diff --git a/src/main/wacc/Main.scala b/src/main/wacc/Main.scala
index e78d4bd..c29de9d 100644
--- a/src/main/wacc/Main.scala
+++ b/src/main/wacc/Main.scala
@@ -1,6 +1,5 @@
 package wacc
 
-import scala.collection.mutable
 import cats.data.{Chain, NonEmptyList}
 import parsley.{Failure, Success}
 
@@ -19,6 +18,7 @@ import org.typelevel.log4cats.Logger
 import assemblyIR as asm
 import cats.data.ValidatedNel
 import java.io.File
+import cats.data.NonEmptySeq
 
 /*
 TODO:
@@ -71,22 +71,15 @@ val outputOpt: Opts[Option[Path]] =
 def frontend(
     contents: String,
     file: File
-): IO[Either[NonEmptyList[Error], microWacc.Program]] =
+): IO[Either[NonEmptySeq[Error], microWacc.Program]] =
   parser.parse(contents) match {
-    case Failure(msg) => IO.pure(Left(NonEmptyList.one(Error.SyntaxError(file, msg))))
+    case Failure(msg) => IO.pure(Left(NonEmptySeq.one(Error.SyntaxError(file, msg))))
     case Success(fn) =>
       val partialProg = fn(file)
-      given errors: mutable.Builder[Error, List[Error]] = List.newBuilder
 
       for {
-        (prog, renameErrors) <- renamer.rename(partialProg)
-        _ = errors.addAll(renameErrors.toList)
-        typedProg = typeChecker.check(prog, errors)
-
-        res = NonEmptyList.fromList(errors.result) match {
-          case Some(errors) => Left(errors)
-          case None         => Right(typedProg)
-        }
+        (typedProg, errors) <- semantics.check(partialProg)
+        res = NonEmptySeq.fromSeq(errors.iterator.toSeq).map(Left(_)).getOrElse(Right(typedProg))
       } yield res
   }
 
@@ -107,12 +100,23 @@ def compile(
 
   // TODO: path, file , the names are confusing (when Path is the type but we are working with files)
   def writeOutputFile(typedProg: microWacc.Program, outputPath: Path): IO[Unit] =
-    writer.writeTo(backend(typedProg), outputPath) *>
-      logger.info(s"Success: ${outputPath.toAbsolutePath}")
+    val backendStart = System.nanoTime()
+    val asmLines = backend(typedProg)
+    val backendEnd = System.nanoTime()
+    writer.writeTo(asmLines, outputPath) *>
+      logAction(
+        s"Backend time (${filePath.toRealPath()}): ${(backendEnd - backendStart).toFloat / 1e6} ms"
+      ) *>
+      IO.blocking(println(s"Success: ${outputPath.toRealPath()}"))
 
   def processProgram(contents: String, file: File, outDir: Path): IO[Int] =
+    val frontendStart = System.nanoTime()
     for {
       frontendResult <- frontend(contents, file)
+      frontendEnd = System.nanoTime()
+      _ <- logAction(
+        s"Frontend time (${filePath.toRealPath()}): ${(frontendEnd - frontendStart).toFloat / 1e6} ms"
+      )
       res <- frontendResult match {
         case Left(errors) =>
           val code = errors.map(err => err.exitCode).toList.min
diff --git a/src/main/wacc/frontend/microWacc.scala b/src/main/wacc/frontend/microWacc.scala
index e2c1bdc..36c5d16 100644
--- a/src/main/wacc/frontend/microWacc.scala
+++ b/src/main/wacc/frontend/microWacc.scala
@@ -1,5 +1,7 @@
 package wacc
 
+import cats.data.Chain
+
 object microWacc {
   import wacc.types._
 
@@ -78,12 +80,12 @@ object microWacc {
   }
 
   case class Assign(lhs: LValue, rhs: Expr) extends Stmt
-  case class If(cond: Expr, thenBranch: List[Stmt], elseBranch: List[Stmt]) extends Stmt
-  case class While(cond: Expr, body: List[Stmt]) extends Stmt
+  case class If(cond: Expr, thenBranch: Chain[Stmt], elseBranch: Chain[Stmt]) extends Stmt
+  case class While(cond: Expr, body: Chain[Stmt]) extends Stmt
   case class Call(target: CallTarget, args: List[Expr]) extends Stmt with Expr(target.retTy)
   case class Return(expr: Expr) extends Stmt
 
   // Program
-  case class FuncDecl(name: Ident, params: List[Ident], body: List[Stmt])
-  case class Program(funcs: List[FuncDecl], stmts: List[Stmt])
+  case class FuncDecl(name: Ident, params: List[Ident], body: Chain[Stmt])
+  case class Program(funcs: Chain[FuncDecl], stmts: Chain[Stmt])
 }
diff --git a/src/main/wacc/frontend/renamer.scala b/src/main/wacc/frontend/renamer.scala
index 4893d42..b16df04 100644
--- a/src/main/wacc/frontend/renamer.scala
+++ b/src/main/wacc/frontend/renamer.scala
@@ -3,27 +3,26 @@ package wacc
 import java.io.File
 import scala.collection.mutable
 import cats.effect.IO
-import cats.syntax.all._
 import cats.implicits._
 import cats.data.Chain
 import cats.data.NonEmptyList
 import parsley.{Failure, Success}
 
-private val MAIN = "$main"
-
 object renamer {
   import ast._
   import types._
 
