Implement a very dumb import system.

examples/imports.pork

@@ -0,0 +1,5 @@
+import "module.pork"
+
+func main() {
+  hello()
+}

examples/module.pork

@@ -0,0 +1,3 @@
+func hello() {
+  println("Hello World")
+}

src/main/kotlin/gay/pizza/pork/ast/NodeCoalescer.kt

@@ -9,15 +9,16 @@ class NodeCoalescer(val handler: (Node) -> Unit) : NodeVisitor<Unit> {
   override fun visitListLiteral(node: ListLiteral): Unit = handle(node)
   override fun visitSymbol(node: Symbol): Unit = handle(node)
   override fun visitFunctionCall(node: FunctionCall): Unit = handle(node)
-  override fun visitDefine(node: Define): Unit = handle(node)
+  override fun visitDefine(node: Assignment): Unit = handle(node)
   override fun visitSymbolReference(node: SymbolReference): Unit = handle(node)
   override fun visitLambda(node: Lambda): Unit = handle(node)
   override fun visitParentheses(node: Parentheses): Unit = handle(node)
   override fun visitPrefixOperation(node: PrefixOperation): Unit = handle(node)
   override fun visitIf(node: If): Unit = handle(node)
   override fun visitInfixOperation(node: InfixOperation): Unit = handle(node)
-  override fun visitFunctionDeclaration(node: FunctionDeclaration): Unit = handle(node)
+  override fun visitFunctionDeclaration(node: FunctionDefinition): Unit = handle(node)
   override fun visitBlock(node: Block): Unit = handle(node)
+  override fun visitImportDeclaration(node: ImportDeclaration): Unit = handle(node)
 
   override fun visitCompilationUnit(node: CompilationUnit): Unit = handle(node)
 

src/main/kotlin/gay/pizza/pork/ast/NodeType.kt

@@ -12,13 +12,14 @@ enum class NodeType(val parent: NodeType? = null) {
   ListLiteral(Expression),
   StringLiteral(Expression),
   Parentheses(Expression),
-  Define(Expression),
+  Assignment(Expression),
   Lambda(Expression),
   PrefixOperation(Expression),
   InfixOperation(Expression),
   SymbolReference(Expression),
   FunctionCall(Expression),
   If(Expression),
+  ImportDeclaration(Declaration),
   FunctionDeclaration(Declaration);
 
   val parents: Set<NodeType>

src/main/kotlin/gay/pizza/pork/ast/NodeVisitor.kt

@@ -9,16 +9,17 @@ interface NodeVisitor<T> {
   fun visitListLiteral(node: ListLiteral): T
   fun visitSymbol(node: Symbol): T
   fun visitFunctionCall(node: FunctionCall): T
-  fun visitDefine(node: Define): T
+  fun visitDefine(node: Assignment): T
   fun visitSymbolReference(node: SymbolReference): T
   fun visitLambda(node: Lambda): T
   fun visitParentheses(node: Parentheses): T
   fun visitPrefixOperation(node: PrefixOperation): T
   fun visitIf(node: If): T
   fun visitInfixOperation(node: InfixOperation): T
-  fun visitFunctionDeclaration(node: FunctionDeclaration): T
+  fun visitFunctionDeclaration(node: FunctionDefinition): T
   fun visitBlock(node: Block): T
 
+  fun visitImportDeclaration(node: ImportDeclaration): T
   fun visitCompilationUnit(node: CompilationUnit): T
 
   fun visitExpression(node: Expression): T = when (node) {
@@ -27,7 +28,7 @@ interface NodeVisitor<T> {
     is BooleanLiteral -> visitBooleanLiteral(node)
     is ListLiteral -> visitListLiteral(node)
     is FunctionCall -> visitFunctionCall(node)
-    is Define -> visitDefine(node)
+    is Assignment -> visitDefine(node)
     is SymbolReference -> visitSymbolReference(node)
     is Lambda -> visitLambda(node)
     is Parentheses -> visitParentheses(node)
@@ -37,7 +38,11 @@ interface NodeVisitor<T> {
   }
 
   fun visitDeclaration(node: Declaration): T = when (node) {
-    is FunctionDeclaration -> visitFunctionDeclaration(node)
+    is ImportDeclaration -> visitImportDeclaration(node)
+  }
+
+  fun visitDefinition(node: Definition): T = when (node) {
+    is FunctionDefinition -> visitFunctionDeclaration(node)
   }
 
   fun visit(node: Node): T = when (node) {
@@ -46,6 +51,7 @@ interface NodeVisitor<T> {
     is CompilationUnit -> visitCompilationUnit(node)
     is Block -> visitBlock(node)
     is Declaration -> visitDeclaration(node)
+    is Definition -> visitDefinition(node)
   }
 
   fun visitNodes(vararg nodes: Node?): List<T> =

src/main/kotlin/gay/pizza/pork/ast/Printer.kt

@@ -72,7 +72,7 @@ class Printer(buffer: StringBuilder) : NodeVisitor<Unit> {
     append(")")
   }
 
