feat: add file input flag (#12)

## Description

The lambda CLI previously only supported inline string expressions and stdin input.
This PR adds support for reading lambda expressions from files using the `-f` flag.
This makes it easier to work with larger programs stored in files.

Changes:
- Added `FileSource` type to `internal/config/source.go` for reading from file paths.
- Added `-f` flag to CLI argument parser with validation to prevent conflicting inputs.
- Updated Makefile targets (`run`, `profile`, `explain`) to use `-f` flag instead of stdin redirection.

### Decisions

The `-f` flag takes precedence over positional arguments.
If both are specified, an error is returned to avoid ambiguity.

## Benefits

- More intuitive workflow for file-based lambda programs.
- Cleaner Makefile targets without stdin redirection.
- Consistent with common CLI conventions (e.g., `grep -f`, `awk -f`).

## Checklist

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

Reviewed-on: #12
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
-	./${BINARY_NAME} - < ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -f ./samples/$(TEST).txt > program.out
 
 profile: build
-	./${BINARY_NAME} -p profile/cpu.prof - < ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -p profile/cpu.prof -f ./samples/$(TEST).txt > program.out
 
 explain: build
-	./${BINARY_NAME} -x -p profile/cpu.prof - < ./samples/$(TEST).txt > program.out
+	./${BINARY_NAME} -x -p profile/cpu.prof -f ./samples/$(TEST).txt > program.out
 
 graph:
 	go tool pprof -raw -output=profile/cpu.raw profile/cpu.prof

internal/config/parse_from_args.go

@@ -12,21 +12,28 @@ 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 flag.Arg(0) == "-" {
-		source = StdinSource{}
+	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 {
-		source = StringSource{Data: flag.Arg(0)}
+		// Positional argument.
+		if flag.Arg(0) == "-" {
+			source = StdinSource{}
+		} else {
+			source = StringSource{Data: flag.Arg(0)}
+		}
 	}
 
 	return &Config{

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
+}