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feat: add optional argument builder #471

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feat: add optional argument builder #471

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5 changes: 5 additions & 0 deletions src/Options/Applicative/Builder.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@ module Options.Applicative.Builder (
subparser,
strArgument,
argument,
optionalArg,
flag,
flag',
switch,
Expand Down Expand Up @@ -297,6 +298,10 @@ argument p m = mkParser d g (ArgReader rdr)
strArgument :: IsString s => Mod ArgumentFields s -> Parser s
strArgument = argument str

-- | Builder for an optional argument.
optionalArg :: ReadM a -> Mod ArgumentFields (Maybe a) -> Parser (Maybe a)
optionalArg p m = argument (Just <$> p) (m <> value Nothing)

-- | Builder for a flag parser.
--
-- A flag that switches from a \"default value\" to an \"active value\" when
Expand Down