Add `Fmt.if'`.

[rizo]

I was surprised this isn't present. This is similar to https://erratique.ch/software/bos/doc/Bos/Cmd/index.html#val-on

[dbuenzli]

I don't remember why but nowadays I rather moved to use if', see for example:

https://erratique.ch/software/b0/doc/B0_std/Cmd/index.html#val-if' https://erratique.ch/software/brr/doc/Brr/At/index.html#val-if' https://erratique.ch/software/webs/doc/Webs_html/At/index.html#val-if'

what do you think ?

[rizo]

No strong opinions, but my thoughts are:

• consistency/compatibility – on is already used by at least bos which is why I proposed Fmt.on;
• if' closer indicates that there's a boolean involved, while on is often times used for states or even event triggers;
• if' bares a more imperative meaning (especially without else) while on is more declarative:
  • Cmd.on verbose (Cmd.v "--verbosity=debug") – suggests that there's a state mapping;
  • Cmd.if' verbose (Cmd.v "--verbosity=debug") – might be perceived to suggest an action;
• on avoids the ' by not being a keyword;
• we already have Fm.on_bytes and Fmt.on_string, just Fmt.on has nothing to do with them which feels inconsistent.

Happy to rename this to Fmt.if'.

[dbuenzli]

I grepped a bit my code bases and even in the Cmd case (which often happens inside a Cmd.()) I definitively find if' clearer to read and, considering the following progression, more natural:

Fmt.(if cond then that_way else this_way)
Fmt.(if cond then that_way else nop)
Fmt.(if' cond that_way)

I rather keep on for labelling FRP event values.

[rizo]

That makes sense. Pushed Fmt.if'.

Would you consider adding Cmd.if' to Bos in that case and deprecating Cmd.on?

[dbuenzli]

Thanks your patch is in as 9db1a3c058973

Would you consider adding Cmd.if' to Bos in that case and deprecating Cmd.on?

https://github.com/dbuenzli/bos/commit/6320f302eb9487518491511292a11ee6f04bd6e9