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Derive Show instances for Sigma/SSigma when possible #634

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Merged
RyanGlScott merged 1 commit into from
May 20, 2026
Merged

Derive Show instances for Sigma/SSigma when possible #634

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14 changes: 14 additions & 0 deletions singletons/src/Data/Singletons/Sigma.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
Expand Down Expand Up @@ -196,6 +197,18 @@ uncurrySigma :: forall a (b :: a ~> Type) (c :: Sigma a b ~> Type).
uncurrySigma f (x :%&: y) = f x y

#if __GLASGOW_HASKELL__ >= 806
#if __GLASGOW_HASKELL__ >= 902
deriving instance (ShowSing s, ShowApply t) => Show (Sigma s t)
deriving instance forall s (t :: s ~> Type) (sig :: Sigma s t).
(ShowSing s, ShowSingApply t)
=> Show (SSigma sig)
#else
-- For technical reasons, the derived Show instances won't typecheck on pre-9.2
-- versions of GHC.
-- (See https://gitlab.haskell.org/ghc/ghc/-/issues/14860#note_454218.)
-- To work around this, we hand-write the code that would have been generated in
-- the derived Show instances, but with extra type annotations to guide the
-- typechecker along.
instance (ShowSing s, ShowApply t) => Show (Sigma s t) where
showsPrec p ((a :: Sing (fst :: s)) :&: b) = showParen (p >= 5) $
showsPrec 5 a . showString " :&: " . showsPrec 5 b
Expand All @@ -208,6 +221,7 @@ instance forall s (t :: s ~> Type) (sig :: Sigma s t).
showParen (p >= 5) $
showsPrec 5 sa . showString " :&: " . showsPrec 5 sb
:: ShowSingApply' t fst snd => ShowS
#endif

------------------------------------------------------------
-- Internal utilities
Expand Down
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