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1 | --- | ||
2 | title: Building an Extensible Framework for Specifying Compile-Time Configuration using Universal Quantification | ||
3 | tags: Thermoprint | ||
4 | published: 2016-01-24 | ||
5 | --- | ||
6 | |||
7 | When I write *Universal Quantification* I mean what is commonly referred to as | ||
8 | [existintial quantification](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/data-type-extensions.html#existential-quantification), | ||
9 | which I think is a misnomer. To wit: | ||
10 | |||
11 | $( \exists x \ldotp f(x) ) \to y$ is isomorphic to $\forall x \ldotp (f(x) \to y)$ (I | ||
12 | won´t try to back this claim up with actual category theory just now. You might want to | ||
13 | nag me occasionally if this bothers you -- I really should invest some more time into | ||
14 | category theory). Since haskell does not support `exists` we´re required to use the | ||
15 | `forall`-version, which really is universally quantified. | ||
16 | |||
17 | What we want is to have the user provide us with a set of specifications of how to | ||
18 | interact with one printer each. | ||
19 | Something like the following: | ||
20 | |||
21 | ~~~ {.haskell} | ||
22 | newtype PrinterMethod = PM { unPM :: Printout -> IO (Maybe PrintingError) } | ||
23 | |||
24 | data Printer = Printer | ||
25 | { print :: PrinterMethod | ||
26 | , queue :: TVar Queue | ||
27 | } | ||
28 | ~~~ | ||
29 | |||
30 | The first step in refining this is necessitated by having the user provide the | ||
31 | [monad-transformer-stack](http://book.realworldhaskell.org/read/monad-transformers.html) | ||
32 | to use at compile time. | ||
33 | Thus we introduce our first universal quantification (in conjunction with | ||
34 | [polymorphic components](https://prime.haskell.org/wiki/PolymorphicComponents)): | ||
35 | |||
36 | ~~~ {.haskell} | ||
37 | newtype PrinterMethod = PM { unPm :: forall m. MonadResource m => Printout -> m (Maybe PrintingError) } | ||
38 | ~~~ | ||
39 | |||
40 | Since we don´t want to *burden* the user with the details of setting up `TVar Queue`{.haskell} we | ||
41 | also introduce function to help with that: | ||
42 | |||
43 | ~~~ {.haskell} | ||
44 | printer :: MonadResource m => PrinterMethod -> m Printer | ||
45 | printer p = Printer p <$> liftIO (newTVarIO def) | ||
46 | ~~~ | ||
47 | |||
48 | We could at this point provide ways to set up `PrinterMethod`{.haskell}s and have the user | ||
49 | provide us with a list of them. | ||
50 | |||
51 | We, however, have numerous examples of printers which require some setup (such opening a | ||
52 | file descriptor). The idiomatic way to handle this is to decorate that setup with some | ||
53 | constraints and construct our list of printers in an | ||
54 | [`Applicative`{.haskell}](https://hackage.haskell.org/package/base/docs/Control-Applicative.html#t:Applicative) | ||
55 | fashion: | ||
56 | |||
57 | ~~~ {.haskell} | ||
58 | printer :: MonadResource m => m PrinterMethod -> m Printer | ||
59 | printer p = Printer <$> p <*> liftIO (newTVarIO def) | ||
60 | ~~~ | ||
61 | |||
62 | At this point a toy implementation of a printer we might provide looks like this: | ||
63 | |||
64 | ~~~ {.haskell} | ||
65 | debugPrint :: Applicative m => m PrinterMethod | ||
66 | debugPrint = pure . PM $ const return Nothing <=< liftIO . putStrLn . toString | ||
67 | |||
68 | toString :: Printout -> String | ||
69 | toString = undefined | ||
70 | ~~~ | ||