src/main/wacc/Error.scala

@@ -0,0 +1,8 @@
+package wacc
+
+enum Error {
+  case DuplicateDeclaration(ident: ast.Ident)
+  case UndefinedIdentifier(ident: ast.Ident, identType: renamer.IdentType)
+  case FunctionParamsMismatch(expected: Int, got: Int)
+  case TypeMismatch(expected: types.SemType, got: types.SemType)
+}

src/main/wacc/Main.scala

@@ -1,5 +1,6 @@
 package wacc
 
+import scala.collection.mutable
 import parsley.{Failure, Success}
 import scopt.OParser
 import java.io.File
@@ -32,8 +33,15 @@ val cliParser = {
 def compile(contents: String): Int = {
   parser.parse(contents) match {
     case Success(ast) =>
-      // TODO: Do semantics things
+      given errors: mutable.Builder[Error, List[Error]] = List.newBuilder
-      0
+      renamer.rename(ast)
+      // given ctx: types.TypeCheckerCtx[List[Error]] =
+      //   types.TypeCheckerCtx(names, errors)
+      // types.check(ast)
+      if (errors.result.nonEmpty) {
+        errors.result.foreach(println)
+        200
+      } else 0
     case Failure(msg) =>
       println(msg)
       100

src/main/wacc/ast.scala

@@ -1,6 +1,10 @@
 package wacc
 
-import parsley.generic._
+import parsley.Parsley
+import parsley.generic.ErrorBridge
+import parsley.ap._
+import parsley.position._
+import parsley.syntax.zipped._
 import cats.data.NonEmptyList
 
 object ast {
@@ -14,80 +18,104 @@ object ast {
   sealed trait Expr6 extends Expr5
 
   // Atoms
-  case class IntLiter(v: Int) extends Expr6
+  case class IntLiter(v: Int)(pos: Position) extends Expr6
-  object IntLiter extends ParserBridge1[Int, IntLiter]
+  object IntLiter extends ParserBridgePos1[Int, IntLiter]
-  case class BoolLiter(v: Boolean) extends Expr6
+  case class BoolLiter(v: Boolean)(pos: Position) extends Expr6
-  object BoolLiter extends ParserBridge1[Boolean, BoolLiter]
+  object BoolLiter extends ParserBridgePos1[Boolean, BoolLiter]
-  case class CharLiter(v: Char) extends Expr6
+  case class CharLiter(v: Char)(pos: Position) extends Expr6
-  object CharLiter extends ParserBridge1[Char, CharLiter]
+  object CharLiter extends ParserBridgePos1[Char, CharLiter]
-  case class StrLiter(v: String) extends Expr6
+  case class StrLiter(v: String)(pos: Position) extends Expr6
-  object StrLiter extends ParserBridge1[String, StrLiter]
+  object StrLiter extends ParserBridgePos1[String, StrLiter]
-  case object PairLiter extends Expr6 with ParserBridge0[PairLiter.type]
+  case class PairLiter()(pos: Position) extends Expr6
-  case class Ident(v: String) extends Expr6 with LValue
+  object PairLiter extends ParserBridgePos0[PairLiter]
-  object Ident extends ParserBridge1[String, Ident]
+  case class Ident(v: String, var uid: Int = -1)(pos: Position) extends Expr6 with LValue
-  case class ArrayElem(name: Ident, indices: NonEmptyList[Expr]) extends Expr6 with LValue
+  object Ident extends ParserBridgePos1[String, Ident] {
-  object ArrayElem extends ParserBridge2[Ident, NonEmptyList[Expr], ArrayElem]
+    def apply(v: String)(pos: Position): Ident = new Ident(v)(pos)
-  case class Parens(expr: Expr) extends Expr6
+  }
-  object Parens extends ParserBridge1[Expr, Parens]
+  case class ArrayElem(name: Ident, indices: NonEmptyList[Expr])(pos: Position)
+      extends Expr6
+      with LValue
+  object ArrayElem extends ParserBridgePos1[NonEmptyList[Expr], Ident => ArrayElem] {
+    def apply(a: NonEmptyList[Expr])(pos: Position): Ident => ArrayElem =
+      name => ArrayElem(name, a)(pos)
+  }
+  case class Parens(expr: Expr)(pos: Position) extends Expr6
+  object Parens extends ParserBridgePos1[Expr, Parens]
 
   // Unary operators
-  case class Negate(x: Expr6) extends Expr6
+  sealed trait UnaryOp extends Expr {
-  object Negate extends ParserBridge1[Expr6, Negate]
+    val x: Expr
-  case class Not(x: Expr6) extends Expr6
+  }
-  object Not extends ParserBridge1[Expr6, Not]
+  case class Negate(x: Expr6)(pos: Position) extends Expr6 with UnaryOp
-  case class Len(x: Expr6) extends Expr6
+  object Negate extends ParserBridgePos1[Expr6, Negate]
-  object Len extends ParserBridge1[Expr6, Len]
+  case class Not(x: Expr6)(pos: Position) extends Expr6 with UnaryOp
-  case class Ord(x: Expr6) extends Expr6
+  object Not extends ParserBridgePos1[Expr6, Not]
-  object Ord extends ParserBridge1[Expr6, Ord]
+  case class Len(x: Expr6)(pos: Position) extends Expr6 with UnaryOp
-  case class Chr(x: Expr6) extends Expr6
+  object Len extends ParserBridgePos1[Expr6, Len]
-  object Chr extends ParserBridge1[Expr6, Chr]
+  case class Ord(x: Expr6)(pos: Position) extends Expr6 with UnaryOp
+  object Ord extends ParserBridgePos1[Expr6, Ord]
+  case class Chr(x: Expr6)(pos: Position) extends Expr6 with UnaryOp
+  object Chr extends ParserBridgePos1[Expr6, Chr]
 
