The Great Renaming, Chapter 9: Monadic and applicative operators

[jmid]

Having slayed the int dragon, we are left with a only a few less interesting challengesm including the monadic and applicative operators. The current status here is as follows:

Combinator name	QCheck.Gen	QCheck.arbitrary	QCheck2.Gen
always	-	?print:'a Print.t -> 'a -> 'a arbitrary	-
return	'a -> 'a t	-	'a -> 'a t
pure	'a -> 'a t	-	'a -> 'a t
bind	-	-	'a t -> ('a -> 'b t) -> 'b t
(>>=)	'a t -> ('a -> 'b t) -> 'b t	-	'a t -> ('a -> 'b t) -> 'b t
ap	-	-	('a -> 'b) t -> 'a t -> 'b t
(<*>)	('a -> 'b) t -> 'a t -> 'b t	-	('a -> 'b) t -> 'a t -> 'b t
(>	=)	'a t -> ('a -> 'b) -> 'b t	-
(<$>)	('a -> 'b) -> 'a t -> 'b t	-	('a -> 'b) -> 'a t -> 'b t
join	'a t t -> 'a t	-	'a t t -> 'a t

Observations:

• QCheck.Gen and QCheck2.Gen offer a monadic and applicative interface with similar operators whereas QCheck.arbitrary doesn't
• bind is missing from QCheck.Gen
• ap missing from QCheck.Gen
• QCheck.always sticks out - it is an equivalent to return/pure. A quick search shows that it is used in Tezos and Goblint.

i initially went for adding always as a third alias to both QCheck.Gen and QCheck2.Gen for consistency, but then regretted and rolled back. My thinking is:

• since QCheck.arbitrary doesn't and cannot offer a monadic or applicative interface it doesn't make sense to provide return or pure for it, as it might confuse more than it helps.
• addng a third name always to both QCheck.Gen and QCheck2.Gen for existing functionality doesn't bring any direct value, more likely it will just make it harder to read 3 names for the same thing.

Alas, I've gone only with adding the missing bind and ap names to QCheck.Gen resulting in the following:

Combinator name	QCheck.Gen	QCheck.arbitrary	QCheck2.Gen
always	-	?print:'a Print.t -> 'a -> 'a arbitrary	-
return	'a -> 'a t	-	'a -> 'a t
pure	'a -> 'a t	-	'a -> 'a t
bind	'a t -> ('a -> 'b t) -> 'b t	-	'a t -> ('a -> 'b t) -> 'b t
(>>=)	'a t -> ('a -> 'b t) -> 'b t	-	'a t -> ('a -> 'b t) -> 'b t
ap	('a -> 'b) t -> 'a t -> 'b t	-	('a -> 'b) t -> 'a t -> 'b t
(<*>)	('a -> 'b) t -> 'a t -> 'b t	-	('a -> 'b) t -> 'a t -> 'b t
(>	=)	'a t -> ('a -> 'b) -> 'b t	-
(<$>)	('a -> 'b) -> 'a t -> 'b t	-	('a -> 'b) -> 'a t -> 'b t
join	'a t t -> 'a t	-	'a t t -> 'a t

This result is consistent across QCheck.Gen and QCheck2.Gen 🤷