71 lines
1.6 KiB
Haskell
71 lines
1.6 KiB
Haskell
{- Some stuff from Prelude should not be used, as it tends to be a source
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- of 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.BadPrelude where
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{- read should be avoided, as it throws an error
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- Instead, use: readMaybe -}
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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 :: (Read a) => String -> Maybe a
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readMaybe 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 [] = Nothing
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headMaybe v = Just $ Prelude.head v
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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 (x:xs) = beginning' x xs
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where
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beginning' _ [] = []
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beginning' y (z:zs) = y : beginning' z zs
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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 (x:xs) = end' x xs
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where
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end' y [] = [y]
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end' _ (y:ys) = end' y ys
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