-  private enum IdentType {
+  val MAIN = "$main"
+
+  enum IdentType {
     case Func
     case Var
   }
 
-  private case class ScopeKey(path: String, name: String, identType: IdentType)
-  private case class ScopeValue(id: Ident, public: Boolean)
+  case class ScopeKey(path: String, name: String, identType: IdentType)
+  case class ScopeValue(id: Ident, public: Boolean)
 
-  private class Scope(
+  class Scope(
       private val current: mutable.Map[ScopeKey, ScopeValue],
       private val parent: Map[ScopeKey, ScopeValue],
       guidStart: Int = 0,
@@ -153,7 +152,7 @@ object renamer {
       get(name.pos.file.getCanonicalPath, name.v, IdentType.Func).map(_.id)
   }
 
-  private def prepareGlobalScope(
+  def prepareGlobalScope(
       partialProg: PartialProgram
   )(using scope: Scope): IO[(FuncDecl, Chain[FuncDecl], Chain[Error])] = {
     def readImportFile(file: File): IO[String] =
@@ -267,26 +266,13 @@ object renamer {
     * @return
     *   (flattenedProg, errors)
     */
-  private def renameFunction(funcScopePair: (FuncDecl, Scope)): IO[Chain[Error]] = {
+  def renameFunction(funcScopePair: (FuncDecl, Scope)): IO[Chain[Error]] = {
     val (FuncDecl(_, _, params, body), subscope) = funcScopePair
     val paramErrors = params.foldMap(param => subscope.add(param.name))
     IO(subscope.withSubscope { s => body.foldMap(rename(s)) })
       .map(bodyErrors => paramErrors ++ bodyErrors)
   }
 
-  def rename(partialProg: PartialProgram): IO[(Program, Chain[Error])] = {
-    given scope: Scope = Scope(mutable.Map.empty, Map.empty)
-
-    for {
-      (main, chunks, globalErrors) <- prepareGlobalScope(partialProg)
-      toRename = (main +: chunks).toList
-      allErrors <- toRename
-        .zip(scope.subscopes(toRename.size))
-        .parFoldMapA(renameFunction)
-      // .map(x => x.combineAll)
-    } yield (Program(chunks.toList, main.body)(main.pos), globalErrors ++ allErrors)
-  }
-
   /** Check scoping of all identifies in a given AST node.
     *
     * @param scope
diff --git a/src/main/wacc/frontend/semantics.scala b/src/main/wacc/frontend/semantics.scala
new file mode 100644
index 0000000..dcc5b94
--- /dev/null
+++ b/src/main/wacc/frontend/semantics.scala
@@ -0,0 +1,42 @@
+package wacc
+
+import scala.collection.mutable
+import cats.implicits._
+import cats.data.Chain
+import cats.effect.IO
+
+object semantics {
+  import renamer.{Scope, prepareGlobalScope, renameFunction}
+  import typeChecker.checkFuncDecl
+
+  private def checkFunc(
+      funcDecl: ast.FuncDecl,
+      scope: Scope
+  ): IO[(microWacc.FuncDecl, Chain[Error])] = {
+    for {
+      renamerErrors <- renameFunction(funcDecl, scope)
+      (microWaccFunc, typeErrors) = checkFuncDecl(funcDecl)
+    } yield (microWaccFunc, renamerErrors ++ typeErrors)
+  }
+
+  def check(partialProg: ast.PartialProgram): IO[(microWacc.Program, Chain[Error])] = {
+    given scope: Scope = Scope(mutable.Map.empty, Map.empty)
+
+    for {
+      (main, chunks, globalErrors) <- prepareGlobalScope(partialProg)
+      toRename = (main +: chunks).toList
+      res <- toRename
+        .zip(scope.subscopes(toRename.size))
+        .parTraverse(checkFunc)
+      (typedChunks, errors) = res.foldLeft((Chain.empty[microWacc.FuncDecl], Chain.empty[Error])) {
+        case ((acc, err), (funcDecl, errors)) =>
+          (acc :+ funcDecl, err ++ errors)
+      }
+      (typedMain, funcs) = typedChunks.uncons match {
+        case Some((head, tail)) => (head.body, tail)
+        case None               => (Chain.empty, Chain.empty)
+      }
+    } yield (microWacc.Program(funcs, typedMain), globalErrors ++ errors)
+  }
+
+}
diff --git a/src/main/wacc/frontend/typeChecker.scala b/src/main/wacc/frontend/typeChecker.scala
index 6f5804b..60fd924 100644
--- a/src/main/wacc/frontend/typeChecker.scala
+++ b/src/main/wacc/frontend/typeChecker.scala
@@ -1,20 +1,12 @@
 package wacc
 
 import cats.syntax.all._
-import scala.collection.mutable
 import cats.data.NonEmptyList
+import cats.data.Chain
 
