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Existentials #8

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Existentials #8

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af294c4
final form of meta constructors
ggreif Mar 4, 2015
f6c814c
rename Retro --> Meta
ggreif Mar 4, 2015
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5 changes: 5 additions & 0 deletions src/Data/Generic/Diff.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -216,9 +216,13 @@ class (Family f) => Type f t where
--
-- Use 'Abstr' for abstract constructors (e.g., for built-in types or types with many
-- (or infinite) constructors)
--
-- Use 'Meta' for abstract constructors whose subtrees are of existential type
-- (which means exposing 'ts' is forbidden by the type checker)
data Con :: (* -> * -> *) -> * -> * where
Concr :: (List f ts) => f t ts -> Con f t
Abstr :: (List f ts) => (t -> f t ts) -> Con f t
Meta :: (t -> Con f t) -> Con f t

class List f ts where
list :: IsList f ts
Expand Down Expand Up @@ -313,6 +317,7 @@ matchConstructor = tryEach constructors
(forall ts. f t ts -> IsList f ts -> ts -> r) -> r
tryEach (Concr c : cs) x k = matchOrRetry c cs x k
tryEach (Abstr c : cs) x k = matchOrRetry (c x) cs x k
tryEach (Meta c : cs) x k = tryEach (c x : cs) x k
tryEach [] _ _ = error "Incorrect Family or Type instance."

matchOrRetry :: (List f ts, Type f t) => f t ts -> [Con f t] -> t ->
Expand Down