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Add Elem type family and ElemSym data type #4

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Add Elem type family and ElemSym data type #4

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8 changes: 8 additions & 0 deletions src/Symparsec/Parser/While/Predicates.hs
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Expand Up @@ -116,3 +116,11 @@ type family IsDecDigit ch where
type IsDecDigitSym :: Char ~> Bool
data IsDecDigitSym ch
type instance App IsDecDigitSym ch = IsDecDigit ch

type Elem :: [a] -> a -> Bool
type family Elem where
Elem (_:xs) x = Elem xs x
Elem '[] _ = 'False
Elem (x':_) x = 'True
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@raehik raehik Oct 28, 2025

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The above two equations match both x:xs and [] cases, so this equation will never be reached. Also, did you intend to assert that x' and x are equivalent? In which case, it should be Elem (x:_) x = True.

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@ashokkimmel ashokkimmel Oct 28, 2025

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I meant to do `Elem (x ': xs) x = 'True

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@raehik raehik Oct 29, 2025

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Ahhh, of course. I tend to avoid ticked promoted constructors except where required to disambiguate. Further reading: https://gitlab.haskell.org/ghc/ghc/-/issues/20531

data ElemSym lst ch
type instance (App (ElemSym lst)) a = Elem lst a