 object typeChecker {
   import wacc.types._
 
-  case class TypeCheckerCtx(
-      errors: mutable.Builder[Error, List[Error]]
-  ) {
-    def error(err: Error): SemType =
-      errors += err
-      ?
-  }
-
   private enum Constraint {
     case Unconstrained
     // Allows weakening in one direction
@@ -38,31 +30,29 @@ object typeChecker {
       * @return
       *   The type if the constraint was satisfied, or ? if it was not.
       */
-    private def satisfies(constraint: Constraint, pos: ast.Position)(using
-        ctx: TypeCheckerCtx
-    ): SemType =
+    private def satisfies(constraint: Constraint, pos: ast.Position): (SemType, Chain[Error]) =
       (ty, constraint) match {
         case (KnownType.Array(KnownType.Char), Constraint.Is(KnownType.String, _)) =>
-          KnownType.String
+          (KnownType.String, Chain.empty)
         case (
               KnownType.String,
               Constraint.IsSymmetricCompatible(KnownType.Array(KnownType.Char), _)
             ) =>
-          KnownType.String
+          (KnownType.String, Chain.empty)
         case (ty, Constraint.IsSymmetricCompatible(ty2, msg)) =>
           ty.satisfies(Constraint.Is(ty2, msg), pos)
         // Change to IsUnweakenable to disallow recursive weakening
         case (ty, Constraint.Is(ty2, msg)) => ty.satisfies(Constraint.IsUnweakenable(ty2, msg), pos)
-        case (ty, Constraint.Unconstrained) => ty
+        case (ty, Constraint.Unconstrained) => (ty, Chain.empty)
         case (ty, Constraint.Never(msg)) =>
-          ctx.error(Error.SemanticError(pos, msg))
+          (?, Chain.one(Error.SemanticError(pos, msg)))
         case (ty, Constraint.IsEither(ty1, ty2, msg)) =>
-          (ty moreSpecific ty1).orElse(ty moreSpecific ty2).getOrElse {
-            ctx.error(Error.TypeMismatch(pos, ty1, ty, msg))
+          (ty moreSpecific ty1).orElse(ty moreSpecific ty2).map((_, Chain.empty)).getOrElse {
+            (?, Chain.one(Error.TypeMismatch(pos, ty1, ty, msg)))
           }
         case (ty, Constraint.IsUnweakenable(ty2, msg)) =>
-          (ty moreSpecific ty2).getOrElse {
-            ctx.error(Error.TypeMismatch(pos, ty2, ty, msg))
+          (ty moreSpecific ty2).map((_, Chain.empty)).getOrElse {
+            (?, Chain.one(Error.TypeMismatch(pos, ty2, ty, msg)))
           }
       }
 
@@ -86,35 +76,29 @@ object typeChecker {
       }
   }
 
-  /** Type-check a WACC program.
+  /** Type-check a function declaration.
     *
-    * @param prog
-    *   The AST of the program to type-check.
-    * @param ctx
-    *   The type checker context which includes the global names and functions, and an errors
-    *   builder.
+    * @param func
+    *   The AST of the function to type-check.
     */
-  def check(prog: ast.Program, errors: mutable.Builder[Error, List[Error]]): microWacc.Program =
-    given ctx: TypeCheckerCtx = TypeCheckerCtx(errors)
-    microWacc.Program(
-      // Ignore function syntax types for return value and params, since those have been converted
-      // to SemTypes by the renamer.
-      prog.funcs.map { case ast.FuncDecl(_, name, params, stmts) =>
-        val FuncType(retType, paramTypes) = name.ty.asInstanceOf[FuncType]
-        microWacc.FuncDecl(
-          microWacc.Ident(name.v, name.guid)(retType),
-          params.zip(paramTypes).map { case (ast.Param(_, ident), ty) =>
-            microWacc.Ident(ident.v, ident.guid)(ty)
-          },
-          stmts.toList
-            .flatMap(
-              checkStmt(_, Constraint.Is(retType, s"function ${name.v} must return $retType"))
-            )
-        )
-      },
-      prog.main.toList
-        .flatMap(checkStmt(_, Constraint.Never("main function must not return")))
+  def checkFuncDecl(func: ast.FuncDecl): (microWacc.FuncDecl, Chain[Error]) = {
+    val ast.FuncDecl(_, name, params, stmts) = func
+    val FuncType(retType, paramTypes) = name.ty.asInstanceOf[FuncType]
+    val returnConstraint =
+      if func.name.v == renamer.MAIN then Constraint.Never("main body must not return")
+      else Constraint.Is(retType, s"function ${name.v} must return $retType")
+    val (body, bodyErrors) = stmts.foldMap(checkStmt(_, returnConstraint))
+    (
+      microWacc.FuncDecl(
+        microWacc.Ident(name.v, name.guid)(retType),
+        params.zip(paramTypes).map { case (ast.Param(_, ident), ty) =>
+          microWacc.Ident(ident.v, ident.guid)(ty)
+        },
+        body
+      ),
+      bodyErrors
     )
+  }
 
