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Add combinator for optional properties #7

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Add combinator for optional properties #7

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24 changes: 24 additions & 0 deletions src/Language/JsonGrammar/Grammar.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -186,6 +186,30 @@ liftAeson = Pure f g
prop :: Json a => Text -> Grammar Obj t (a :- t)
prop n = Property n grammar

-- | Like 'prop', but for optional properties
optProp :: Json a => Text -> Grammar Obj t (Maybe a :- t)
optProp propName = just `mappend` nothing
where
just :: Json a => Grammar Obj t (Maybe a :- t)
just = pure fr to . prop propName
where
fr :: (a :- t) -> Parser (Maybe a :- t)
fr (a :- t) = return (Just a :- t)

to :: (Maybe a :- t) -> Maybe (a :- t)
to (Just a :- t) = Just (a :- t)
to (Nothing :- _) = Nothing

nothing :: Grammar Obj t (Maybe a :- t)
nothing = pure fr to
where
fr :: t -> Parser (Maybe a :- t)
fr t = return (Nothing :- t)

to :: (Maybe a :- t) -> Maybe t
to (Nothing :- t) = Just t
to (Just _ :- _) = Nothing

-- | Expect or produce an array 'element' whose value grammar is specified by 'grammar'.
el :: Json a => Grammar Arr t (a :- t)
el = Element grammar
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