Re:Tutorial: `Forall` in Haskell – Wasp
by josephcsible on Thursday December 01, 2022 @11:39PM
(#33826855)
> While normaly forall plays a role of the "universal quantifier", it can also play a role of the "existential quantifier" (depends on the situation).
> What is interesting is that in this case, forall plays the role of an existential quantifier (therefore the name of extension, ExistentialQuantification), unlike the role of universal quantifier it normally plays.
This isn't really true. While you can indeed use "forall" to write something equivalent to an existentially quantified type, that doesn't mean it's playing that role. That'd be like saying that the English words "all" and "some" mean the same thing just because "not all X are Y" and "some X are not Y" mean the same thing.
> What is interesting is that in this case, forall plays the role of an existential quantifier (therefore the name of extension, ExistentialQuantification), unlike the role of universal quantifier it normally plays.
This isn't really true. While you can indeed use "forall" to write something equivalent to an existentially quantified type, that doesn't mean it's playing that role. That'd be like saying that the English words "all" and "some" mean the same thing just because "not all X are Y" and "some X are not Y" mean the same thing.