-  override fun visitDefine(node: Define) {
+  override fun visitDefine(node: Assignment) {
     visit(node.symbol)
     append(" = ")
     visit(node.value)
@@ -147,7 +147,7 @@ class Printer(buffer: StringBuilder) : NodeVisitor<Unit> {
     visit(node.right)
   }
 
-  override fun visitFunctionDeclaration(node: FunctionDeclaration) {
+  override fun visitFunctionDeclaration(node: FunctionDefinition) {
     append("fn ")
     visit(node.symbol)
     append("(")
@@ -175,10 +175,24 @@ class Printer(buffer: StringBuilder) : NodeVisitor<Unit> {
     append("}")
   }
 
+  override fun visitImportDeclaration(node: ImportDeclaration) {
+    append("import ")
+    visit(node.path)
+  }
+
   override fun visitCompilationUnit(node: CompilationUnit) {
     for (declaration in node.declarations) {
       visit(declaration)
       appendLine()
     }
+
+    if (node.declarations.isNotEmpty()) {
+      appendLine()
+    }
+
+    for (definition in node.definitions) {
+      visit(definition)
+      appendLine()
+    }
   }
 }

src/main/kotlin/gay/pizza/pork/ast/nodes/Assignment.kt

@@ -6,15 +6,15 @@ import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
 @Serializable
-@SerialName("define")
-class Define(val symbol: Symbol, val value: Expression) : Expression() {
-  override val type: NodeType = NodeType.Define
+@SerialName("assignment")
+class Assignment(val symbol: Symbol, val value: Expression) : Expression() {
+  override val type: NodeType = NodeType.Assignment
 
   override fun <T> visitChildren(visitor: NodeVisitor<T>): List<T> =
     visitor.visitNodes(symbol, value)
 
   override fun equals(other: Any?): Boolean {
-    if (other !is Define) return false
+    if (other !is Assignment) return false
     return other.symbol == symbol && other.value == value
   }
 

src/main/kotlin/gay/pizza/pork/ast/nodes/CompilationUnit.kt

@@ -7,7 +7,7 @@ import kotlinx.serialization.Serializable
 
 @Serializable
 @SerialName("compilationUnit")
-class CompilationUnit(val declarations: List<Declaration>) : Node() {
+class CompilationUnit(val declarations: List<Declaration>, val definitions: List<Definition>) : Node() {
   override val type: NodeType = NodeType.CompilationUnit
 
   override fun <T> visitChildren(visitor: NodeVisitor<T>): List<T> =

src/main/kotlin/gay/pizza/pork/ast/nodes/Definition.kt

@@ -0,0 +1,6 @@
+package gay.pizza.pork.ast.nodes
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+sealed class Definition : Node()

src/main/kotlin/gay/pizza/pork/ast/nodes/FunctionDefinition.kt

@@ -6,23 +6,23 @@ import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
 @Serializable
-@SerialName("functionDeclaration")
-class FunctionDeclaration(val symbol: Symbol, val arguments: List<Symbol>, val block: Block) : Declaration() {
+@SerialName("functionDefinition")
+class FunctionDefinition(val symbol: Symbol, val arguments: List<Symbol>, val block: Block) : Definition() {
   override val type: NodeType = NodeType.FunctionDeclaration
 
   override fun <T> visitChildren(visitor: NodeVisitor<T>): List<T> =
     visitor.visitNodes(symbol)
 
   override fun equals(other: Any?): Boolean {
-    if (other !is FunctionDeclaration) return false
+    if (other !is FunctionDefinition) return false
     return other.symbol == symbol && other.arguments == arguments && other.block == block
   }
 
   override fun hashCode(): Int {
     var result = symbol.hashCode()
-    result = 31 * result + symbol.hashCode()
     result = 31 * result + arguments.hashCode()
     result = 31 * result + block.hashCode()
+    result = 31 * result + type.hashCode()
     return result
   }
 }