   // Binary operators
-  case class Add(x: Expr4, y: Expr5) extends Expr4
+  sealed trait BinaryOp extends Expr {
-  object Add extends ParserBridge2[Expr4, Expr5, Add]
+    val x: Expr
-  case class Sub(x: Expr4, y: Expr5) extends Expr4
+    val y: Expr
-  object Sub extends ParserBridge2[Expr4, Expr5, Sub]
+  }
-  case class Mul(x: Expr5, y: Expr6) extends Expr5
+  case class Add(x: Expr4, y: Expr5)(pos: Position) extends Expr4 with BinaryOp
-  object Mul extends ParserBridge2[Expr5, Expr6, Mul]
+  object Add extends ParserBridgePos2[Expr4, Expr5, Add]
-  case class Div(x: Expr5, y: Expr6) extends Expr5
+  case class Sub(x: Expr4, y: Expr5)(pos: Position) extends Expr4 with BinaryOp
-  object Div extends ParserBridge2[Expr5, Expr6, Div]
+  object Sub extends ParserBridgePos2[Expr4, Expr5, Sub]
-  case class Mod(x: Expr5, y: Expr6) extends Expr5
+  case class Mul(x: Expr5, y: Expr6)(pos: Position) extends Expr5 with BinaryOp
-  object Mod extends ParserBridge2[Expr5, Expr6, Mod]
+  object Mul extends ParserBridgePos2[Expr5, Expr6, Mul]
-  case class Greater(x: Expr4, y: Expr4) extends Expr3
+  case class Div(x: Expr5, y: Expr6)(pos: Position) extends Expr5 with BinaryOp
-  object Greater extends ParserBridge2[Expr4, Expr4, Greater]
+  object Div extends ParserBridgePos2[Expr5, Expr6, Div]
-  case class GreaterEq(x: Expr4, y: Expr4) extends Expr3
+  case class Mod(x: Expr5, y: Expr6)(pos: Position) extends Expr5 with BinaryOp
-  object GreaterEq extends ParserBridge2[Expr4, Expr4, GreaterEq]
+  object Mod extends ParserBridgePos2[Expr5, Expr6, Mod]
-  case class Less(x: Expr4, y: Expr4) extends Expr3
+  case class Greater(x: Expr4, y: Expr4)(pos: Position) extends Expr3 with BinaryOp
-  object Less extends ParserBridge2[Expr4, Expr4, Less]
+  object Greater extends ParserBridgePos2[Expr4, Expr4, Greater]
-  case class LessEq(x: Expr4, y: Expr4) extends Expr3
+  case class GreaterEq(x: Expr4, y: Expr4)(pos: Position) extends Expr3 with BinaryOp
-  object LessEq extends ParserBridge2[Expr4, Expr4, LessEq]
+  object GreaterEq extends ParserBridgePos2[Expr4, Expr4, GreaterEq]
-  case class Eq(x: Expr3, y: Expr3) extends Expr2
+  case class Less(x: Expr4, y: Expr4)(pos: Position) extends Expr3 with BinaryOp
-  object Eq extends ParserBridge2[Expr3, Expr3, Eq]
+  object Less extends ParserBridgePos2[Expr4, Expr4, Less]
-  case class Neq(x: Expr3, y: Expr3) extends Expr2
+  case class LessEq(x: Expr4, y: Expr4)(pos: Position) extends Expr3 with BinaryOp
-  object Neq extends ParserBridge2[Expr3, Expr3, Neq]
+  object LessEq extends ParserBridgePos2[Expr4, Expr4, LessEq]
-  case class And(x: Expr2, y: Expr1) extends Expr1
+  case class Eq(x: Expr3, y: Expr3)(pos: Position) extends Expr2 with BinaryOp
-  object And extends ParserBridge2[Expr2, Expr1, And]
+  object Eq extends ParserBridgePos2[Expr3, Expr3, Eq]
-  case class Or(x: Expr1, y: Expr) extends Expr
+  case class Neq(x: Expr3, y: Expr3)(pos: Position) extends Expr2 with BinaryOp
-  object Or extends ParserBridge2[Expr1, Expr, Or]
+  object Neq extends ParserBridgePos2[Expr3, Expr3, Neq]
+  case class And(x: Expr2, y: Expr1)(pos: Position) extends Expr1 with BinaryOp
+  object And extends ParserBridgePos2[Expr2, Expr1, And]
+  case class Or(x: Expr1, y: Expr)(pos: Position) extends Expr with BinaryOp
+  object Or extends ParserBridgePos2[Expr1, Expr, Or]
 