   /** Type-check an AST statement node.
     *
@@ -123,45 +107,51 @@ object typeChecker {
     * @param returnConstraint
     *   The constraint that any `return <expr>` statements must satisfy.
     */
-  private def checkStmt(stmt: ast.Stmt, returnConstraint: Constraint)(using
-      ctx: TypeCheckerCtx
-  ): List[microWacc.Stmt] = stmt match {
+  private def checkStmt(
+      stmt: ast.Stmt,
+      returnConstraint: Constraint
+  ): (Chain[microWacc.Stmt], Chain[Error]) = stmt match {
     // Ignore the type of the variable, since it has been converted to a SemType by the renamer.
     case ast.VarDecl(_, name, value) =>
       val expectedTy = name.ty
-      val typedValue = checkValue(
+      val (typedValue, valueErrors) = checkValue(
         value,
         Constraint.Is(
           expectedTy.asInstanceOf[SemType],
           s"variable ${name.v} must be assigned a value of type $expectedTy"
         )
       )
-      List(
-        microWacc.Assign(
-          microWacc.Ident(name.v, name.guid)(expectedTy.asInstanceOf[SemType]),
-          typedValue
-        )
+      (
+        Chain.one(
+          microWacc.Assign(
+            microWacc.Ident(name.v, name.guid)(expectedTy.asInstanceOf[SemType]),
+            typedValue
+          )
+        ),
+        valueErrors
       )
     case ast.Assign(lhs, rhs) =>
-      val lhsTyped = checkLValue(lhs, Constraint.Unconstrained)
-      val rhsTyped =
+      val (lhsTyped, lhsErrors) = checkLValue(lhs, Constraint.Unconstrained)
+      val (rhsTyped, rhsErrors) =
         checkValue(rhs, Constraint.Is(lhsTyped.ty, s"assignment must have type ${lhsTyped.ty}"))
-      (lhsTyped.ty, rhsTyped.ty) match {
+      val unknownError = (lhsTyped.ty, rhsTyped.ty) match {
         case (?, ?) =>
-          ctx.error(
+          Chain.one(
             Error.SemanticError(lhs.pos, "assignment with both sides of unknown type is illegal")
           )
-        case _ => ()
+        case _ => Chain.empty
       }
-      List(microWacc.Assign(lhsTyped, rhsTyped))
+      (Chain.one(microWacc.Assign(lhsTyped, rhsTyped)), lhsErrors ++ rhsErrors ++ unknownError)
     case ast.Read(dest) =>
-      val destTyped = checkLValue(dest, Constraint.Unconstrained)
-      val destTy = destTyped.ty match {
+      val (destTyped, destErrors) = checkLValue(dest, Constraint.Unconstrained)
+      val (destTy, destTyErrors) = destTyped.ty match {
         case ? =>
-          ctx.error(
-            Error.SemanticError(dest.pos, "cannot read into a destination with an unknown type")
+          (
+            ?,
+            Chain.one(
+              Error.SemanticError(dest.pos, "cannot read into a destination with an unknown type")
+            )
           )
-          ?
         case destTy =>
           destTy.satisfies(
             Constraint.IsEither(
@@ -172,49 +162,44 @@ object typeChecker {
             dest.pos
           )
       }
-      List(
-        microWacc.Assign(
-          destTyped,
-          microWacc.Call(
-            microWacc.Builtin.Read,
-            List(
-              destTy match {
-                case KnownType.Int      => " %d".toMicroWaccCharArray
-                case KnownType.Char | _ => " %c".toMicroWaccCharArray
-              },
-              destTyped
-            )
-          )
-        )
-      )
-    case ast.Free(lhs) =>
-      List(
-        microWacc.Call(
-          microWacc.Builtin.Free,
-          List(
-            checkValue(
-              lhs,
-              Constraint.IsEither(
-                KnownType.Array(?),
-                KnownType.Pair(?, ?),
-                "free must be applied to an array or pair"
+      (
+        Chain.one(
+          microWacc.Assign(
+            destTyped,
+            microWacc.Call(
+              microWacc.Builtin.Read,
+              List(
+                destTy match {
+                  case KnownType.Int      => " %d".toMicroWaccCharArray
+                  case KnownType.Char | _ => " %c".toMicroWaccCharArray
+                },
+                destTyped
               )
             )
           )
+        ),
+        destErrors ++ destTyErrors
+      )
+    case ast.Free(lhs) =>
+      val (lhsTyped, lhsErrors) = checkValue(
+        lhs,
+        Constraint.IsEither(
+          KnownType.Array(?),
+          KnownType.Pair(?, ?),
+          "free must be applied to an array or pair"
         )
       )
+      (Chain.one(microWacc.Call(microWacc.Builtin.Free, List(lhsTyped))), lhsErrors)
     case ast.Return(expr) =>
-      List(microWacc.Return(checkValue(expr, returnConstraint)))
+      val (exprTyped, exprErrors) = checkValue(expr, returnConstraint)
+      (Chain.one(microWacc.Return(exprTyped)), exprErrors)
     case ast.Exit(expr) =>
-      List(
-        microWacc.Call(
-          microWacc.Builtin.Exit,
-          List(checkValue(expr, Constraint.Is(KnownType.Int, "exit value must be int")))
-        )
-      )
+      val (exprTyped, exprErrors) =
+        checkValue(expr, Constraint.Is(KnownType.Int, "exit value must be int"))
+      (Chain.one(microWacc.Call(microWacc.Builtin.Exit, List(exprTyped))), exprErrors)
     case ast.Print(expr, newline) =>
       // This constraint should never fail, the scope-checker should have caught it already
-      val exprTyped = checkValue(expr, Constraint.Unconstrained)
+      val (exprTyped, exprErrors) = checkValue(expr, Constraint.Unconstrained)
       val exprFormat = exprTyped.ty match {
         case KnownType.Bool | KnownType.String             => "%s"
         case KnownType.Array(KnownType.Char)               => "%.*s"
@@ -223,7 +208,7 @@ object typeChecker {
         case KnownType.Pair(_, _) | KnownType.Array(_) | ? => "%p"
       }
       val printfCall = { (func: microWacc.Builtin, value: microWacc.Expr) =>
-        List(
+        Chain.one(
           microWacc.Call(
             func,
             List(
@@ -233,36 +218,38 @@ object typeChecker {
           )
         )
       }
-      exprTyped.ty match {
-        case KnownType.Bool =>
-          List(
-            microWacc.If(
-              exprTyped,
-              printfCall(microWacc.Builtin.Printf, "true".toMicroWaccCharArray),
-              printfCall(microWacc.Builtin.Printf, "false".toMicroWaccCharArray)
+      (
+        exprTyped.ty match {
+          case KnownType.Bool =>
+            Chain.one(
+              microWacc.If(
+                exprTyped,
+                printfCall(microWacc.Builtin.Printf, "true".toMicroWaccCharArray),
+                printfCall(microWacc.Builtin.Printf, "false".toMicroWaccCharArray)
+              )
             )
-          )
-        case KnownType.Array(KnownType.Char) =>
-          printfCall(microWacc.Builtin.PrintCharArray, exprTyped)
-        case _ => printfCall(microWacc.Builtin.Printf, exprTyped)
-      }
+          case KnownType.Array(KnownType.Char) =>
+            printfCall(microWacc.Builtin.PrintCharArray, exprTyped)
+          case _ => printfCall(microWacc.Builtin.Printf, exprTyped)
+        },
+        exprErrors
+      )
     case ast.If(cond, thenStmt, elseStmt) =>
-      List(
-        microWacc.If(
-          checkValue(cond, Constraint.Is(KnownType.Bool, "if condition must be a bool")),
-          thenStmt.toList.flatMap(checkStmt(_, returnConstraint)),
-          elseStmt.toList.flatMap(checkStmt(_, returnConstraint))
-        )
+      val (condTyped, condErrors) =
+        checkValue(cond, Constraint.Is(KnownType.Bool, "if condition must be a bool"))
+      val (thenStmtTyped, thenErrors) = thenStmt.foldMap(checkStmt(_, returnConstraint))
+      val (elseStmtTyped, elseErrors) = elseStmt.foldMap(checkStmt(_, returnConstraint))
+      (
+        Chain.one(microWacc.If(condTyped, thenStmtTyped, elseStmtTyped)),
+        condErrors ++ thenErrors ++ elseErrors
       )
     case ast.While(cond, body) =>
-      List(
-        microWacc.While(
-          checkValue(cond, Constraint.Is(KnownType.Bool, "while condition must be a bool")),
-          body.toList.flatMap(checkStmt(_, returnConstraint))
-        )
-      )
-    case ast.Block(body)   => body.toList.flatMap(checkStmt(_, returnConstraint))
-    case skip @ ast.Skip() => List.empty
+      val (condTyped, condErrors) =
+        checkValue(cond, Constraint.Is(KnownType.Bool, "while condition must be a bool"))
+      val (bodyTyped, bodyErrors) = body.foldMap(checkStmt(_, returnConstraint))
+      (Chain.one(microWacc.While(condTyped, bodyTyped)), condErrors ++ bodyErrors)
+    case ast.Block(body)   => body.foldMap(checkStmt(_, returnConstraint))
+    case skip @ ast.Skip() => (Chain.empty, Chain.empty)
   }
 
