Test Trac #5315

testsuite/tests/ghc-regress/simplCore/should_run/T5315.hs

+{-# LANGUAGE UnicodeSyntax #-}
+{-# LANGUAGE EmptyDataDecls #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+infixr 7 :*, .*
+infix  8 :*:, .*.
+
+data HNil
+data α :* β
+type HSingle α = α :* HNil
+type α :*: β = α :* β :* HNil
+
+data HList l where
+  HNil  ∷ HList HNil
+  (:*) ∷ α → HList t → HList (α :* t)
+
+(.*) ∷ α → HList t → HList (α :* t)
+(.*) = (:*)
+
+(.*.) ∷ α → β → HList (α :*: β)
+a .*. b = a .* b .* HNil
+
+data First
+data Next p
+
+data HIndex i where
+  First ∷ HIndex First
+  Next  ∷ HIndex p → HIndex (Next p)
+
+class (l ~ (HHead l :* HTail l)) ⇒ HNonEmpty l where
+  type HHead l
+  type HTail l
+
+instance HNonEmpty (h :* t) where
+  type HHead (h :* t) = h
+  type HTail (h :* t) = t
+
+hHead ∷ HNonEmpty l ⇒ HList l → HHead l
+hHead (h :* _) = h
+hHead _        = undefined
+
+hTail ∷ HNonEmpty l ⇒ HList l → HList (HTail l)
+hTail (_ :* t) = t
+hTail _        = undefined
+
+data HFromWitness n l where
+  HFromFirst ∷ HFromWitness First l
+  HFromNext  ∷ (HNonEmpty l, HFromClass p (HTail l),
+                HTail (HFrom (Next p) l) ~ HFrom (Next p) (HTail l))
+             ⇒ HFromWitness (Next p) l
+
+class HFromClass n l where
+  type HFrom n l
+  hFromWitness ∷ HFromWitness n l
+
+instance HFromClass First l where
+  type HFrom First l = l
+  hFromWitness = HFromFirst
+
+instance (HNonEmpty l, HFromClass p (HTail l)) ⇒ HFromClass (Next p) l where
+  type HFrom (Next p) l = HFrom p (HTail l)
+  hFromWitness = case hFromWitness ∷ HFromWitness p (HTail l) of
+    HFromFirst → HFromNext
+    HFromNext  → HFromNext
+
+hFrom ∷ ∀ n l . HFromClass n l ⇒ HIndex n → HList l → HList (HFrom n l)
+hFrom First    l = l
+hFrom (Next p) l = case hFromWitness ∷ HFromWitness n l of
+  HFromNext  → hFrom p (hTail l)
+  _          → undefined
+
+type HNth n l = HHead (HFrom n l)
+
+hNth ∷ ∀ n l . (HFromClass n l, HNonEmpty (HFrom n l))
+     ⇒ HIndex n → HList l → HNth n l
+hNth First l    = hHead l
+hNth (Next p) l = case hFromWitness ∷ HFromWitness n l of
+  HFromNext → hNth p (hTail l)
+  _         → undefined
+
+main = putStrLn $ hNth (Next First) (0 .*. "Test")
+

testsuite/tests/ghc-regress/simplCore/should_run/T5315.stdout

+Test

testsuite/tests/ghc-regress/simplCore/should_run/all.T

@@ -44,3 +44,4 @@ test('T3983', [only_ways(['normal','optasm']),
 test('T3972', extra_clean(['T3972A.hi', 'T3972A.o']),
               compile_and_run,
               [''])
+test('T5315', normal, compile_and_run, [''])