   // Types
   sealed trait Type
   sealed trait BaseType extends Type with PairElemType
-  case object IntType extends BaseType with ParserBridge0[IntType.type]
+  case class IntType()(pos: Position) extends BaseType
-  case object BoolType extends BaseType with ParserBridge0[BoolType.type]
+  object IntType extends ParserBridgePos0[IntType]
-  case object CharType extends BaseType with ParserBridge0[CharType.type]
+  case class BoolType()(pos: Position) extends BaseType
-  case object StringType extends BaseType with ParserBridge0[StringType.type]
+  object BoolType extends ParserBridgePos0[BoolType]
-  case class ArrayType(elemType: Type, dimensions: Int) extends Type with PairElemType
+  case class CharType()(pos: Position) extends BaseType
-  object ArrayType extends ParserBridge2[Type, Int, ArrayType]
+  object CharType extends ParserBridgePos0[CharType]
-  case class PairType(fst: PairElemType, snd: PairElemType) extends Type
+  case class StringType()(pos: Position) extends BaseType
-  object PairType extends ParserBridge2[PairElemType, PairElemType, PairType]
+  object StringType extends ParserBridgePos0[StringType]
+  case class ArrayType(elemType: Type, dimensions: Int)(pos: Position)
+      extends Type
+      with PairElemType
+  object ArrayType extends ParserBridgePos1[Int, Type => ArrayType] {
+    def apply(a: Int)(pos: Position): Type => ArrayType = elemType => ArrayType(elemType, a)(pos)
+  }
+  case class PairType(fst: PairElemType, snd: PairElemType)(pos: Position) extends Type
+  object PairType extends ParserBridgePos2[PairElemType, PairElemType, PairType]
 
   sealed trait PairElemType
-  case object UntypedPairType extends PairElemType with ParserBridge0[UntypedPairType.type]
+  case class UntypedPairType()(pos: Position) extends PairElemType
+  object UntypedPairType extends ParserBridgePos0[UntypedPairType]
 
   // waccadoodledo
-  case class Program(funcs: List[FuncDecl], main: NonEmptyList[Stmt])
+  case class Program(funcs: List[FuncDecl], main: NonEmptyList[Stmt])(pos: Position)
-  object Program extends ParserBridge2[List[FuncDecl], NonEmptyList[Stmt], Program]
+  object Program extends ParserBridgePos2[List[FuncDecl], NonEmptyList[Stmt], Program]
 
   // Function Definitions
   case class FuncDecl(
@@ -95,49 +123,106 @@ object ast {
       name: Ident,
       params: List[Param],
       body: NonEmptyList[Stmt]
-  )
+  )(pos: Position)
-  object FuncDecl extends ParserBridge4[Type, Ident, List[Param], NonEmptyList[Stmt], FuncDecl]
+  object FuncDecl
+      extends ParserBridgePos2[
+        List[Param],
+        NonEmptyList[Stmt],
+        ((Type, Ident)) => FuncDecl
+      ] {
+    def apply(params: List[Param], body: NonEmptyList[Stmt])(
+        pos: Position
+    ): ((Type, Ident)) => FuncDecl =
+      (returnType, name) => FuncDecl(returnType, name, params, body)(pos)
+  }
 
-  case class Param(paramType: Type, name: Ident)
+  case class Param(paramType: Type, name: Ident)(pos: Position)
-  object Param extends ParserBridge2[Type, Ident, Param]
+  object Param extends ParserBridgePos2[Type, Ident, Param]
 
   // Statements
   sealed trait Stmt
-  case object Skip extends Stmt with ParserBridge0[Skip.type]
+  case class Skip()(pos: Position) extends Stmt
-  case class VarDecl(varType: Type, name: Ident, value: RValue) extends Stmt
+  object Skip extends ParserBridgePos0[Skip]
-  object VarDecl extends ParserBridge3[Type, Ident, RValue, VarDecl]
+  case class VarDecl(varType: Type, name: Ident, value: RValue)(pos: Position) extends Stmt
-  case class Assign(lhs: LValue, value: RValue) extends Stmt
+  object VarDecl extends ParserBridgePos3[Type, Ident, RValue, VarDecl]
-  object Assign extends ParserBridge2[LValue, RValue, Assign]
+  case class Assign(lhs: LValue, value: RValue)(pos: Position) extends Stmt
-  case class Read(lhs: LValue) extends Stmt
+  object Assign extends ParserBridgePos2[LValue, RValue, Assign]
-  object Read extends ParserBridge1[LValue, Read]
+  case class Read(lhs: LValue)(pos: Position) extends Stmt
-  case class Free(expr: Expr) extends Stmt
+  object Read extends ParserBridgePos1[LValue, Read]
-  object Free extends ParserBridge1[Expr, Free]
+  case class Free(expr: Expr)(pos: Position) extends Stmt
-  case class Return(expr: Expr) extends Stmt
+  object Free extends ParserBridgePos1[Expr, Free]
-  object Return extends ParserBridge1[Expr, Return]
+  case class Return(expr: Expr)(pos: Position) extends Stmt
-  case class Exit(expr: Expr) extends Stmt
+  object Return extends ParserBridgePos1[Expr, Return]
-  object Exit extends ParserBridge1[Expr, Exit]
+  case class Exit(expr: Expr)(pos: Position) extends Stmt
-  case class Print(expr: Expr, newline: Boolean) extends Stmt
+  object Exit extends ParserBridgePos1[Expr, Exit]
-  object Print extends ParserBridge2[Expr, Boolean, Print]
+  case class Print(expr: Expr, newline: Boolean)(pos: Position) extends Stmt
-  case class If(cond: Expr, thenStmt: NonEmptyList[Stmt], elseStmt: NonEmptyList[Stmt]) extends Stmt
+  object Print extends ParserBridgePos2[Expr, Boolean, Print]
-  object If extends ParserBridge3[Expr, NonEmptyList[Stmt], NonEmptyList[Stmt], If]
+  case class If(cond: Expr, thenStmt: NonEmptyList[Stmt], elseStmt: NonEmptyList[Stmt])(
-  case class While(cond: Expr, body: NonEmptyList[Stmt]) extends Stmt
+      pos: Position
-  object While extends ParserBridge2[Expr, NonEmptyList[Stmt], While]
+  ) extends Stmt
-  case class Block(stmt: NonEmptyList[Stmt]) extends Stmt
+  object If extends ParserBridgePos3[Expr, NonEmptyList[Stmt], NonEmptyList[Stmt], If]
-  object Block extends ParserBridge1[NonEmptyList[Stmt], Block]
+  case class While(cond: Expr, body: NonEmptyList[Stmt])(pos: Position) extends Stmt
+  object While extends ParserBridgePos2[Expr, NonEmptyList[Stmt], While]
+  case class Block(stmt: NonEmptyList[Stmt])(pos: Position) extends Stmt
+  object Block extends ParserBridgePos1[NonEmptyList[Stmt], Block]
 
