70 lines
1.7 KiB
Haskell
70 lines
1.7 KiB
Haskell
{- 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.
|
|
-}
|
|
|
|
{-# OPTIONS_GHC -fno-warn-tabs #-}
|
|
|
|
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]
|