Command Line Flags in Lisp

Here’s the translation of the Go code to Lisp, with explanations in Markdown format suitable for Hugo:

(defun main ()
  (let ((word-ptr (make-flag:string "word" "foo" "a string"))
        (numb-ptr (make-flag:integer "numb" 42 "an int"))
        (fork-ptr (make-flag:boolean "fork" nil "a bool"))
        (svar "bar"))
    (flag:string-var 'svar "svar" "bar" "a string var")
    (flag:parse)
    (format t "word: ~A~%" (deref word-ptr))
    (format t "numb: ~A~%" (deref numb-ptr))
    (format t "fork: ~A~%" (deref fork-ptr))
    (format t "svar: ~A~%" svar)
    (format t "tail: ~A~%" (flag:args))))

(main)

Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

In this example, we’ll use a hypothetical flag package to implement basic command-line flag parsing. This package would provide similar functionality to Go’s flag package.

We start by defining our main function. Inside this function, we declare our flags:

1. We declare a string flag word with a default value “foo” and a short description.
2. We declare an integer flag numb with a default value 42.
3. We declare a boolean flag fork with a default value of nil (false in Lisp).
4. We also demonstrate how to declare an option that uses an existing variable svar.

After declaring the flags, we call flag:parse to execute the command-line parsing.

Finally, we print out the parsed options and any trailing positional arguments. Note that we need to dereference the pointers with deref to get the actual option values.

To experiment with this program, you would compile it and run the resulting binary. Here are some example usages:

$ sbcl --script command-line-flags.lisp -word=opt -numb=7 -fork -svar=flag
word: opt
numb: 7
fork: T
svar: flag
tail: NIL

$ sbcl --script command-line-flags.lisp -word=opt
word: opt
numb: 42
fork: NIL
svar: bar
tail: NIL

$ sbcl --script command-line-flags.lisp -word=opt a1 a2 a3
word: opt
numb: 42
fork: NIL
svar: bar
tail: (a1 a2 a3)

Note that if you omit flags, they automatically take their default values. Trailing positional arguments can be provided after any flags.

In Lisp, you would typically use libraries like command-line-arguments or write custom parsing logic for more complex command-line interfaces. This example provides a simplified illustration of how you might implement similar functionality to Go’s flag package in Lisp.