68 lines
1.6 KiB
Haskell
68 lines
1.6 KiB
Haskell
{- Parts of the Prelude are partial functions, which are a common source of
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- bugs.
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-
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- This exports functions that conflict with the prelude, which avoids
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- them being accidentially used.
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-}
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module Utility.PartialPrelude where
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import qualified Data.Maybe
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{- read should be avoided, as it throws an error
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- Instead, use: readish -}
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read :: Read a => String -> a
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read = Prelude.read
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{- head is a partial function; head [] is an error
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- Instead, use: take 1 or headMaybe -}
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head :: [a] -> a
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head = Prelude.head
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{- tail is also partial
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- Instead, use: drop 1 -}
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tail :: [a] -> [a]
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tail = Prelude.tail
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{- init too
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- Instead, use: beginning -}
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init :: [a] -> [a]
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init = Prelude.init
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{- last too
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- Instead, use: end or lastMaybe -}
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last :: [a] -> a
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last = Prelude.last
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{- Attempts to read a value from a String.
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-
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- Ignores leading/trailing whitespace, and throws away any trailing
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- text after the part that can be read.
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-
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- readMaybe is available in Text.Read in new versions of GHC,
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- but that one requires the entire string to be consumed.
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-}
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readish :: Read a => String -> Maybe a
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readish s = case reads s of
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((x,_):_) -> Just x
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_ -> Nothing
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{- Like head but Nothing on empty list. -}
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headMaybe :: [a] -> Maybe a
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headMaybe = Data.Maybe.listToMaybe
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{- Like last but Nothing on empty list. -}
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lastMaybe :: [a] -> Maybe a
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lastMaybe [] = Nothing
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lastMaybe v = Just $ Prelude.last v
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{- All but the last element of a list.
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- (Like init, but no error on an empty list.) -}
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beginning :: [a] -> [a]
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beginning [] = []
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beginning l = Prelude.init l
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{- Like last, but no error on an empty list. -}
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end :: [a] -> [a]
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end [] = []
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end l = [Prelude.last l]
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