   sealed trait LValue
 
   sealed trait RValue
-  case class ArrayLiter(elems: List[Expr]) extends RValue
+  case class ArrayLiter(elems: List[Expr])(pos: Position) extends RValue
-  object ArrayLiter extends ParserBridge1[List[Expr], ArrayLiter]
+  object ArrayLiter extends ParserBridgePos1[List[Expr], ArrayLiter]
-  case class NewPair(fst: Expr, snd: Expr) extends RValue
+  case class NewPair(fst: Expr, snd: Expr)(pos: Position) extends RValue
-  object NewPair extends ParserBridge2[Expr, Expr, NewPair]
+  object NewPair extends ParserBridgePos2[Expr, Expr, NewPair]
-  case class Call(name: Ident, args: List[Expr]) extends RValue
+  case class Call(name: Ident, args: List[Expr])(pos: Position) extends RValue
-  object Call extends ParserBridge2[Ident, List[Expr], Call]
+  object Call extends ParserBridgePos2[Ident, List[Expr], Call]
 
   sealed trait PairElem extends LValue with RValue
-  case class Fst(elem: LValue) extends PairElem
+  case class Fst(elem: LValue)(pos: Position) extends PairElem
-  object Fst extends ParserBridge1[LValue, Fst]
+  object Fst extends ParserBridgePos1[LValue, Fst]
-  case class Snd(elem: LValue) extends PairElem
+  case class Snd(elem: LValue)(pos: Position) extends PairElem
-  object Snd extends ParserBridge1[LValue, Snd]
+  object Snd extends ParserBridgePos1[LValue, Snd]
+
+  // Parser bridges
+  case class Position(line: Int, column: Int, offset: Int)
+
+  trait ParserSingletonBridgePos[+A] extends ErrorBridge {
+    protected def con(pos: (Int, Int), offset: Int): A
+    infix def from(op: Parsley[?]): Parsley[A] = error((pos, offset).zipped(con) <~ op)
+    final def <#(op: Parsley[?]): Parsley[A] = this from op
+  }
+
+  trait ParserBridgePos0[+A] extends ParserSingletonBridgePos[A] {
+    def apply()(pos: Position): A
+
+    override final def con(pos: (Int, Int), offset: Int): A =
+      apply()(Position(pos._1, pos._2, offset))
+  }
+
+  trait ParserBridgePos1[-A, +B] extends ParserSingletonBridgePos[A => B] {
+    def apply(a: A)(pos: Position): B
+    def apply(a: Parsley[A]): Parsley[B] = error(ap1((pos, offset).zipped(con), a))
+
+    override final def con(pos: (Int, Int), offset: Int): A => B =
+      this.apply(_)(Position(pos._1, pos._2, offset))
+  }
+
+  trait ParserBridgePos2[-A, -B, +C] extends ParserSingletonBridgePos[(A, B) => C] {
+    def apply(a: A, b: B)(pos: Position): C
+    def apply(a: Parsley[A], b: => Parsley[B]): Parsley[C] = error(
+      ap2((pos, offset).zipped(con), a, b)
+    )
+
+    override final def con(pos: (Int, Int), offset: Int): (A, B) => C =
+      apply(_, _)(Position(pos._1, pos._2, offset))
+  }
+
+  trait ParserBridgePos3[-A, -B, -C, +D] extends ParserSingletonBridgePos[(A, B, C) => D] {
+    def apply(a: A, b: B, c: C)(pos: Position): D
+    def apply(a: Parsley[A], b: => Parsley[B], c: => Parsley[C]): Parsley[D] = error(
+      ap3((pos, offset).zipped(con), a, b, c)
+    )
+
+    override final def con(pos: (Int, Int), offset: Int): (A, B, C) => D =
+      apply(_, _, _)(Position(pos._1, pos._2, offset))
+  }
 }

src/main/wacc/lexer.scala

@@ -4,7 +4,36 @@ import parsley.Parsley
 import parsley.character
 import parsley.token.{Basic, Lexer}
 import parsley.token.descriptions.*
+import parsley.token.errors._
 
