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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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