Created
March 25, 2019 11:07
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Comonadic builders
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#!/usr/bin/env cabal | |
{- cabal: | |
build-depends: | |
, base ^>= 4.12.0.0 | |
, comonad ^>= 5.0 | |
, pretty-simple ^>= 2.2 | |
, text | |
-} | |
{-# LANGUAGE OverloadedStrings #-} | |
{-# LANGUAGE RecordWildCards #-} | |
import Control.Comonad (Comonad (..), (=>>)) | |
import Data.Semigroup (Any (..)) | |
import Data.Text (Text) | |
import Text.Pretty.Simple (pPrint) | |
data Settings = Settings | |
{ settingsHasLibrary :: Any | |
, settingsGitHub :: Any | |
, settingsTravis :: Any | |
} deriving (Show) | |
instance Semigroup Settings where | |
Settings a1 b1 c1 <> Settings a2 b2 c2 = | |
Settings (a1 <> a2) (b1 <> b2) (c1 <> c2) | |
instance Monoid Settings where | |
mempty = Settings mempty mempty mempty | |
data Project = Project | |
{ projectName :: !Text | |
, projectHasLibrary :: !Bool | |
, projectGitHub :: !Bool | |
, projectTravis :: !Bool | |
} deriving (Show) | |
type ProjectBuilder = Settings -> Project | |
buildProject :: Text -> ProjectBuilder | |
buildProject projectName Settings{..} = Project | |
{ projectHasLibrary = getAny settingsHasLibrary | |
, projectGitHub = getAny settingsGitHub | |
, projectTravis = getAny settingsTravis | |
, .. | |
} | |
{- | |
builder =>> f | |
:: Settings -> Project | |
= flip extend builder f -- (=>>) definition | |
= extend f builder -- flip definition | |
= fmap f (duplicate builder) -- default implementation of extend | |
= fmap f (\settings1 -> builder . mappend settings1) -- duplicate for arrow | |
= f . (\settings1 -> builder . mappend settings1) -- Functor instance for arrow | |
= f . (\settings1 settings2 -> builder $ settings1 <> settings2) -- eta-expanding internal lambda | |
= \settings -> f $ (\settings1 settings2 -> builder $ settings1 <> settings2) settings -- eta-expanding outer lambda | |
= \settings -> f $ \settings2 -> builder $ settings <> settings2 -- partially applying lambda | |
-} | |
append :: ProjectBuilder -> (ProjectBuilder -> Project) -> ProjectBuilder | |
append = (=>>) | |
{- | |
buildProject "foo" =>> hasLibraryB | |
:: Settings -> Project | |
= \settings -> hasLibraryB $ \settings2 -> buildProject "foo" $ settings <> settings2 | |
= \settings -> (\settings2 -> buildProject "foo" $ settings <> settings2) (mempty { settingsHasLibrary = Any True }) | |
= \settings -> buildProject "foo" $ settings <> mempty { settingsHasLibrary = Any True } | |
-} | |
hasLibraryB :: ProjectBuilder -> Project | |
hasLibraryB builder = builder $ mempty { settingsHasLibrary = Any True } | |
gitHubB :: ProjectBuilder -> Project | |
gitHubB builder = builder $ mempty { settingsGitHub = Any True } | |
alwaysTravisB :: ProjectBuilder -> Project | |
alwaysTravisB builder = builder $ mempty { settingsTravis = Any True } | |
travisB :: ProjectBuilder -> Project | |
travisB builder = | |
let project = extract builder | |
in project { projectTravis = projectGitHub project } | |
main :: IO () | |
main = do | |
-- plain | |
pPrint $ extract $ buildProject "minimal-project" | |
pPrint $ extract $ buildProject "only-library" =>> hasLibraryB | |
pPrint $ extract $ buildProject "library-github" =>> hasLibraryB =>> gitHubB | |
-- dependent: 1 level | |
pPrint $ extract $ buildProject "travis" =>> travisB | |
pPrint $ extract $ buildProject "always-travis" =>> alwaysTravisB | |
pPrint $ extract $ buildProject "github-travis" =>> gitHubB =>> travisB | |
pPrint $ extract $ buildProject "travis-github" =>> travisB =>> gitHubB |
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