   /** Type-check an AST LValue, RValue or Expr node. This function does all 3 since these traits
@@ -275,127 +262,142 @@ object typeChecker {
     * @return
     *   The most specific type of the value if it could be determined, or ? if it could not.
     */
-  private def checkValue(value: ast.LValue | ast.RValue | ast.Expr, constraint: Constraint)(using
-      ctx: TypeCheckerCtx
-  ): microWacc.Expr = value match {
+  private def checkValue(
+      value: ast.LValue | ast.RValue | ast.Expr,
+      constraint: Constraint
+  ): (microWacc.Expr, Chain[Error]) = value match {
     case l @ ast.IntLiter(v) =>
-      KnownType.Int.satisfies(constraint, l.pos)
-      microWacc.IntLiter(v)
+      val (_, errors) = KnownType.Int.satisfies(constraint, l.pos)
+      (microWacc.IntLiter(v), errors)
     case l @ ast.BoolLiter(v) =>
-      KnownType.Bool.satisfies(constraint, l.pos)
-      microWacc.BoolLiter(v)
+      val (_, errors) = KnownType.Bool.satisfies(constraint, l.pos)
+      (microWacc.BoolLiter(v), errors)
     case l @ ast.CharLiter(v) =>
-      KnownType.Char.satisfies(constraint, l.pos)
-      microWacc.CharLiter(v)
+      val (_, errors) = KnownType.Char.satisfies(constraint, l.pos)
+      (microWacc.CharLiter(v), errors)
     case l @ ast.StrLiter(v) =>
-      KnownType.String.satisfies(constraint, l.pos)
-      v.toMicroWaccCharArray
+      val (_, errors) = KnownType.String.satisfies(constraint, l.pos)
+      (v.toMicroWaccCharArray, errors)
     case l @ ast.PairLiter() =>
-      microWacc.NullLiter()(KnownType.Pair(?, ?).satisfies(constraint, l.pos))
+      val (ty, errors) = KnownType.Pair(?, ?).satisfies(constraint, l.pos)
+      (microWacc.NullLiter()(ty), errors)
     case ast.Parens(expr) => checkValue(expr, constraint)
     case l @ ast.ArrayLiter(elems) =>
-      val (elemTy, elemsTyped) = elems.mapAccumulate[SemType, microWacc.Expr](?) {
-        case (acc, elem) =>
-          val elemTyped = checkValue(
-            elem,
-            Constraint.IsSymmetricCompatible(acc, s"array elements must have the same type")
-          )
-          (elemTyped.ty, elemTyped)
-      }
-      val arrayTy = KnownType
+      val ((elemTy, elemsErrors), elemsTyped) =
+        elems.mapAccumulate[(SemType, Chain[Error]), microWacc.Expr]((?, Chain.empty)) {
+          case ((acc, errors), elem) =>
+            val (elemTyped, elemErrors) = checkValue(
+              elem,
+              Constraint.IsSymmetricCompatible(acc, s"array elements must have the same type")
+            )
+            ((elemTyped.ty, errors ++ elemErrors), elemTyped)
+        }
+      val (arrayTy, arrayErrors) = KnownType
         // Start with an unknown param type, make it more specific while checking the elements.
         .Array(elemTy)
         .satisfies(constraint, l.pos)
-      microWacc.ArrayLiter(elemsTyped)(arrayTy)
+      (microWacc.ArrayLiter(elemsTyped)(arrayTy), elemsErrors ++ arrayErrors)
     case l @ ast.NewPair(fst, snd) =>
-      val fstTyped = checkValue(fst, Constraint.Unconstrained)
-      val sndTyped = checkValue(snd, Constraint.Unconstrained)
-      microWacc.ArrayLiter(List(fstTyped, sndTyped))(
+      val (fstTyped, fstErrors) = checkValue(fst, Constraint.Unconstrained)
+      val (sndTyped, sndErrors) = checkValue(snd, Constraint.Unconstrained)
+      val (pairTy, pairErrors) =
         KnownType.Pair(fstTyped.ty, sndTyped.ty).satisfies(constraint, l.pos)
-      )
+      (microWacc.ArrayLiter(List(fstTyped, sndTyped))(pairTy), fstErrors ++ sndErrors ++ pairErrors)
     case ast.Call(id, args) =>
       val funcTy @ FuncType(retTy, paramTys) = id.ty.asInstanceOf[FuncType]
-      if (args.length != paramTys.length) {
-        ctx.error(Error.FunctionParamsMismatch(id, paramTys.length, args.length, funcTy))
-      }
+      val lenError =
+        if (args.length == paramTys.length) then Chain.empty
+        else Chain.one(Error.FunctionParamsMismatch(id, paramTys.length, args.length, funcTy))
       // Even if the number of arguments is wrong, we still check the types of the arguments
       // in the best way we can (by taking a zip).
-      val argsTyped = args.zip(paramTys).map { case (arg, paramTy) =>
-        checkValue(arg, Constraint.Is(paramTy, s"argument type mismatch in function ${id.v}"))
-      }
-      microWacc.Call(microWacc.Ident(id.v, id.guid)(retTy.satisfies(constraint, id.pos)), argsTyped)
+      val (argsErrors, argsTyped) =
+        args.zip(paramTys).mapAccumulate(Chain.empty[Error]) { case (errors, (arg, paramTy)) =>
+          val (argTyped, argErrors) =
+            checkValue(arg, Constraint.Is(paramTy, s"argument type mismatch in function ${id.v}"))
+          (errors ++ argErrors, argTyped)
+        }
+      val (retTyChecked, retErrors) = retTy.satisfies(constraint, id.pos)
+      (
+        microWacc.Call(microWacc.Ident(id.v, id.guid)(retTyChecked), argsTyped),
+        lenError ++ argsErrors ++ retErrors
+      )
 
