Optional labelled parameter in functions

[yawaramin]

I've come across a couple of functions in QCheck.Gen which have only optional parameters:

val string : ?⁠gen:char t ‑> string t
val small_string : ?⁠gen:char t ‑> string t

Not sure if there are more throughout the library. Functions with only optional labelled parameters are tricky to use :-) would there be some receptiveness to adding a 'bookend' () parameter to them for 1.0? If we're going by SemVer, backward-incompatible changes are allowed in a major version...

[c-cube]

That's a good remark indeed. Perhaps it's time we start designing 1.0, the library has been mostly stable for years now. I'd probably rather have string : string t and string_of : char t -> string t (similarly for small_string) rather than optional arguments, btw.

[yawaramin]

Yes, that's a good design for sure.

[jmid]

Can you elaborate why you find them tricky to use?
I don't follow the concern we are trying to address with such a backward-incompatible change:

# Gen.generate1 Gen.string;;
- : string = ":Í\0025�gª;N"
# Gen.generate1 (Gen.string ~gen:(Gen.oneofl ['a';'b']));;
- : string = "aaabbababbbbbbbaabbbbaabbbbabbbabaaaaabaabaaabaababaabaaabba"

[yawaramin]

@jmid the trickiness comes from trying to get a Gen.t with the default generator, from Gen.string, without running it immediately with Gen.generate1. E.g.:

(* This doesn't give me a `string Gen.t`: *)
utop # Gen.string;;
- : ?gen:char Gen.t -> string Gen.t = <fun>

(* This does but it defeats the purpose of having an _optional_ parameter: *)
utop # Gen.string ~gen:Gen.char;;
- : string Gen.t = <fun>

[c-cube]

optional parameters not followed by a positional parameters are, in effect, not really optional. So that's a design mistake that's on my. However we could simply provide new functions (like string_readable : string t and string_of : char t -> string t) without breaking the existing API.

[jmid]

OK. Thanks for clarifying.
I've previously solved a similar issue by adding a type annotation:

# (Gen.string : string Gen.t);;
- : string Gen.t = <fun>

[yawaramin]

@jmid that's a good workaround, thanks.

[c-cube]

a PR to enrich the API would be seen favorably 😉

[jmid]

optional parameters not followed by a positional parameters are, in effect, not really optional. So that's a design mistake that's on my.

@c-cube But they are followed by a positional parameter aren't they? A generator type is, by definition, taking a state parameter:

# #show Gen.t;;
type nonrec 'a t = Random.State.t -> 'a

[c-cube]

@jmid yes, but in this case it'll always be used as a partial application, so the argument still stands.