+val errConfig = new ErrorConfig {
+  override def labelSymbol = Map(
+    "!=" -> Label("binary operator"),
+    "%" -> Label("binary operator"),
+    "&&" -> Label("binary operator"),
+    "*" -> Label("binary operator"),
+    "/" -> Label("binary operator"),
+    "<" -> Label("binary operator"),
+    "<=" -> Label("binary operator"),
+    "==" -> Label("binary operator"),
+    ">" -> Label("binary operator"),
+    ">=" -> Label("binary operator"),
+    "||" -> Label("binary operator"),
+    "!" -> Label("unary operator"),
+    "len" -> Label("unary operator"),
+    "ord" -> Label("unary operator"),
+    "chr" -> Label("unary operator"),
+    "bool" -> Label("valid type"),
+    "char" -> Label("valid type"),
+    "int" -> Label("valid type"),
+    "pair" -> Label("valid type"),
+    "string" -> Label("valid type"),
+    "fst" -> Label("pair extraction"),
+    "snd" -> Label("pair extraction"),
+    "false" -> Label("boolean literal"),
+    "true" -> Label("boolean literal")
+  )
+}
 object lexer {
   private val desc = LexicalDesc.plain.copy(
     nameDesc = NameDesc.plain.copy(
@@ -43,7 +72,7 @@ object lexer {
     )
   )
 
-  private val lexer = Lexer(desc)
+  private val lexer = Lexer(desc, errConfig)
   val ident = lexer.lexeme.names.identifier
   val integer = lexer.lexeme.integer.decimal32[Int]
   val negateCheck = lexer.nonlexeme.symbol("-") ~> character.digit
@@ -51,5 +80,15 @@ object lexer {
   val stringLit = lexer.lexeme.string.ascii
   val implicits = lexer.lexeme.symbol.implicits
 
+  val errTokens = Seq(
+    lexer.nonlexeme.names.identifier.map(v => s"identifier $v"),
+    lexer.nonlexeme.integer.decimal32[Int].map(n => s"integer $n"),
+    lexer.nonlexeme.character.ascii.map(c => s"character literal $c"),
+    lexer.nonlexeme.string.ascii.map(s => s"string literal $s"),
+    character.whitespace.map(_ => "")
+  ) ++ desc.symbolDesc.hardKeywords.map { k =>
+    lexer.nonlexeme.symbol(k).as(s"keyword $k")
+  }
+
   def fully[A](p: Parsley[A]): Parsley[A] = lexer.fully(p)
 }

src/main/wacc/parser.scala

@@ -6,14 +6,51 @@ import parsley.Parsley.{atomic, many, notFollowedBy, pure}
 import parsley.combinator.{countSome, sepBy}
 import parsley.expr.{precedence, SOps, InfixL, InfixN, InfixR, Prefix, Atoms}
 import parsley.errors.combinator._
-import parsley.cats.combinator.{sepBy1, some}
+import parsley.syntax.zipped._
+import parsley.cats.combinator.{some}
 import cats.data.NonEmptyList
+import parsley.errors.DefaultErrorBuilder
+import parsley.errors.ErrorBuilder
+import parsley.errors.tokenextractors.LexToken
 
 object parser {
   import lexer.implicits.implicitSymbol
-  import lexer.{ident, integer, charLit, stringLit, negateCheck}
+  import lexer.{ident, integer, charLit, stringLit, negateCheck, errTokens}
   import ast._
 
+  // error extensions
+  extension [A](p: Parsley[A]) {
+    // combines label and explain together into one function call
+    def labelAndExplain(label: String, explanation: String): Parsley[A] = {
+      p.label(label).explain(explanation)
+    }
+    def labelAndExplain(t: LabelType): Parsley[A] = {
+      t match {
+        case LabelType.Expr =>
+          labelWithType(t).explain(
+            "a valid expression can start with: null, literals, identifiers, unary operators, or parentheses. " +
+              "Expressions can also contain array indexing and binary operators. " +
+              "Pair extraction is not allowed in expressions, only in assignments."
+          )
+        case _ => labelWithType(t)
+      }
+    }
+
+    def labelWithType(t: LabelType): Parsley[A] = {
+      t match {
+        case LabelType.Expr => p.label("valid expression")
+        case LabelType.Pair => p.label("valid pair")
+      }
+    }
+  }
+
+  enum LabelType:
+    case Expr
+    case Pair
+
+  implicit val builder: ErrorBuilder[String] = new DefaultErrorBuilder with LexToken {
+    def tokens = errTokens
+  }
   def parse(input: String): Result[String, Program] = parser.parse(input)
   private val parser = lexer.fully(`<program>`)
 
@@ -28,11 +65,14 @@ object parser {
         Greater from ">",
         GreaterEq from ">="
       ) +:
-      SOps(InfixL)(Add from "+", Sub from "-") +:
+      SOps(InfixL)(
+        (Add from "+").label("binary operator"),
+        (Sub from "-").label("binary operator")
+      ) +:
       SOps(InfixL)(Mul from "*", Div from "/", Mod from "%") +:
       SOps(Prefix)(
         Not from "!",
-        Negate from (notFollowedBy(negateCheck) ~> "-"),
+        (Negate from (notFollowedBy(negateCheck) ~> "-")).hide,
         Len from "len",
         Ord from "ord",
         Chr from "chr"
@@ -42,10 +82,10 @@ object parser {
 