     // Unary operators
     case ast.Negate(x) =>
-      microWacc.UnaryOp(
-        checkValue(x, Constraint.Is(KnownType.Int, "negation must be applied to an int")),
-        microWacc.UnaryOperator.Negate
-      )(KnownType.Int.satisfies(constraint, x.pos))
+      val (argTyped, argErrors) =
+        checkValue(x, Constraint.Is(KnownType.Int, "negation must be applied to an int"))
+      val (retTy, retErrors) = KnownType.Int.satisfies(constraint, x.pos)
+      (microWacc.UnaryOp(argTyped, microWacc.UnaryOperator.Negate)(retTy), argErrors ++ retErrors)
     case ast.Not(x) =>
-      microWacc.UnaryOp(
-        checkValue(x, Constraint.Is(KnownType.Bool, "logical not must be applied to a bool")),
-        microWacc.UnaryOperator.Not
-      )(KnownType.Bool.satisfies(constraint, x.pos))
+      val (argTyped, argErrors) =
+        checkValue(x, Constraint.Is(KnownType.Bool, "logical not must be applied to a bool"))
+      val (retTy, retErrors) = KnownType.Bool.satisfies(constraint, x.pos)
+      (microWacc.UnaryOp(argTyped, microWacc.UnaryOperator.Not)(retTy), argErrors ++ retErrors)
     case ast.Len(x) =>
-      microWacc.UnaryOp(
-        checkValue(x, Constraint.Is(KnownType.Array(?), "len must be applied to an array")),
-        microWacc.UnaryOperator.Len
-      )(KnownType.Int.satisfies(constraint, x.pos))
+      val (argTyped, argErrors) =
+        checkValue(x, Constraint.Is(KnownType.Array(?), "len must be applied to an array"))
+      val (retTy, retErrors) = KnownType.Int.satisfies(constraint, x.pos)
+      (microWacc.UnaryOp(argTyped, microWacc.UnaryOperator.Len)(retTy), argErrors ++ retErrors)
     case ast.Ord(x) =>
-      microWacc.UnaryOp(
-        checkValue(x, Constraint.Is(KnownType.Char, "ord must be applied to a char")),
-        microWacc.UnaryOperator.Ord
-      )(KnownType.Int.satisfies(constraint, x.pos))
+      val (argTyped, argErrors) =
+        checkValue(x, Constraint.Is(KnownType.Char, "ord must be applied to a char"))
+      val (retTy, retErrors) = KnownType.Int.satisfies(constraint, x.pos)
+      (microWacc.UnaryOp(argTyped, microWacc.UnaryOperator.Ord)(retTy), argErrors ++ retErrors)
     case ast.Chr(x) =>
-      microWacc.UnaryOp(
-        checkValue(x, Constraint.Is(KnownType.Int, "chr must be applied to an int")),
-        microWacc.UnaryOperator.Chr
-      )(KnownType.Char.satisfies(constraint, x.pos))
+      val (argTyped, argErrors) =
+        checkValue(x, Constraint.Is(KnownType.Int, "chr must be applied to an int"))
+      val (retTy, retErrors) = KnownType.Char.satisfies(constraint, x.pos)
+      (microWacc.UnaryOp(argTyped, microWacc.UnaryOperator.Chr)(retTy), argErrors ++ retErrors)
 
