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2020-09-21 02:32:10 × Guest42888 quits (~lagothrix@unaffiliated/lagothrix) (Killed (rothfuss.freenode.net (Nickname regained by services)))
2020-09-21 02:32:16 → lagothrix joins (~lagothrix@unaffiliated/lagothrix)
2020-09-21 02:32:18 <ski> (if there's several `forall's in a type, then we say the overall rank is the maximum of the individual ranks of the `forall's in it. ordinary polymorphic operations are rank-one. monomorphic operations could be said to be rank-zero, i suppose ..)
2020-09-21 02:32:26 <ski> inkbottle : is this making sense ?
2020-09-21 02:34:05 <inkbottle> I'm not done reading, I've been interrupted
2020-09-21 02:34:30 × thir quits (~thir@p200300f27f0fc600ed2222922a5678d5.dip0.t-ipconnect.de) (Ping timeout: 244 seconds)
2020-09-21 02:35:00 <ski> take your time. if anything is unclear, if you want further elaboration, or you have questions or comments, please do tell
2020-09-21 02:36:02 <ski> here's one other example of a rank-two (and in this case also polymorphic) operation :
2020-09-21 02:36:13 <ski> runST :: forall a. (forall s. ST s a) -> a
2020-09-21 02:38:23 <inkbottle> The explanation in terms of "who pick 'a" is very interesting
2020-09-21 02:38:37 <ski> for "running" (executing/performing) an `ST s'-action (`ST s' expressing a kind of state effects, where one can dynamically allocate mutable reference cells (and arrays), and read from and write to them. implemented in terms of actual update-in-place, unlike say `State s' (which has a single global state `s', no dynamic allocation))
2020-09-21 02:39:26 <ski> yea, that's the main thing that happens, as we pass to higher ranks, we alternate between caller and callee picking
2020-09-21 02:40:04 <ski> the usual reason for wanting to define a rank-two operation is that you actually want to use a callback with multiple choices for your type variable(s)
2020-09-21 02:40:20 <ski> (as in my `blah' example a bit up)
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2020-09-21 02:41:19 <ski> another reason is to want to hide some implementation detail of the function, from its caller. that is, hide which (possiby single) type that the implementation uses
2020-09-21 02:42:17 <ski> here's some kind of example of that. consider
2020-09-21 02:42:28 hackage monad-memo 0.5.2 - Memoization monad transformer https://hackage.haskell.org/package/monad-memo-0.5.2 (EduardSergeev)
2020-09-21 02:42:45 <ski> newtype StateT s m a = MkStateT (s -> m (a,s))
2020-09-21 02:43:08 → asan joins (~yan4138@114.84.148.87)
2020-09-21 02:43:14 <ski> there is a function
2020-09-21 02:43:16 <ski> @type mapStateT
2020-09-21 02:43:18 <lambdabot> (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
2020-09-21 02:43:46 → howdoi joins (uid224@gateway/web/irccloud.com/x-mtwkarqdregvoupa)
2020-09-21 02:44:00 <ski> that can be used to change the `m' in `StateT s m a' to another monad, `n' (also here swapping `a' out for `b')
2020-09-21 02:44:47 <ski> however, in the callback, we can see that we're handed an action of type `m (a,s)', that will produce a pair. and we're to give back a new action of type `n (b,s)', that will also produce a pair
2020-09-21 02:45:01 <ski> one might consider this as "implementation detail leaking"
2020-09-21 02:45:08 <ski> so, one could make a variant
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2020-09-21 02:45:32 × mariatsji quits (~mariatsji@2a01:79d:53aa:c66c:342b:5324:f59b:5b0e) (Ping timeout: 260 seconds)
2020-09-21 02:45:48 <ski> mapStateT' :: (forall h. m h -> n h) -> StateT s m a -> StateT s n a
2020-09-21 02:46:46 <ski> where `h' here hides the pair type from the caller of the function. in some cases, this can be useful to prevent the callback that the caller provides, from messing around with the internal implementation (the values of the "hidden" type `h')
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2020-09-21 02:48:07 <ski> inkbottle : is that reasonably clear ?
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2020-09-21 02:49:59 <ski> (there are probably more suggestive examples of this "information hiding" capability of rank-two .. but i can't think offhand of one that would be quick to showcase)
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2020-09-21 03:12:56 <ski> inkbottle : anyway .. if you want to, perhaps we could continue this conversation later (getting to existentials, hopefully), as it's quite late here
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2020-09-21 03:45:28 hackage stm-actor 0.1.0.0 - See the README https://hackage.haskell.org/package/stm-actor-0.1.0.0 (sgschlesinger)
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