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docs/src/haskell/developers-roadmap/TypeFamilies/TaggedClasses.md
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```haskell | ||
{-# LANGUAGE AllowAmbiguousTypes #-} | ||
{-# LANGUAGE ConstraintKinds #-} | ||
{-# LANGUAGE DataKinds #-} | ||
{-# LANGUAGE FlexibleContexts #-} | ||
{-# LANGUAGE FlexibleInstances #-} | ||
{-# LANGUAGE MultiParamTypeClasses #-} | ||
{-# LANGUAGE StandaloneKindSignatures #-} | ||
{-# LANGUAGE TypeFamilies #-} | ||
{-# LANGUAGE UndecidableInstances #-} | ||
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module Try.TypeFamilies.TaggedClasses where | ||
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-- Rewriting instance Chains using Tagged Classes | ||
-- https://github.com/sigma-andex/purescript-barlow-lens/blob/295c4b32fbeca052ebfd3665a9071012e654b9c0/src/Data/Lens/Barlow/Parser.purs#L11 | ||
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import Data.Kind (Constraint) | ||
import GHC.TypeLits (AppendSymbol, Symbol) | ||
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data Tag | ||
= WhenDot | ||
| WhenSpace | ||
| WhenSame | ||
| Else | ||
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type SelectTag :: Symbol -> Symbol -> Symbol -> Symbol -> Tag | ||
type family SelectTag head tail out rest where | ||
SelectTag "." t "." t = WhenDot | ||
SelectTag " " t "" t = WhenSpace | ||
SelectTag h "" h "" = WhenSame | ||
SelectTag _ _ _ _ = Else | ||
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type ParsePercentageSymbol' :: Tag -> Symbol -> Symbol -> Symbol -> Symbol -> Constraint | ||
class ParsePercentageSymbol' tag head tail out rest | ||
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instance ParsePercentageSymbol' WhenDot "." t "" t | ||
instance ParsePercentageSymbol' WhenSpace " " t "" t | ||
instance ParsePercentageSymbol' WhenSame h "" h "" | ||
instance (ParsePercentageSymbol th tt tout trest, t ~ AppendSymbol th tt, out ~ AppendSymbol h tout) => ParsePercentageSymbol' Else h t out trest | ||
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type ParsePercentageSymbol :: Symbol -> Symbol -> Symbol -> Symbol -> Constraint | ||
type ParsePercentageSymbol h t o r = ParsePercentageSymbol' (SelectTag h t o r) h t o r | ||
``` |
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