ref: 5cd0b7d8139f0cd5f4af1f9f3b82fe2cd3f37288
dir: /Args/README.md/
# Command-Line Arguments Inferno has a dedicated module for processing commandline flags and arguments, [arg(2)](http://man.cat-v.org/inferno/2/arg). ## Source ### args.b:17,22 The module `Arg` is loaded and initialized. As per the manual, the `arg->init()` function must be called before any other functions can be called from `Arg`. The usage message is also set pre-emptively for use later, if necessary. ### args.b:24,37 There are two flag arguments, `r` and `c` which set reversal of arguments and the list item indicator mark, respectively. The flag `r` is a binary option toggle, the flag's presence as an argument is sufficient to change the value of `rev`. The flag `c` takes an argument. The `arg->earg()` function is used to pop the flag's argument out of the list. Specifically with regards to `earg()` rather than `arg()` as per the manual, `earg()` will call `arg->usage()` if the relevant argument does not exist. Note: After the processing of all flags, `argv` is re-set from the value contained within `arg` to remove the elements (if any) utilized by flags and their arguments. ### args.b:41,51 This section utilizes the `rev` variable to (naively) reverse the list `argv`. After the `rev` check, the list `argv` is printed in order from beginning to end with each element being printed on a new line with the `mark` variable being prefixed to the list element currently at the head of the `argv` list each iteration during printout. ## Demo ; limbo args.b ; args -h usage: args [-r] [-c mark] words... ; args a b c d Argc after flags: 4 → a → b → c → d ; args -r a b c d Argc after flags: 4 → d → c → b → a ; args -c 'quack: ' a b c d Argc after flags: 4 quack: a quack: b quack: c quack: d ; args -r -c '-> ' d c b a Argc after flags: 4 -> a -> b -> c -> d ; ## Exercises - Can you trick `earg()` into accepting a nil value? - How would you pull an `int` out of `earg()`? - How would you trip the `usage()` message call-able from `earg()`?