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author | Gregor Kleen <gkleen@yggdrasil.li> | 2016-05-28 16:05:39 +0200 |
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committer | Gregor Kleen <gkleen@yggdrasil.li> | 2016-05-28 16:05:39 +0200 |
commit | d65d4d606a9f4b2035ca74a3323da61804049bc6 (patch) | |
tree | 3d41474bb647e5ac2cc282c5ac71573dfa5ee37e /provider | |
parent | c297ee8043bc38b58c466974f700d9cdb8e1f1b9 (diff) | |
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events-01
Reviewed-by: Gregor Kleen <gkleen@yggdrasil.li>
Diffstat (limited to 'provider')
-rw-r--r-- | provider/posts/events/01.lhs | 142 |
1 files changed, 142 insertions, 0 deletions
diff --git a/provider/posts/events/01.lhs b/provider/posts/events/01.lhs new file mode 100644 index 0000000..fd1bd03 --- /dev/null +++ b/provider/posts/events/01.lhs | |||
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1 | --- | ||
2 | title: "On the Design of a Turing-Complete Appointment Book" | ||
3 | published: 2016-05-27 | ||
4 | tags: events | ||
5 | repo: https://git.yggdrasil.li/gkleen/pub/events | ||
6 | base: https://git.yggdrasil.li/gkleen/pub/events/tree/events/src/Events/Types.hs?id=e04e707b1fb63b7857878e2d77c560abe3efd51b | ||
7 | --- | ||
8 | |||
9 | I have a long history of using digital appointment books (calendars) | ||
10 | and not being satisfied with them (I´m currently being frustrated by | ||
11 | [Google Calendar][] after a long history of using [Remind][]). | ||
12 | Thus, of course, I had to implement my own, because, as always, all | ||
13 | existing software does not fullfill my exceedingly unrealistic | ||
14 | expectations with respect to customizability and extendability. | ||
15 | |||
16 | For now all I want from my appointment book is the ability to, given | ||
17 | an interval of time, print a list of events in some machine-parsable | ||
18 | format (probably [JSON][]). | ||
19 | |||
20 | > {-# LANGUAGE TemplateHaskell #-} | ||
21 | > {-# LANGUAGE MultiParamTypeClasses #-} | ||
22 | > {-# LANGUAGE FlexibleInstances #-} | ||
23 | > {-# LANGUAGE UndecidableInstances #-} | ||
24 | > | ||
25 | > module Events.Types | ||
26 | > ( TimeRange(..), rangeStart, rangeDuration | ||
27 | > , Event(..), payload, occursWithin | ||
28 | > , EvalCtx(..), ctxVars, ctxEvents | ||
29 | > , ObjCtx(..), objOccurs, objPayload | ||
30 | > , Eval, evaluate | ||
31 | > , module Data.Aeson | ||
32 | > , module Data.Time.Clock | ||
33 | > , module Data.Default.Class | ||
34 | > ) where | ||
35 | > | ||
36 | > import Control.Lens.TH | ||
37 | > | ||
38 | > import Data.Aeson (Object) | ||
39 | > | ||
40 | > import Data.Time.Clock | ||
41 | > | ||
42 | > import Control.Monad.State.Lazy | ||
43 | > import ListT (ListT) | ||
44 | > import qualified ListT | ||
45 | > | ||
46 | > import Data.Default.Class | ||
47 | > | ||
48 | > import Data.Monoid | ||
49 | > import Control.Monad.Fix | ||
50 | > import Control.Lens | ||
51 | > import Data.Maybe | ||
52 | |||
53 | We can quite easily encode an interval of time as a lower bound and a | ||
54 | duration: | ||
55 | |||
56 | > data TimeRange = TimeRange | ||
57 | > { _rangeStart :: UTCTime | ||
58 | > , _rangeDuration :: NominalDiffTime | ||
59 | > } | ||
60 | > makeLenses ''TimeRange | ||
61 | |||
62 | For our purposes it´s sufficient to consider an event to be some data | ||
