diff --git a/cmd/lambda/lambda.go b/cmd/lambda/lambda.go
index 1ea2978..9900c5e 100644
--- a/cmd/lambda/lambda.go
+++ b/cmd/lambda/lambda.go
@@ -7,35 +7,37 @@ import (
 	"time"
 
 	"git.maximhutz.com/max/lambda/internal/cli"
+	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/parser"
 	"git.maximhutz.com/max/lambda/pkg/tokenizer"
 )
 
+// Run main application.
 func main() {
-
-	options, err := cli.ParseOptions(os.Args[1:])
+	options, err := config.FromArgs()
 	cli.HandleError(err)
 
-	logger := cli.GetLogger(*options)
+	logger := options.GetLogger()
 	logger.Info("Using program arguments.", "args", os.Args)
 	logger.Info("Parsed CLI options.", "options", options)
 
-	if options.Input == "-" {
-		options.Input, err = cli.ReadInput()
-		cli.HandleError(err)
-	}
+	input, err := options.Source.Pull()
+	cli.HandleError(err)
 
-	tokens, fails := tokenizer.GetTokens([]rune(options.Input))
+	// Parse tokens.
+	tokens, fails := tokenizer.GetTokens([]rune(input))
 	if len(fails) > 0 {
 		cli.HandleError(errors.Join(fails...))
 	}
 	logger.Info("Parsed tokens.", "tokens", tokens)
 
+	// Turn tokens into syntax tree.
 	expression, err := parser.GetTree(tokens)
 	cli.HandleError(err)
 	logger.Info("Parsed syntax tree.", "tree", lambda.Stringify(expression))
 
+	// Reduce expression.
 	start := time.Now()
 
 	if options.Explanation {
diff --git a/internal/cli/arguments.go b/internal/cli/arguments.go
index 56ed199..5249317 100644
--- a/internal/cli/arguments.go
+++ b/internal/cli/arguments.go
@@ -5,18 +5,24 @@ import (
 	"fmt"
 )
 
+// Arguments given to program.
 type CLIOptions struct {
-	Input       string
-	Verbose     bool
+	// The source code given to the program.
+	Input string
+	// Whether or not to print debug logs.
+	Verbose bool
+	// Whether or not to print an explanation of the reduction.
 	Explanation bool
 }
 
-func ParseOptions(args []string) (*CLIOptions, error) {
-	// Parse flags and arguments.
+// Extract the program configuration from the command-line arguments.
+func ParseOptions() (*CLIOptions, error) {
+	// Parse flags.
 	verbose := flag.Bool("v", false, "Verbosity. If set, the program will print logs.")
 	explanation := flag.Bool("x", false, "Explanation. Whether or not to show all reduction steps.")
 	flag.Parse()
 
+	// Parse non-flag arguments.
 	if flag.NArg() == 0 {
 		return nil, fmt.Errorf("No input given.")
 	} else if flag.NArg() > 1 {
diff --git a/internal/cli/exit.go b/internal/cli/exit.go
index 6ede677..32cfc5e 100644
--- a/internal/cli/exit.go
+++ b/internal/cli/exit.go
@@ -5,6 +5,8 @@ import (
 	"os"
 )
 
+// A helper function to handle errors in the program. If it is given an error,
+// the program will exist, and print the error.
 func HandleError(err error) {
 	if err == nil {
 		return
@@ -12,4 +14,4 @@ func HandleError(err error) {
 
 	fmt.Fprintln(os.Stderr, err)
 	os.Exit(1)
-}
\ No newline at end of file
+}
diff --git a/internal/cli/read.go b/internal/cli/read.go
deleted file mode 100644
index 93b762c..0000000
--- a/internal/cli/read.go
+++ /dev/null
@@ -1,15 +0,0 @@
-package cli
-
-import (
-	"io"
-	"os"
-)
-
-func ReadInput() (string, error) {
-	data, err := io.ReadAll(os.Stdin)
-	if err != nil {
-		return "", err
-	}
-
-	return string(data), nil
-}
diff --git a/internal/config/config.go b/internal/config/config.go
new file mode 100644
index 0000000..cd74cef
--- /dev/null
+++ b/internal/config/config.go
@@ -0,0 +1,11 @@
+package config
+
+// Arguments given to program.
+type Config struct {
+	// The source code given to the program.
+	Source Source
+	// Whether or not to print debug logs.
+	Verbose bool
+	// Whether or not to print an explanation of the reduction.
+	Explanation bool
+}
diff --git a/internal/cli/logger.go b/internal/config/get_logger.go
similarity index 62%
rename from internal/cli/logger.go
rename to internal/config/get_logger.go
index 97437a4..ea7fb5e 100644
--- a/internal/cli/logger.go
+++ b/internal/config/get_logger.go
@@ -1,13 +1,14 @@
-package cli
+package config
 
 import (
 	"log/slog"
 	"os"
 )
 
-func GetLogger(o CLIOptions) *slog.Logger {
+// Define the correct logger for the program to use.
+func (this Config) GetLogger() *slog.Logger {
 	var level slog.Level
-	if o.Verbose {
+	if this.Verbose {
 		level = slog.LevelInfo
 	} else {
 		level = slog.LevelError
diff --git a/internal/config/parse_from_args.go b/internal/config/parse_from_args.go
new file mode 100644
index 0000000..f2bf8dd
--- /dev/null
+++ b/internal/config/parse_from_args.go
@@ -0,0 +1,35 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+)
+
+// Extract the program configuration from the command-line arguments.
+func FromArgs() (*Config, error) {
+	// Parse flags.
+	verbose := flag.Bool("v", false, "Verbosity. If set, the program will print logs.")
+	explanation := flag.Bool("x", false, "Explanation. Whether or not to show all reduction steps.")
+	flag.Parse()
+
+	// Parse non-flag arguments.
+	if flag.NArg() == 0 {
+		return nil, fmt.Errorf("No input given.")
+	} else if flag.NArg() > 1 {
+		return nil, fmt.Errorf("More than 1 command-line argument.")
+	}
+
+	// Parse source type.
+	var source Source
+	if flag.Arg(0) == "-" {
+		source = StdinSource{}
+	} else {
+		source = StringSource{data: flag.Arg(0)}
+	}
+
+	return &Config{
+		Source:      source,
+		Verbose:     *verbose,
+		Explanation: *explanation,
+	}, nil
+}
diff --git a/internal/config/source.go b/internal/config/source.go
new file mode 100644
index 0000000..f4b07b3
--- /dev/null
+++ b/internal/config/source.go
@@ -0,0 +1,29 @@
+package config
+
+import (
+	"io"
+	"os"
+)
+
+// Defines the consumption of different types of input sources.
+type Source interface {
+	// Get the data.
+	Pull() (string, error)
+}
+
+// A source coming from a string.
+type StringSource struct{ data string }
+
+func (this StringSource) Pull() (string, error) { return this.data, nil }
+
+// A source coming from standard input.
+type StdinSource struct{}
+
+func (this StdinSource) Pull() (string, error) {
+	data, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}