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sirlensalot/recover-cons.hs

Created April 14, 2016 00:30
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Recover Cons in a newtype
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recover-cons.hs
newtype Ledger a = Ledger { _lEntries :: Seq a }
deriving (Eq,Show,Generic,Monoid)
makeLenses ''Ledger
instance Cons (Ledger a) (Ledger a) a a where
_Cons = prism (\(a,s) -> over lEntries (review _Cons . (a,)) s) $
\s -> case firstOf _Cons (view lEntries s) of
Nothing -> Left mempty
(Just (a,as)) -> Right (a,set lEntries as s)
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