fun load(path: String): CompilationUnit {
-    val unit = loadOneUnit(path)
+  fun load(importLocator: ImportLocator): CompilationUnit {
+    val unit = loadOneUnit(importLocator)
     resolveAllImports(unit)
     return unit
   }
+
+  private fun pickContentSource(form: String? = null): ContentSource {
+    if (form != null) {
+      return importSource.provideContentSource(form)
+    }
+    return importSource.fileContentSource
+  }
+
+  fun stableIdentity(
+    importLocator: ImportLocator,
+    contentSource: ContentSource = pickContentSource(importLocator.form)
+  ): String {
+    val formKey = importLocator.form ?: "file"
+    val stableIdentity = contentSource.stableContentIdentity(importLocator.path)
+    return "[${formKey}][${stableIdentity}]"
+  }
 }