     // Binary operators
     case op: (ast.Add | ast.Sub | ast.Mul | ast.Div | ast.Mod) =>
       val operand = Constraint.Is(KnownType.Int, s"${op.name} operator must be applied to an int")
-      microWacc.BinaryOp(
-        checkValue(op.x, operand),
-        checkValue(op.y, operand),
-        microWacc.BinaryOperator.fromAst(op)
-      )(KnownType.Int.satisfies(constraint, op.pos))
+      val (xTyped, xErrors) = checkValue(op.x, operand)
+      val (yTyped, yErrors) = checkValue(op.y, operand)
+      val (retTy, retErrors) = KnownType.Int.satisfies(constraint, op.pos)
+      (
+        microWacc.BinaryOp(xTyped, yTyped, microWacc.BinaryOperator.fromAst(op))(retTy),
+        xErrors ++ yErrors ++ retErrors
+      )
     case op: (ast.Eq | ast.Neq) =>
-      val xTyped = checkValue(op.x, Constraint.Unconstrained)
-      microWacc.BinaryOp(
-        xTyped,
-        checkValue(
-          op.y,
-          Constraint
-            .Is(xTyped.ty, s"${op.name} operator must be applied to values of the same type")
-        ),
-        microWacc.BinaryOperator.fromAst(op)
-      )(KnownType.Bool.satisfies(constraint, op.pos))
+      val (xTyped, xErrors) = checkValue(op.x, Constraint.Unconstrained)
+      val (yTyped, yErrors) = checkValue(
+        op.y,
+        Constraint.Is(xTyped.ty, s"${op.name} operator must be applied to values of the same type")
+      )
+      val (retTy, retErrors) = KnownType.Bool.satisfies(constraint, op.pos)
+      (
+        microWacc.BinaryOp(xTyped, yTyped, microWacc.BinaryOperator.fromAst(op))(retTy),
+        xErrors ++ yErrors ++ retErrors
+      )
     case op: (ast.Less | ast.LessEq | ast.Greater | ast.GreaterEq) =>
       val xConstraint = Constraint.IsEither(
         KnownType.Int,
         KnownType.Char,
         s"${op.name} operator must be applied to an int or char"
       )
-      val xTyped = checkValue(op.x, xConstraint)
+      val (xTyped, xErrors) = checkValue(op.x, xConstraint)
       // If x type-check failed, we still want to check y is an Int or Char (rather than ?)
       val yConstraint = xTyped.ty match {
         case ? => xConstraint
         case xTy =>
           Constraint.Is(xTy, s"${op.name} operator must be applied to values of the same type")
       }
-      microWacc.BinaryOp(
-        xTyped,
-        checkValue(op.y, yConstraint),
-        microWacc.BinaryOperator.fromAst(op)
-      )(KnownType.Bool.satisfies(constraint, op.pos))
+      val (yTyped, yErrors) = checkValue(op.y, yConstraint)
+      val (retTy, retErrors) = KnownType.Bool.satisfies(constraint, op.pos)
+      (
+        microWacc.BinaryOp(xTyped, yTyped, microWacc.BinaryOperator.fromAst(op))(retTy),
+        xErrors ++ yErrors ++ retErrors
+      )
     case op: (ast.And | ast.Or) =>
       val operand = Constraint.Is(KnownType.Bool, s"${op.name} operator must be applied to a bool")
-      microWacc.BinaryOp(
-        checkValue(op.x, operand),
-        checkValue(op.y, operand),
-        microWacc.BinaryOperator.fromAst(op)
-      )(KnownType.Bool.satisfies(constraint, op.pos))
+      val (xTyped, xErrors) = checkValue(op.x, operand)
+      val (yTyped, yErrors) = checkValue(op.y, operand)
+      val (retTy, retErrors) = KnownType.Bool.satisfies(constraint, op.pos)
+      (
+        microWacc.BinaryOp(xTyped, yTyped, microWacc.BinaryOperator.fromAst(op))(retTy),
+        xErrors ++ yErrors ++ retErrors
+      )
 