   // Atoms
   private lazy val `<atom>`: Atoms[Expr6] = Atoms(
-    IntLiter(integer),
+    IntLiter(integer).label("integer literal"),
-    BoolLiter(("true" as true) | ("false" as false)),
+    BoolLiter(("true" as true) | ("false" as false)).label("boolean literal"),
-    CharLiter(charLit),
+    CharLiter(charLit).label("character literal"),
-    StrLiter(stringLit),
+    StrLiter(stringLit).label("string literal"),
     PairLiter from "null",
     `<ident-or-array-elem>`,
     Parens("(" ~> `<expr>` <~ ")")
@@ -53,10 +93,7 @@ object parser {
   private val `<ident>` = Ident(ident)
   private lazy val `<ident-or-array-elem>` =
     `<ident>` <**> (`<array-indices>` </> identity)
-  private val `<array-indices>` =
+  private val `<array-indices>` = ArrayElem(some("[" ~> `<expr>` <~ "]"))
-    some("[" ~> `<expr>` <~ "]") map { indices =>
-      ArrayElem((_: Ident), indices)
-    }
 
   // Types
   private lazy val `<type>`: Parsley[Type] =
@@ -64,7 +101,7 @@ object parser {
   private val `<base-type>` =
     (IntType from "int") | (BoolType from "bool") | (CharType from "char") | (StringType from "string")
   private lazy val `<array-type>` =
-    countSome("[" ~> "]") map { cnt => ArrayType((_: Type), cnt) }
+    ArrayType(countSome("[" ~> "]"))
   private val `<pair-type>` = "pair"
   private val `<pair-elems-type>`: Parsley[PairType] = PairType(
     "(" ~> `<pair-elem-type>` <~ ",",
@@ -72,38 +109,48 @@ object parser {
   )
   private lazy val `<pair-elem-type>` =
     (`<base-type>` <**> (`<array-type>` </> identity)) |
-      `<pair-type>` ~> ((`<pair-elems-type>` <**> `<array-type>`) </> UntypedPairType)
+      ((UntypedPairType from `<pair-type>`) <**>
+        ((`<pair-elems-type>` <**> `<array-type>`)
+          .map(arr => (_: UntypedPairType) => arr) </> identity))
 
   // Statements
   private lazy val `<program>` = Program(
-    "begin" ~> many(atomic(`<type>` <~> `<ident>` <~ "(") <**> `<partial-func-decl>`),
+    "begin" ~> many(
-    `<stmt>` <~ "end"
+      atomic(`<type>`.label("function declaration") <~> `<ident>` <~ "(") <**> `<partial-func-decl>`
+    ).label("function declaration"),
+    `<stmt>`.label("main program body") <~ "end"
   )
   private lazy val `<partial-func-decl>` =
-    (sepBy(`<param>`, ",") <~ ")" <~ "is" <~> `<stmt>`.guardAgainst {
+    FuncDecl(
+      sepBy(`<param>`, ",") <~ ")" <~ "is",
+      `<stmt>`.guardAgainst {
         case stmts if !stmts.isReturning => Seq("All functions must end in a returning statement")
-    } <~ "end") map { (params, stmt) =>
+      } <~ "end"
-      (FuncDecl((_: Type), (_: Ident), params, stmt)).tupled
+    )
-    }
   private lazy val `<param>` = Param(`<type>`, `<ident>`)
   private lazy val `<stmt>`: Parsley[NonEmptyList[Stmt]] =
-    sepBy1(`<basic-stmt>`, ";")
+    (
+      `<basic-stmt>`.label("main program body"),
+      (many(";" ~> `<basic-stmt>`.label("statement after ';'"))) </> Nil
+    ).zipped(NonEmptyList.apply)
+
   private lazy val `<basic-stmt>` =
     (Skip from "skip")
       | Read("read" ~> `<lvalue>`)
-      | Free("free" ~> `<expr>`)
+      | Free("free" ~> `<expr>`.labelAndExplain(LabelType.Expr))
-      | Return("return" ~> `<expr>`)
+      | Return("return" ~> `<expr>`.labelAndExplain(LabelType.Expr))
-      | Exit("exit" ~> `<expr>`)
+      | Exit("exit" ~> `<expr>`.labelAndExplain(LabelType.Expr))
-      | Print("print" ~> `<expr>`, pure(false))
+      | Print("print" ~> `<expr>`.labelAndExplain(LabelType.Expr), pure(false))
-      | Print("println" ~> `<expr>`, pure(true))
+      | Print("println" ~> `<expr>`.labelAndExplain(LabelType.Expr), pure(true))
       | If(
-        "if" ~> `<expr>` <~ "then",
+        "if" ~> `<expr>`.labelWithType(LabelType.Expr) <~ "then",
         `<stmt>` <~ "else",
         `<stmt>` <~ "fi"
       )
-      | While("while" ~> `<expr>` <~ "do", `<stmt>` <~ "done")
+      | While("while" ~> `<expr>`.labelWithType(LabelType.Expr) <~ "do", `<stmt>` <~ "done")
       | Block("begin" ~> `<stmt>` <~ "end")
-      | VarDecl(`<type>`, `<ident>` <~ "=", `<rvalue>`)
+      | VarDecl(`<type>`, `<ident>` <~ "=", `<rvalue>`.label("valid initial value for variable"))
+      // TODO: Can we inline the name of the variable in the message
       | Assign(`<lvalue>` <~ "=", `<rvalue>`)
   private lazy val `<lvalue>`: Parsley[LValue] =
     `<pair-elem>` | `<ident-or-array-elem>`
@@ -117,9 +164,10 @@ object parser {
       Call(
         "call" ~> `<ident>` <~ "(",
         sepBy(`<expr>`, ",") <~ ")"
-      ) | `<expr>`
+      ) | `<expr>`.labelWithType(LabelType.Expr)
   private lazy val `<pair-elem>` =
-    Fst("fst" ~> `<lvalue>`) | Snd("snd" ~> `<lvalue>`)
+    Fst("fst" ~> `<lvalue>`.label("valid pair"))
+      | Snd("snd" ~> `<lvalue>`.label("valid pair"))
   private lazy val `<array-liter>` = ArrayLiter(
     "[" ~> sepBy(`<expr>`, ",") <~ "]"
   )

