diff --git a/.golangci.yml b/.golangci.yml
index a9a6f07..9c9494b 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -48,7 +48,7 @@ linters:
       # More information: https://golangci-lint.run/usage/false-positives/#comments
       #
       # Please uncomment the following line if your code is not using the godoc format
-      - comments
+      # - comments
 
       # Common false positives
       # feel free to remove this if you don't have any false positives
@@ -126,6 +126,9 @@ linters:
         # Blank import should be only in a main or test package, or have a comment justifying it.
         - name: blank-imports
 
+        # Packages should have comments of the form "Package x ...".
+        - name: package-comments
+
         # context.Context() should be the first parameter of a function when provided as argument.
         - name: context-as-argument
           arguments:
diff --git a/Makefile b/Makefile
index 2ce49e2..c4ee60d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 BINARY_NAME=lambda
 TEST=simple
 
-.PHONY: help build run profile explain graph docs test bench clean
+.PHONY: help build run profile explain graph docs test bench lint clean
 .DEFAULT_GOAL := help
 .SILENT:
 
@@ -15,6 +15,7 @@ help:
 	echo "  docs     - Start local godoc server on port 6060"
 	echo "  test     - Run tests for all samples"
 	echo "  bench    - Run benchmarks for all samples"
+	echo "  lint     - Run golangci-lint on all packages"
 	echo "  clean    - Remove all build artifacts"
 
 build:
@@ -45,6 +46,9 @@ test:
 bench:
 	go test -bench=. -benchtime=10x -cpu=4 ./cmd/lambda
 
+lint:
+	go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest run ./...
+
 clean:
 	rm -f ${BINARY_NAME}
 	rm -f program.out
diff --git a/pkg/codec/codec.go b/pkg/codec/codec.go
new file mode 100644
index 0000000..7fc8a7f
--- /dev/null
+++ b/pkg/codec/codec.go
@@ -0,0 +1,12 @@
+package codec
+
+import "git.maximhutz.com/max/lambda/pkg/repr"
+
+type String string
+
+func (s String) Id() string { return "string" }
+
+type Codec[T, U repr.Repr] interface {
+	Encode(T) (U, error)
+	Decode(U) (T, error)
+}
diff --git a/pkg/expr/expr.go b/pkg/expr/expr.go
deleted file mode 100644
index b45a515..0000000
--- a/pkg/expr/expr.go
+++ /dev/null
@@ -1,15 +0,0 @@
-// Package expr provides the abstract Expression interface for all evaluatable
-// expression types in the lambda runtime.
-package expr
-
-import (
-	"fmt"
-)
-
-// Expression is the base interface for all evaluatable expression types.
-// Different evaluation modes (lambda calculus, SKI combinators, typed lambda
-// calculus, etc.) implement this interface with their own concrete types.
-type Expression interface {
-	// The expression should have a human-readable representation.
-	fmt.Stringer
-}
diff --git a/pkg/lambda/expression.go b/pkg/lambda/expression.go
index dd28fe6..3b78f13 100644
--- a/pkg/lambda/expression.go
+++ b/pkg/lambda/expression.go
@@ -1,14 +1,17 @@
 package lambda
 
 import (
-	"git.maximhutz.com/max/lambda/pkg/expr"
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/repr"
 	"git.maximhutz.com/max/lambda/pkg/set"
 )
 
 // Expression is the interface for all lambda calculus expression types.
 // It embeds the general expr.Expression interface for cross-mode compatibility.
 type Expression interface {
-	expr.Expression
+	repr.Repr
+	fmt.Stringer
 
 	// Substitute replaces all free occurrences of the target variable with the
 	// replacement expression. Alpha-renaming is performed automatically to
@@ -37,6 +40,8 @@ type Abstraction struct {
 
 var _ Expression = Abstraction{}
 
+func (a Abstraction) Id() string { return "lambda" }
+
 func (a Abstraction) Parameter() string {
 	return a.parameter
 }
@@ -62,6 +67,8 @@ type Application struct {
 
 var _ Expression = Application{}
 
+func (a Application) Id() string { return "lambda" }
+
 func (a Application) Abstraction() Expression {
 	return a.abstraction
 }
@@ -86,6 +93,8 @@ type Variable struct {
 
 var _ Expression = Variable{}
 
+func (a Variable) Id() string { return "lambda" }
+
 func (v Variable) Name() string {
 	return v.name
 }
diff --git a/pkg/normalorder/runtime.go b/pkg/normalorder/runtime.go
index 37d38e4..c014e3d 100644
--- a/pkg/normalorder/runtime.go
+++ b/pkg/normalorder/runtime.go
@@ -2,8 +2,8 @@ package normalorder
 
 import (
 	"git.maximhutz.com/max/lambda/pkg/emitter"
-	"git.maximhutz.com/max/lambda/pkg/expr"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/repr"
 	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
@@ -23,7 +23,7 @@ func NewRuntime(expression lambda.Expression) *Runtime {
 }
 
 // Expression returns the current expression state.
-func (r *Runtime) Expression() expr.Expression {
+func (r *Runtime) Expression() repr.Repr {
 	return r.expression
 }
 
diff --git a/pkg/repr/repr.go b/pkg/repr/repr.go
new file mode 100644
index 0000000..a1248f0
--- /dev/null
+++ b/pkg/repr/repr.go
@@ -0,0 +1,15 @@
+// Package repr defines a general definition of a representation of lambda
+// calculus.
+package repr
+
+import "fmt"
+
+// Repr is a representation of lambda calculus.
+type Repr interface {
+	fmt.Stringer
+
+	// Id returns to name of the objects underlying representation. If is
+	// assumed that if two Repr objects have the same Id(), they share the same
+	// representation.
+	Id() string
+}
diff --git a/pkg/runtime/runtime.go b/pkg/runtime/runtime.go
index c2aafad..5749045 100644
--- a/pkg/runtime/runtime.go
+++ b/pkg/runtime/runtime.go
@@ -4,7 +4,7 @@ package runtime
 
 import (
 	"git.maximhutz.com/max/lambda/pkg/emitter"
-	"git.maximhutz.com/max/lambda/pkg/expr"
+	"git.maximhutz.com/max/lambda/pkg/repr"
 )
 
 // Runtime defines the interface for expression reduction strategies.
@@ -23,5 +23,5 @@ type Runtime interface {
 	Run()
 
 	// Copy the state of the runtime.
-	Expression() expr.Expression
+	Expression() repr.Repr
 }