     case lvalue: ast.LValue => checkLValue(lvalue, constraint)
   }
@@ -412,68 +414,69 @@ object typeChecker {
     * @return
     *   The most specific type of the value if it could be determined, or ? if it could not.
     */
-  private def checkLValue(value: ast.LValue, constraint: Constraint)(using
-      ctx: TypeCheckerCtx
-  ): microWacc.LValue = value match {
+  private def checkLValue(
+      value: ast.LValue,
+      constraint: Constraint
+  ): (microWacc.LValue, Chain[Error]) = value match {
     case id @ ast.Ident(name, guid, ty) =>
-      microWacc.Ident(name, guid)(ty.asInstanceOf[SemType].satisfies(constraint, id.pos))
+      val (idTy, idErrors) = ty.asInstanceOf[SemType].satisfies(constraint, id.pos)
+      (microWacc.Ident(name, guid)(idTy), idErrors)
     case ast.ArrayElem(id, indices) =>
       val arrayTy = id.ty.asInstanceOf[SemType]
-      val (elemTy, indicesTyped) = indices.mapAccumulate(arrayTy.asInstanceOf[SemType]) {
-        (acc, elem) =>
-          val idxTyped =
-            checkValue(elem, Constraint.Is(KnownType.Int, "array index must be an int"))
-          val next = acc match {
-            case KnownType.Array(innerTy) => innerTy
-            case ?                        => ? // we can keep indexing an unknown type
-            case nonArrayTy =>
-              ctx.error(
-                Error.TypeMismatch(
-                  elem.pos,
-                  KnownType.Array(?),
-                  acc,
-                  "cannot index into a non-array"
+      val ((elemTy, elemErrors), indicesTyped) =
+        indices.mapAccumulate((arrayTy.asInstanceOf[SemType], Chain.empty[Error])) {
+          case ((acc, errors), elem) =>
+            val (idxTyped, idxErrors) =
+              checkValue(elem, Constraint.Is(KnownType.Int, "array index must be an int"))
+            val (next, nextError) = acc match {
+              case KnownType.Array(innerTy) => (innerTy, Chain.empty)
+              case ? => (?, Chain.empty) // we can keep indexing an unknown type
+              case nonArrayTy =>
+                (
+                  ?,
+                  Chain.one(
+                    Error.TypeMismatch(
+                      elem.pos,
+                      KnownType.Array(?),
+                      acc,
+                      "cannot index into a non-array"
+                    )
+                  )
                 )
-              )
-              ?
-          }
-          (next, idxTyped)
-      }
+            }
+            ((next, errors ++ idxErrors ++ nextError), idxTyped)
+        }
+      val (retTy, retErrors) = elemTy.satisfies(constraint, value.pos)
       val firstArrayElem = microWacc.ArrayElem(
         microWacc.Ident(id.v, id.guid)(arrayTy),
         indicesTyped.head
-      )(elemTy.satisfies(constraint, value.pos))
+      )(retTy)
       val arrayElem = indicesTyped.tail.foldLeft(firstArrayElem) { (acc, idx) =>
         microWacc.ArrayElem(acc, idx)(KnownType.Array(acc.ty))
       }
       // Need to type-check the final arrayElem with the constraint
-      microWacc.ArrayElem(arrayElem.value, arrayElem.index)(elemTy.satisfies(constraint, value.pos))
+      // TODO: What
+      (microWacc.ArrayElem(arrayElem.value, arrayElem.index)(retTy), elemErrors ++ retErrors)
     case ast.Fst(elem) =>
-      val elemTyped = checkLValue(
+      val (elemTyped, elemErrors) = checkLValue(
         elem,
         Constraint.Is(KnownType.Pair(?, ?), "fst must be applied to a pair")
       )
-      microWacc.ArrayElem(
-        elemTyped,
-        microWacc.IntLiter(0)
-      )(elemTyped.ty match {
-        case KnownType.Pair(left, _) =>
-          left.satisfies(constraint, elem.pos)
-        case _ => ctx.error(Error.InternalError(elem.pos, "fst must be applied to a pair"))
-      })
+      val (retTy, retErrors) = elemTyped.ty match {
+        case KnownType.Pair(left, _) => left.satisfies(constraint, elem.pos)
+        case _ => (?, Chain.one(Error.InternalError(elem.pos, "fst must be applied to a pair")))
+      }
+      (microWacc.ArrayElem(elemTyped, microWacc.IntLiter(0))(retTy), elemErrors ++ retErrors)
     case ast.Snd(elem) =>
-      val elemTyped = checkLValue(
+      val (elemTyped, elemErrors) = checkLValue(
         elem,
         Constraint.Is(KnownType.Pair(?, ?), "snd must be applied to a pair")
       )
-      microWacc.ArrayElem(
-        elemTyped,
-        microWacc.IntLiter(1)
-      )(elemTyped.ty match {
-        case KnownType.Pair(_, right) =>
-          right.satisfies(constraint, elem.pos)
-        case _ => ctx.error(Error.InternalError(elem.pos, "snd must be applied to a pair"))
-      })
+      val (retTy, retErrors) = elemTyped.ty match {
+        case KnownType.Pair(_, right) => right.satisfies(constraint, elem.pos)
+        case _ => (?, Chain.one(Error.InternalError(elem.pos, "snd must be applied to a pair")))
+      }
+      (microWacc.ArrayElem(elemTyped, microWacc.IntLiter(1))(retTy), elemErrors ++ retErrors)
   }
 
   extension (s: String) {