63 | we´ll display when needed and some way to determine whether the given | ||
64 | `TimeRange`{.haskell} intersects it: | ||
65 | |||
66 | > data Event = Event | ||
67 | > { _payload :: Object | ||
68 | > , _occursWithin :: TimeRange -> Bool | ||
69 | > } | ||
70 | > makeLenses ''Event | ||
71 | |||
72 | We are going to want to parse a specification of some kind into a form we can run. | ||
73 | Below we see one such form. | ||
74 | |||
75 | `ListT`{.haskell} allows for nondeterministic computation – it allows us to split | ||
76 | our wordline and continue depth-first much like `[]`{.haskell}. | ||
77 | |||
78 | Within every wordline we modify a distinct snapshot of `ObjCtx`{.haskell} we took | ||
79 | while branching. | ||
80 | |||
81 | We also share one `EvalCtx`{.haskell} across all worldlines. | ||
82 | |||
83 | > type Eval m a = StateT ObjCtx (ListT (StateT EvalCtx m)) a | ||
84 | |||
85 | `ListT`{.haskell} does not ship with extensive support for the | ||
86 | [transformers package][transformers]: | ||
87 | |||
88 | > instance MonadState s m => MonadState s (ListT m) where | ||
89 | > get = lift get | ||
90 | > put = lift . put | ||
91 | |||
92 | The context shared among all worldlines mainly contains all objects that | ||
93 | eventually get computed – haskells lazyness ensures that we terminate as | ||
94 | long as we don´t have objects depend on themselves in a sufficiently | ||
95 | degenerate way. | ||
96 | |||
97 | > data EvalCtx = EvalCtx | ||
98 | > { _ctxEvents :: [Object] | ||
99 | > } deriving (Show) | ||
100 | > makeLenses ''EvalCtx | ||
101 | > | ||
102 | > instance Default EvalCtx where | ||
103 | > def = EvalCtx | ||
104 | > { _ctxEvents = mempty | ||
105 | > } | ||
106 | |||
107 | Every wordline constructs exactly one object while having access to a set | ||
108 | of declarations that can occur anywhere on the wordline. | ||
109 | |||
110 | > data ObjCtx = ObjCtx | ||
111 | > { _objOccurs :: Bool | ||
112 | > , _objPayload :: Maybe Object | ||
113 | > , _objVars :: Object | ||
114 | > } | ||
115 | > makeLenses ''ObjCtx | ||
116 | > | ||
117 | > instance Default ObjCtx where | ||
118 | > def = ObjCtx | ||
119 | > { _objOccurs = False | ||
120 | > , _objPayload = Nothing | ||
121 | > , _objVars = mempty | ||
122 | > } | ||
123 | |||
124 | Constructing an `Object`{.haskell} from an `ObjCtx`{.haskell} is straightforward: | ||
125 | |||
126 | > objCtx :: ObjCtx -> Maybe Object | ||
127 | > objCtx (ObjCtx False _) = Nothing | ||
128 | > objCtx (ObjCtx True o) = o | ||
129 | |||
130 | `mfix`{.haskell} allows all values contained within our various `StateT`{.haskell} | ||
131 | layers to depend on one another: | ||
132 | |||
133 | > evaluate :: MonadFix m => Eval m () -> m [Object] | ||
134 | > evaluate x = catMaybes <$> mfix x' | ||
135 | > where | ||
136 | > x' = evalStateT (ListT.toList (objCtx <$> execStateT x def)) . flip (set ctxEvents) def . catMaybes | ||
137 | |||
138 | [Google Calendar]: <https://calendar.google.com> | ||
139 | [Remind]: <https://www.roaringpenguin.com/products/remind> | ||
140 | [JSON]: <https://en.wikipedia.org/wiki/JSON> | ||
141 | [monad transformers]: <https://en.wikibooks.org/wiki/Haskell/Monad_transformers> | ||
142 | [transformers]: <https://hackage.haskell.org/package/transformers> | ||