src/main/kotlin/gay/pizza/pork/ast/nodes/ImportDeclaration.kt

@@ -0,0 +1,26 @@
+package gay.pizza.pork.ast.nodes
+
+import gay.pizza.pork.ast.NodeType
+import gay.pizza.pork.ast.NodeVisitor
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+@Serializable
+@SerialName("importDeclaration")
+class ImportDeclaration(val path: StringLiteral) : Declaration() {
+  override val type: NodeType = NodeType.FunctionDeclaration
+
+  override fun <T> visitChildren(visitor: NodeVisitor<T>): List<T> =
+    visitor.visitNodes(path)
+
+  override fun equals(other: Any?): Boolean {
+    if (other !is ImportDeclaration) return false
+    return other.path == path
+  }
+
+  override fun hashCode(): Int {
+    var result = path.hashCode()
+    result = 31 * result + type.hashCode()
+    return result
+  }
+}

src/main/kotlin/gay/pizza/pork/eval/Evaluator.kt

@@ -3,7 +3,7 @@ package gay.pizza.pork.eval
 import gay.pizza.pork.ast.NodeVisitor
 import gay.pizza.pork.ast.nodes.*
 
-class Evaluator(root: Scope) : NodeVisitor<Any> {
+class Evaluator(root: Scope, val importLoader: ImportLoader) : NodeVisitor<Any> {
   private var currentScope: Scope = root
 
   override fun visitIntLiteral(node: IntLiteral): Any = node.value
@@ -18,7 +18,7 @@ class Evaluator(root: Scope) : NodeVisitor<Any> {
     return currentScope.call(node.symbol.id, Arguments(arguments))
   }
 
-  override fun visitDefine(node: Define): Any {
+  override fun visitDefine(node: Assignment): Any {
     val value = visit(node.value)
     currentScope.define(node.symbol.id, value)
     return value
@@ -102,7 +102,7 @@ class Evaluator(root: Scope) : NodeVisitor<Any> {
     }
   }
 
-  override fun visitFunctionDeclaration(node: FunctionDeclaration): Any {
+  override fun visitFunctionDeclaration(node: FunctionDefinition): Any {
     val blockFunction = visitBlock(node.block) as BlockFunction
     val function = CallableFunction { arguments ->
       currentScope = currentScope.fork()
@@ -127,10 +127,21 @@ class Evaluator(root: Scope) : NodeVisitor<Any> {
     value ?: None
   }
 
+  override fun visitImportDeclaration(node: ImportDeclaration): Any {
+    val importPath = node.path.text
+    val compilationUnit = importLoader.load(importPath)
+    visit(compilationUnit)
+    return None
+  }
+
   override fun visitCompilationUnit(node: CompilationUnit): Any {
     for (declaration in node.declarations) {
       visit(declaration)
     }
+
+    for (definition in node.definitions) {
+      visit(definition)
+    }
     return None
   }
 }

src/main/kotlin/gay/pizza/pork/eval/ImportLoader.kt

@@ -0,0 +1,7 @@
+package gay.pizza.pork.eval
+
+import gay.pizza.pork.ast.nodes.CompilationUnit
+
+interface ImportLoader {
+  fun load(path: String): CompilationUnit
+}

src/main/kotlin/gay/pizza/pork/eval/NullImportLoader.kt

@@ -0,0 +1,9 @@
+package gay.pizza.pork.eval
+
+import gay.pizza.pork.ast.nodes.CompilationUnit
+
+object NullImportLoader : ImportLoader {
+  override fun load(path: String): CompilationUnit {
+    throw RuntimeException("NullImportLoader cannot import compilation units.")
+  }
+}

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

@@ -7,4 +7,6 @@ import kotlin.io.path.readText
 
 class FileFrontend(val path: Path) : Frontend() {
   override fun createCharSource(): CharSource = StringCharSource(path.readText())
+  override fun resolveImportSource(path: String): CharSource =
+    StringCharSource(this.path.parent.resolve(path).readText())
 }

