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40 lines
1.1 KiB
Haskell
40 lines
1.1 KiB
Haskell
{- This file is part of Vervis.
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-
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- Written in 2016, 2018 by fr33domlover <fr33domlover@riseup.net>.
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-
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- ♡ Copying is an act of love. Please copy, reuse and share.
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-
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- The author(s) have dedicated all copyright and related and neighboring
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- rights to this software to the public domain worldwide. This software is
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- distributed without any warranty.
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-
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- You should have received a copy of the CC0 Public Domain Dedication along
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- with this software. If not, see
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- <http://creativecommons.org/publicdomain/zero/1.0/>.
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-}
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{-# LANGUAGE CPP #-}
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module Data.ByteString.Local
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( fromDecimal
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)
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where
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import Data.ByteString (ByteString)
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import qualified Data.ByteString as B
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-- | Given an ASCII string representing an integer in decimal, parse it and
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-- return the number. Return 'Nothing' on invalid digit chars and on an empty
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-- bytestring.
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--
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-- >>> fromDecimal "345"
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-- Just 345
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--
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-- >>> fromDecimal "a1b2c3"
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-- Nothing
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fromDecimal :: Num a => ByteString -> Maybe a
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fromDecimal s =
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if (not . B.null) s && B.all (\ b -> 48 <= b && b <= 57) s
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then Just $ B.foldl' (\ n b -> 10 * n + fromIntegral b - 48) 0 s
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else Nothing
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