In the world of command-line interfaces, consistency and predictability are paramount. A new Go package called argp has recently emerged, promising to provide a GNU-standard command line argument parser. However, its release has sparked significant discussion among developers about CLI standards, compatibility concerns, and the ongoing tension between following established conventions versus introducing new approaches.
The Battle of CLI Standards
The introduction of this new parser has reignited a long-standing debate about which command-line interface standards developers should follow. While the package claims to implement GNU-style argument parsing, several commenters have pointed out inconsistencies with the actual GNU standard. One notable deviation is the handling of multiple values for options. The package documentation suggests that -a 1 2 3 would assign all three values to option a, whereas traditional GNU style would interpret this as option a with value 1, followed by two positional arguments.
In general, command-line argument parsers should just follow the GNU style. No more, no less. Deviations confuse users as it is not immediately obvious to them what rules a parser is imposing.
This sentiment reflects a common frustration among developers who must navigate inconsistent CLI behaviors across different tools. The POSIX standard, which predates GNU conventions, is even more restrictive and doesn't define long options at all. This has led to a fragmented landscape where different communities (Go, Python, Java) have developed their own CLI conventions, often to the dismay of users who expect consistent behavior.
Popular Command Line Parsers in Go
- Standard library flag: Simple Go-style flags (
-flag=value) - Cobra/pflag: GNU-style flags with autocompletion support
- Kong: Advanced functionality like flag groups and environment variable association
- urfave/cli: Popular alternative with good developer experience
- go-flags: Struct-based approach using tags
- argp: New GNU-style parser with struct tag support
GNU Command Line Rules
- Arguments are options when they begin with a hyphen
- - Multiple short options can be combined:
-abcequals-a -b -c - Long options start with two hyphens:
--verbose - Option values can be separated by space, equal sign, or nothing
- Options and non-options can be interleaved
--terminates all options- A single
-is treated as a non-option
Compatibility Concerns and Edge Cases
A particularly contentious issue raised in the discussion involves the handling of equal signs in short options. The package documentation states that -a=1 is equivalent to -a 1, but this can create problems with tools like cut where -d= is a common usage pattern. If the equal sign is eaten by the parser, it changes the behavior in potentially breaking ways.
This highlights a broader challenge: backward compatibility. Shell scripts and user workflows that have relied on specific CLI behaviors for decades can break when new tools implement different parsing rules. Several commenters shared real-world examples where these inconsistencies have caused problems, such as in Gentoo package management where exact package names must start with an equal sign.
The Ecosystem of Command Line Parsers
The discussion revealed that the Go community already has several established command line parsers with different approaches and feature sets. Cobra (with pflag), Kong, and urfave/cli were frequently mentioned as mature alternatives. Each has its strengths – Cobra offers automatic shell completion, Kong handles complex CLI interfaces well, and go-flags provides a struct-based approach similar to the new package.
This fragmentation raises questions about whether another parser is needed, especially one that introduces its own interpretations of GNU standards. Some developers expressed frustration that Go's standard library flag package doesn't follow GNU conventions, while others defended Go's simpler approach as being more consistent within its own ecosystem.
Reflection and Performance Considerations
A technical concern raised by some developers was the use of struct tags and reflection in the new parser. While this approach enables a clean, declarative style of defining command-line options, it comes with potential drawbacks. Reflection can impact performance and disable dead-code elimination during compilation, potentially leading to larger binary sizes. It also moves validation from compile-time to runtime, which some developers consider problematic.
The debate around struct tags highlights a broader tension in Go development between following idiomatic patterns and optimizing for performance and safety. While many popular Go packages use reflection and struct tags (like encoding/json), some developers prefer explicit code over magic runtime behaviors.
In the end, the discussion around this new command line parser reflects the ongoing challenge of balancing standards compliance, user expectations, and language idioms. While having options is valuable, the proliferation of slightly different approaches to CLI parsing continues to create friction for both developers and users. As one commenter noted, perhaps the most important thing is not which standard is best, but that tools consistently follow some standard that users can rely on.
Reference: GNU command line argument parser