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

@@ -4,11 +4,13 @@ import gay.pizza.pork.ast.NodeVisitor
 import gay.pizza.pork.ast.Printer
 import gay.pizza.pork.ast.nodes.CompilationUnit
 import gay.pizza.pork.eval.Evaluator
+import gay.pizza.pork.eval.ImportLoader
 import gay.pizza.pork.eval.Scope
 import gay.pizza.pork.parse.*
 
 abstract class Frontend {
   abstract fun createCharSource(): CharSource
+  abstract fun resolveImportSource(path: String): CharSource
 
   fun tokenize(): TokenStream =
     Tokenizer(createCharSource()).tokenize()
@@ -20,9 +22,16 @@ abstract class Frontend {
     Highlighter(scheme).highlight(tokenize())
 
   fun evaluate(scope: Scope = Scope()): Any =
-    visit(Evaluator(scope))
+    visit(Evaluator(scope, FrontendImportLoader(this)))
 
   fun reprint(): String = buildString { visit(Printer(this)) }
 
   fun <T> visit(visitor: NodeVisitor<T>): T = visitor.visit(parse())
+
+  private class FrontendImportLoader(val frontend: Frontend) : ImportLoader {
+    override fun load(path: String): CompilationUnit {
+      val tokenStream = Tokenizer(frontend.resolveImportSource(path)).tokenize()
+      return Parser(TokenStreamSource(tokenStream), DiscardNodeAttribution).readCompilationUnit()
+    }
+  }
 }

src/main/kotlin/gay/pizza/pork/parse/Parser.kt

@@ -45,7 +45,7 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
       expect(TokenType.RightParentheses)
       FunctionCall(symbol, arguments)
     } else if (next(TokenType.Equals)) {
-      Define(symbol, readExpression())
+      Assignment(symbol, readExpression())
     } else {
       SymbolReference(symbol)
     }
@@ -169,19 +169,44 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
     Block(items)
   }
 
-  private fun readFunctionDeclaration(): FunctionDeclaration = within {
+  private fun readImportDeclaration(): ImportDeclaration = within {
+    expect(TokenType.Import)
+    ImportDeclaration(readStringLiteral())
+  }
+
+  private fun readFunctionDeclaration(): FunctionDefinition = within {
     expect(TokenType.Func)
     val name = readSymbolRaw()
     expect(TokenType.LeftParentheses)
     val arguments = collect(TokenType.RightParentheses, TokenType.Comma) { readSymbolRaw() }
     expect(TokenType.RightParentheses)
-    FunctionDeclaration(name, arguments, readBlock())
+    FunctionDefinition(name, arguments, readBlock())
+  }
+
+  private fun maybeReadDefinition(): Definition? {
+    val token = peek()
+    return when (token.type) {
+      TokenType.Func -> readFunctionDeclaration()
+      else -> null
+    }
+  }
+
+  private fun readDefinition(): Definition {
+    val definition = maybeReadDefinition()
+    if (definition != null) {
+      return definition
+    }
+    val token = peek()
+    throw RuntimeException(
+      "Failed to parse token: ${token.type} '${token.text}' as" +
+        " definition (index ${unsanitizedSource.currentIndex})"
+    )
   }
 
   fun readDeclaration(): Declaration {
     val token = peek()
     return when (token.type) {
-      TokenType.Func -> readFunctionDeclaration()
+      TokenType.Import -> readImportDeclaration()
       else -> throw RuntimeException(
         "Failed to parse token: ${token.type} '${token.text}' as" +
           " declaration (index ${unsanitizedSource.currentIndex})"
@@ -201,9 +226,25 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
     }
 
   fun readCompilationUnit(): CompilationUnit = within {
-    val declarations = collect(TokenType.EndOfFile) { readDeclaration() }
-    expect(TokenType.EndOfFile)
-    CompilationUnit(declarations)
+    val declarations = mutableListOf<Declaration>()
+    val definitions = mutableListOf<Definition>()
+    var declarationAccepted = true
+
+    while (!peek(TokenType.EndOfFile)) {
+      if (declarationAccepted) {
+        val definition = maybeReadDefinition()
+        if (definition != null) {
+          declarationAccepted = false
+          definitions.add(definition)
+          continue
+        }
+        declarations.add(readDeclaration())
+      } else {
+        definitions.add(readDefinition())
+      }
+    }
+
+    CompilationUnit(declarations, definitions)
   }
 
   private fun <T> collect(

src/main/kotlin/gay/pizza/pork/parse/TokenType.kt

@@ -28,6 +28,7 @@ enum class TokenType(vararg properties: TokenTypeProperty) {
   If(Keyword("if"), KeywordFamily),
   Then(Keyword("then"), KeywordFamily),
   Else(Keyword("else"), KeywordFamily),
+  Import(Keyword("import"), KeywordFamily),
   Func(Keyword("func"), KeywordFamily),
   Whitespace(CharConsumer { it == ' ' || it == '\r' || it == '\n' || it == '\t' }),
   BlockComment(CommentFamily),