feat: add file input flag to CLI.

Added -f flag to allow reading lambda expressions from files.

Changes:
- Add FileSource type to read from file paths.
- Add -f flag to command-line parser.
- Implement validation to prevent conflicting -f and positional arguments.

internal/config/parse_from_args.go

@@ -12,22 +12,29 @@ func FromArgs() (*Config, error) {
 	explanation := flag.Bool("x", false, "Explanation. Whether or not to show all reduction steps.")
 	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.")
 	flag.Parse()
 
-	// There must only be one input argument.
-	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 *file != "" {
+		// File flag takes precedence.
+		if flag.NArg() > 0 {
+			return nil, fmt.Errorf("cannot specify both -f flag and positional argument")
+		}
+		source = FileSource{Path: *file}
+	} else 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")
+	} else {
+		// Positional argument.
 		if flag.Arg(0) == "-" {
 			source = StdinSource{}
 		} else {
 			source = StringSource{Data: flag.Arg(0)}
 		}
+	}
 
 	return &Config{
 		Source:      source,

internal/config/source.go

@@ -27,3 +27,15 @@ func (s StdinSource) Extract() (string, error) {
 
 	return string(data), nil
 }
+
+// A source reading from a file.
+type FileSource struct{ Path string }
+
+func (s FileSource) Extract() (string, error) {
+	data, err := os.ReadFile(s.Path)
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}