src/main/wacc/renamer.scala

@@ -0,0 +1,215 @@
+package wacc
+
+import scala.collection.mutable
+
+object renamer {
+  import ast._
+  import types._
+
+  enum IdentType {
+    case Func
+    case Var
+  }
+
+  private case class Scope(
+      current: mutable.Map[(String, IdentType), Ident],
+      parent: Map[(String, IdentType), Ident]
+  ) {
+
+    /** Create a new scope with the current scope as its parent.
+      *
+      * @return
+      *   A new scope with an empty current scope, and this scope flattened into the parent scope.
+      */
+    def subscope: Scope =
+      Scope(mutable.Map.empty, Map.empty.withDefault(current.withDefault(parent)))
+
+    /** Attempt to add a new identifier to the current scope. If the identifier already exists in
+      * the current scope, add an error to the error list.
+      *
+      * @param semType
+      *   The semantic type of the identifier.
+      * @param name
+      *   The name of the identifier.
+      * @param identType
+      *   The identifier type (function or variable).
+      * @param globalNames
+      *   The global map of identifiers to semantic types - the identifier will be added to this
+      *   map.
+      * @param globalNumbering
+      *   The global map of identifier names to the number of times they have been declared - will
+      *   used to rename this identifier, and will be incremented.
+      * @param errors
+      *   The list of errors to append to.
+      */
+    def add(semType: SemType, name: Ident, identType: IdentType)(using
+        globalNames: mutable.Map[Ident, SemType],
+        globalNumbering: mutable.Map[String, Int],
+        errors: mutable.Builder[Error, List[Error]]
+    ) = {
+      if (current.contains((name.v, identType))) {
+        errors += Error.DuplicateDeclaration(name)
+      } else {
+        val uid = globalNumbering.getOrElse(name.v, 0)
+        name.uid = uid
+        current((name.v, identType)) = name
+
+        globalNames(name) = semType
+        globalNumbering(name.v) = uid + 1
+      }
+    }
+  }
+
+  /** Check scoping of all variables and functions in the program. Also generate semantic types for
+    * all identifiers.
+    *
+    * @param prog
+    *   AST of the program
+    * @param errors
+    *   List of errors to append to
+    * @return
+    *   Map of all (renamed) identifies to their semantic types
+    */
+  def rename(prog: Program)(using
+      errors: mutable.Builder[Error, List[Error]]
+  ): Map[Ident, SemType] =
+    given globalNames: mutable.Map[Ident, SemType] = mutable.Map.empty
+    given globalNumbering: mutable.Map[String, Int] = mutable.Map.empty
+    val scope = Scope(mutable.Map.empty, Map.empty)
+    val Program(funcs, main) = prog
+    funcs
+      // First add all function declarations to the scope
+      .map { case FuncDecl(retType, name, params, body) =>
+        val paramTypes = params.map { param =>
+          val paramType = SemType(param.paramType)
+          paramType
+        }
+        scope.add(KnownType.Func(SemType(retType), paramTypes), name, IdentType.Func)
+        (params zip paramTypes, body)
+      }
+      // Only then rename the function bodies
+      // (functions can call one-another regardless of order of declaration)
+      .foreach { case (params, body) =>
+        val functionScope = scope.subscope
+        params.foreach { case (param, paramType) =>
+          functionScope.add(paramType, param.name, IdentType.Var)
+        }
+        body.toList.foreach(rename(functionScope.subscope)) // body can shadow function params
+      }
+    main.toList.foreach(rename(scope))
+    globalNames.toMap
+
+  /** Check scoping of all identifies in a given AST node.
+    *
+    * @param scope
+    *   The current scope and flattened parent scope.
+    * @param node
+    *   The AST node.
+    * @param globalNames
+    *   The global map of identifiers to semantic types - renamed identifiers will be added to this
+    *   map.
+    * @param globalNumbering
+    *   The global map of identifier names to the number of times they have been declared - used and
+    *   updated during identifier renaming.
+    * @param errors
+    */
+  private def rename(scope: Scope)(
+      node: Ident | Stmt | LValue | RValue | Expr
+  )(using
+      globalNames: mutable.Map[Ident, SemType],
+      globalNumbering: mutable.Map[String, Int],
+      errors: mutable.Builder[Error, List[Error]]
+  ): Unit = node match {
+    // These cases are more interesting because the involve making subscopes
+    // or modifying the current scope.
+    case VarDecl(synType, name, value) => {
