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Add maybeValue combinator. #492

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Add maybeValue combinator. #492

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12 changes: 12 additions & 0 deletions src/Options/Applicative/Builder.hs
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Expand Up @@ -177,6 +177,18 @@ long = fieldMod . name . OptLong
value :: HasValue f => a -> Mod f a
value x = Mod id (DefaultProp (Just x) Nothing) id

-- | Specify an optional default value for an option.
--
-- /Note/: Because this modifier means the parser will never fail when the
-- passed value is not Nothing, do not use it with combinators such as 'some' or
-- 'many', as these combinators continue until a failure ocurs. Careless use
-- will thus result in a hang.
--
-- This also sets showDefault to allow the help output to differentiate when the
-- default value was used or not.
maybeValue :: (HasValue f, Show a) => Maybe a -> Mod f a
maybeValue = maybe mempty (\x -> value x <> showDefault)

-- | Specify a function to show the default value for an option.
showDefaultWith :: (a -> String) -> Mod f a
showDefaultWith s = Mod id (DefaultProp Nothing (Just s)) id
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