feat: add output flag (#13)

## Description

The lambda CLI previously only wrote output to stdout using shell redirection.
This PR adds support for writing results to files using the `-o` flag.
This is implemented using a new `Destination` interface that mirrors the existing `Source` pattern.

Changes:
- Added `Destination` interface with `StdoutDestination` and `FileDestination` implementations.
- Added `-o` flag to CLI argument parser for output file specification.
- Updated `Config` to use `Destination` instead of direct output handling.
- Refactored main to use `Destination.Write()` for result output.
- Updated Makefile targets (`run`, `profile`, `explain`) to use `-o` flag instead of shell redirection.

### Decisions

The `-o` flag defaults to stdout when not specified or when set to `-`.
This maintains backward compatibility while providing explicit file output capability.

## Benefits

- Cleaner command-line interface without shell redirection.
- Symmetric design with `Source` interface for input.
- More portable across different shells and environments.
- Explicit output handling improves code clarity.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [ ] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #13
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

Makefile

@@ -20,13 +20,13 @@ build:
 	chmod +x ${BINARY_NAME}
 
 run: build
-	./${BINARY_NAME} -f ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -f ./samples/$(TEST).txt -o program.out
 
 profile: build
-	./${BINARY_NAME} -p profile/cpu.prof -f ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -p profile/cpu.prof -f ./samples/$(TEST).txt -o program.out
 
 explain: build
-	./${BINARY_NAME} -x -p profile/cpu.prof -f ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -x -p profile/cpu.prof -f ./samples/$(TEST).txt -o program.out
 
 graph:
 	go tool pprof -raw -output=profile/cpu.raw profile/cpu.prof

cmd/lambda/lambda.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"fmt"
 	"os"
 
 	"git.maximhutz.com/max/lambda/internal/cli"
@@ -72,5 +71,7 @@ func main() {
 	process.Run()
 
 	// Return the final reduced result.
-	fmt.Println(lambda.Stringify(compiled))
+	result := lambda.Stringify(compiled)
+	err = options.Destination.Write(result)
+	cli.HandleError(err)
 }

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 // Configuration settings for the program.
 type Config struct {
 	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.

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,