feat: add output flag with Destination interface.

Added -o flag to write results to files using a new Destination interface.

Changes:
- Add Destination interface with StdoutDestination and FileDestination implementations.
- Add -o flag to CLI parser for output file specification.
- Update Config to use Destination instead of direct output handling.
- Refactor main to use Destination.Write() for result output.

internal/config/config.go

@@ -3,9 +3,10 @@ package config
 
 // Configuration settings for the program.
 type Config struct {
-	Source      Source // The source code given to the program.
-	Verbose     bool   // Whether or not to print debug logs.
-	Explanation bool   // Whether or not to print an explanation of the reduction.
-	Profile     string // If not nil, print a CPU profile during execution.
-	Statistics  bool   // Whether or not to print statistics.
+	Source      Source      // The source code given to the program.
+	Destination Destination // The destination for the final result.
+	Verbose     bool        // Whether or not to print debug logs.
+	Explanation bool        // Whether or not to print an explanation of the reduction.
+	Profile     string      // If not nil, print a CPU profile during execution.
+	Statistics  bool        // Whether or not to print statistics.
 }

internal/config/destination.go

@@ -0,0 +1,27 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+// A method of writing output to the user.
+type Destination interface {
+	// Write data to this destination.
+	Write(data string) error
+}
+
+// A destination writing to stdout.
+type StdoutDestination struct{}
+
+func (d StdoutDestination) Write(data string) error {
+	fmt.Println(data)
+	return nil
+}
+
+// A destination writing to a file.
+type FileDestination struct{ Path string }
+
+func (d FileDestination) Write(data string) error {
+	return os.WriteFile(d.Path, []byte(data+"\n"), 0644)
+}

internal/config/parse_from_args.go

@@ -13,6 +13,7 @@ func FromArgs() (*Config, error) {
 	statistics := flag.Bool("s", false, "Statistics. If set, the process will print various statistics about the run.")
 	profile := flag.String("p", "", "CPU profiling. If an output file is defined, the program will profile its execution and dump its results into it.")
 	file := flag.String("f", "", "File. If set, read source from the specified file.")
+	output := flag.String("o", "", "Output. If set, write result to the specified file. Use '-' for stdout (default).")
 	flag.Parse()
 
 	// Parse source type.
@@ -36,8 +37,17 @@ func FromArgs() (*Config, error) {
 		}
 	}
 
+	// Parse destination type.
+	var destination Destination
+	if *output == "" || *output == "-" {
+		destination = StdoutDestination{}
+	} else {
+		destination = FileDestination{Path: *output}
+	}
+
 	return &Config{
 		Source:      source,
+		Destination: destination,
 		Verbose:     *verbose,
 		Explanation: *explanation,
 		Profile:     *profile,