What does the `forall` keyword in Haskell/GHC do?
- by JUST MY correct OPINION
I've been banging my head on this one for (quite literally) years now. I'm beginning to kinda/sorta understand how the foreach keyword is used in so-called "existential types" like this:
data ShowBox = forall s. Show s => SB s
(This despite the confusingly-worded explanations of it in the fragments found all around the web.)
This is only a subset, however, of how foreach is used and I simply cannot wrap my mind around its use in things like this:
runST :: forall a. (forall s. ST s a) -> a
Or explaining why these are different:
foo :: (forall a. a -> a) -> (Char,Bool)
bar :: forall a. ((a -> a) -> (Char, Bool))
Or the whole RankNTypes stuff that breaks my brain when "explained" in a way that makes me want to do that Samuel L. Jackson thing from Pulp Fiction. (Don't follow that link if you're easily offended by strong language.)
The problem, really, is that I'm a dullard. I can't fathom the chicken scratchings (some call them "formulae") of the elite mathematicians that created this language seeing as my university years are over two decades behind me and I never actually had to put what I learnt into use in practice. I also tend to prefer clear, jargon-free English rather than the kinds of language which are normal in academic environments. Most of the explanations I attempt to read on this (the ones I can find through search engines) have these problems:
They're incomplete. They explain one part of the use of this keyword (like "existential types") which makes me feel happy until I read code that uses it in a completely different way (like runST, foo and bar above).
They're densely packed with assumptions that I've read the latest in whatever branch of discrete math, category theory or abstract algebra is popular this week. (If I never read the words "consult the paper whatever for details of implementation" again, it will be too soon.)
They're written in ways that frequently turn even simple concepts into tortuously twisted and fractured grammar and semantics.
(I suspect that the last two items are the biggest problem. I wouldn't know, though, since I'm too much a dullard to comprehend them.)
It's been asked why Haskell never really caught on in industry. I suspect, in my own humble, unintelligent way, that my experience in figuring out one stupid little keyword -- a keyword that is increasingly ubiquitous in the libraries being written these days -- are also part of the answer to that question. It's hard for a language to catch on when even its individual keywords cause years-long quests to comprehend. Years-long quests which end in failure.
So...
On to the actual question. Can anybody completely explain the foreach keyword in clear, plain English (or, if it exists somewhere, point to such a clear explanation which I've missed) that doesn't assume I'm a mathematician steeped in the jargon?