+      // Order matters here. Variable isn't declared until after the value is evaluated.
+      rename(scope)(value)
+      // Attempt to add the new variable to the current scope.
+      scope.add(SemType(synType), name, IdentType.Var)
+    }
+    case If(cond, thenStmt, elseStmt) => {
+      rename(scope)(cond)
+      // then and else both have their own scopes
+      thenStmt.toList.foreach(rename(scope.subscope))
+      elseStmt.toList.foreach(rename(scope.subscope))
+    }
+    case While(cond, body) => {
+      rename(scope)(cond)
+      // while bodies have their own scopes
+      body.toList.foreach(rename(scope.subscope))
+    }
+    // begin-end blocks have their own scopes
+    case Block(body) => body.toList.foreach(rename(scope.subscope))
+
+    // These cases are simpler, mostly just recursive calls to rename()
+    case Assign(lhs, value) => {
+      // Variables may be reassigned with their value in the rhs, so order doesn't matter here.
+      rename(scope)(lhs)
+      rename(scope)(value)
+    }
+    case Read(lhs)      => rename(scope)(lhs)
+    case Free(expr)     => rename(scope)(expr)
+    case Return(expr)   => rename(scope)(expr)
+    case Exit(expr)     => rename(scope)(expr)
+    case Print(expr, _) => rename(scope)(expr)
+    case NewPair(fst, snd) => {
+      rename(scope)(fst)
+      rename(scope)(snd)
+    }
+    case Call(name, args) => {
+      renameIdent(scope, name, IdentType.Func)
+      args.foreach(rename(scope))
+    }
+    case Fst(elem)         => rename(scope)(elem)
+    case Snd(elem)         => rename(scope)(elem)
+    case ArrayLiter(elems) => elems.foreach(rename(scope))
+    case ArrayElem(name, indices) => {
+      rename(scope)(name)
+      indices.toList.foreach(rename(scope))
+    }
+    case Parens(expr) => rename(scope)(expr)
+    case op: UnaryOp  => rename(scope)(op.x)
+    case op: BinaryOp => {
+      rename(scope)(op.x)
+      rename(scope)(op.y)
+    }
+    // Default to variables. Only `call` uses IdentType.Func.
+    case id: Ident => renameIdent(scope, id, IdentType.Var)
+    // These literals cannot contain identifies, exit immediately.
+    case IntLiter(_) | BoolLiter(_) | CharLiter(_) | StrLiter(_) | PairLiter() | Skip() => ()
+  }
+
+  /** Lookup an identifier in the current scope and rename it. If the identifier is not found, add
+    * an error to the error list and add it to the current scope with an unknown type.
+    *
+    * @param scope
+    *   The current scope and flattened parent scope.
+    * @param ident
+    *   The identifier to rename.
+    * @param identType
+    *   The type of the identifier (function or variable).
+    * @param globalNames
+    *   Used to add not-found identifiers to scope.
+    * @param globalNumbering
+    *   Used to add not-found identifiers to scope.
+    * @param errors
+    *   The list of errors to append to.
+    */
+  private def renameIdent(scope: Scope, ident: Ident, identType: IdentType)(using
+      globalNames: mutable.Map[Ident, SemType],
+      globalNumbering: mutable.Map[String, Int],
+      errors: mutable.Builder[Error, List[Error]]
+  ): Unit = {
+    // Unfortunately map defaults only work with `.apply()`, which throws an error when the key is not found.
+    // Neither is there a way to check whether a default exists, so we have to use a try-catch.
+    try {
+      val Ident(_, uid) = scope.current.withDefault(scope.parent)((ident.v, identType))
+      ident.uid = uid
+    } catch {
+      case _: NoSuchElementException =>
+        errors += Error.UndefinedIdentifier(ident, identType)
+        scope.add(?, ident, identType)
+    }
+  }
+}

src/main/wacc/types.scala

@@ -0,0 +1,31 @@
+package wacc
+
+object types {
+  import ast._
+
+  sealed trait SemType
+  case object ? extends SemType
+  enum KnownType extends SemType {
+    case Int
+    case Bool
+    case Char
+    case String
+    case Array(elem: SemType)
+    case Pair(left: SemType, right: SemType)
+    case Func(ret: SemType, params: List[SemType])
+  }
+
+  object SemType {
+    def apply(synType: Type | PairElemType): KnownType = synType match {
+      case IntType()    => KnownType.Int
+      case BoolType()   => KnownType.Bool
+      case CharType()   => KnownType.Char
+      case StringType() => KnownType.String
+      // For semantic types it is easier to work with recursion rather than a fixed size
+      case ArrayType(elemType, dimension) =>
+        (0 until dimension).foldLeft(SemType(elemType))((acc, _) => KnownType.Array(acc))
+      case PairType(fst, snd) => KnownType.Pair(SemType(fst), SemType(snd))
+      case UntypedPairType()  => KnownType.Pair(?, ?)
+    }
+  }
+}