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