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1---
2title: "Type level" utilities for an overly complicated feedreader
3published: 2015-08-05
4tags: Beuteltier
5---
6
7By popular (n=1) demand we will, in this post, be taking a look at
8`beuteltier/Beuteltier/Types/Util.hs` the, creatively named, module providing some "type
9level" utilities.
10
11What I mean when I say "type level" is: additional instances (placed here when they
12contain major design decisions and are not "Ord" or "Eq"), utilities not connected to
13beuteltier itself (like the different flavours of `alter` below)
14
15In contrast to the first, this post is straightforward enough to be read linearly.
16
17> {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
18>
19> module Beuteltier.Types.Util
20> ( -- * Constructing structures
21> construct
22> , construct'
23> , alter
24> , alter'
25> -- * Dealing with 'ObjectGen's (here be dragons)
26> , generateObject
27> , liftGen
28> -- * Equivalence on 'Object's (for nubbing)
29> , Equivalent(..)
30> -- * Operations on 'SearchQuery's
31> , runQuery
32> -- , runExpr
33> ) where
34>
35> import Beuteltier.Types
36> import Beuteltier.Types.Lenses
37
38We make use of lenses (as provided by [lens](http://hackage.haskell.org/package/lens))
39extensively.
40We won´t dedicate a post to `beuteltier/Beuteltier/Types/Lenses.hs` because it consists
41mostly of the canonical invocations of
42[makeLenses](http://hackage.haskell.org/package/lens-4.12.3/docs/Control-Lens-TH.html#v:makeLenses).
43
44> import Data.Default
45>
46> import Prelude hiding (sequence)
47> import Data.Traversable (sequence)
48>
49> import Control.Lens
50>
51> import Control.Monad.State.Lazy hiding (sequence) -- Why is this exported?
52>
53> import Data.Map (Map)
54> import qualified Data.Map as Map
55>
56> import Data.Hashable (Hashable(..), hashUsing)
57>
58> import Data.Monoid ((<>))
59>
60> import Data.Function (on)
61> import Data.Maybe (mapMaybe)
62>
63> import Data.BoolExpr
64
65Quite often we find ourselves in the position that we want to alter some small parts of a
66complicated structure. We would therefore like to write the following:
67
68~~~ {.haskell .numberLines}
69updateFoo :: Monad m => Foo -> m Foo
70updateFoo = alter $ do
71 bar <~ constructNewBarInM
72 buz .= makeConstantBuz
73~~~
74
75The definitions below allow us not only to do so, but also provide some convenience
76functions for constructing entirely new values and performing both operations in a pure
77context.
78
79> alter :: Monad m => s -> StateT s m a -> m s
80> -- ^ Alter a complex structure monodically
81> alter = flip execStateT
82>
83> alter' :: s -> State s a -> s
84> -- ^ Specialization of 'alter' to 'Identity'
85> alter' s = runIdentity . alter s
86>
87> construct :: (Monad m, Default s) => StateT s m a -> m s
88> -- ^ Compute a complex structure monadically
89> construct = alter def
90>
91> construct' :: Default s => State s a -> s
92> -- ^ Specialization of 'construct' to 'Identity'
93> construct' = runIdentity . construct
94
95Sometimes we just really want to translate an `ObjectGen` to an `Object`.
96
97> generateObject :: Monad f => ObjectGen f -> f Object
98> -- ^ Run an object generator.
99> -- Use iff /all/ components of an object are needed /in RAM now/.
100> generateObject gen = construct $ do
101> content <- lift $ gen ^. oContent >>= sequence
102> thunks <- lift $ gen ^. oThunks >>= sequence
103> meta <- lift $ gen ^. oMeta
104> oContent .= return (fmap return content)
105> oThunks .= return (fmap return thunks)
106> oMeta .= return meta
107>
108> liftGen :: Monad f => Object -> ObjectGen f
109> -- ^ Lift an 'Object' to be an 'ObjectGen' in any 'Monad' by the power of 'return'
110> liftGen obj = construct' $ do
111> oContent .= return (Map.map return $ obj ^. oContent')
112> oThunks .= return (map return $ obj ^. oThunks')
113> oMeta .= return (obj ^. oMeta')
114
115We expect implementations of `insert` to perform what we call nubbing. That is removal of
116'Object's that are, in some sense, `Equivalent` to the new one we´re currently
117inserting. Thus we provide a definition of what we mean, when we say `Equivalent`.
118
119> class Equivalent a where
120> (~~) :: a -> a -> Bool
121>
122> -- | Two 'Object's are equivalent iff their content is identical as follows:
123> -- the set of 'SubObjectName's both promised and actually occurring is identical
124> -- and all 'SubObject's that actually occurr and share a 'SubObjectName' are
125> -- identical (as per '(==)')
126> --
127> -- Additionally we expect their 'Metadata' to be identical (as per '(==)')
128> instance Equivalent Object where
129> a ~~ b = (contentCompare `on` content) a b && ((==) `on` (^. oMeta')) a b
130> where
131> contentCompare :: (Ord k, Eq v) => Map k (Maybe v) -> Map k (Maybe v) -> Bool
132> contentCompare a b = Map.foldl (&&) True $ Map.mergeWithKey combine setFalse setFalse a b
133> combine _ a b = Just $ cmpMaybes a b
134> setFalse = Map.map $ const False
135>
136> cmpMaybes Nothing _ = True
137> cmpMaybes _ Nothing = True
138> cmpMaybes (Just a) (Just b) = a == b
139
140To speed up nubbing we also provide a quick way to "cache results". To make caching
141meaningful we of course expect the following to hold:
142
143~~~
144a ~~ b ⇒ (hash a) == (hash b)
145~~~
146
147Note that we do not expect the converse to hold. We will thus require a second pass over
148all objects sharing a hash to determine true equivalency.
149
150> -- | Two 'Object's´ hashes are a first indication of whether they are 'Equivalent'
151> instance Hashable Object where
152> hashWithSalt = hashUsing $ \a -> (a ^. oMeta', Map.keys $ content a)
153>
154> content :: Object -> Map SubObjectName (Maybe SubObject)
155> content obj = promised obj <> actual obj
156> actual :: Object -> Map SubObjectName (Maybe SubObject)
157> actual = fmap Just . (^. oContent')
158> promised :: Object -> Map SubObjectName (Maybe SubObject)
159> promised = Map.fromList . map (\n -> (n, Nothing)) . concat . promises
160> promises :: Object -> [[SubObjectName]]
161> promises = mapMaybe (^. tPromises) . (^. oThunks')
162
163Evaluating a `SearchQuery` against an `ObjectGen` is, due to the structure of elementary
164`SearchQuery`s quite straightforward.
165
166> runQuery :: Monad f => SearchQuery f -> ObjectGen f -> f Bool
167> -- ^ Run a 'SearchQuery' against an 'ObjectGen'
168> runQuery query obj = liftM reduceBoolExpr $ sequence $ fmap ($ obj) query
169>
170> -- runExpr :: Monad f => ObjectGen f -> Predicate f -> f Bool
171> -- -- ^ Run a 'Predicate' (»an atomic 'SearchQuery'«) against an 'ObjectGen'
172> -- runExpr obj (Prim f) = f obj
173> -- runExpr obj (Meta f) = liftM f (obj ^. oMeta)
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