listsafe (empty) → 0.1.0.0
raw patch · 4 files changed
+351/−0 lines, 4 filesdep +basedep +exceptionssetup-changed
Dependencies added: base, exceptions
Files
- Data/List/Safe.hs +125/−0
- LICENSE.md +202/−0
- Setup.hs +2/−0
- listsafe.cabal +22/−0
+ Data/List/Safe.hs view
@@ -0,0 +1,125 @@+{-# LANGUAGE DeriveDataTypeable #-} + +-- |Operations on lists. This module re-exports all safe functions of +-- 'Data.List', but wraps all partial functions which may fail. As such, this +-- module can be imported instead of "Data.List". +-- +-- Partial functions are wrapped into the 'MonadThrow'-monad from +-- "Control.Monad.Catch" and as such, have appropriate failure cases for all +-- instances. E.g.: +-- +-- * 'Nothing' for 'Maybe', +-- * the empty list for '[a]', +-- * 'IOException' for 'IO', +-- * lifted exceptions for monad transformers. +module Data.List.Safe ( + module LSafe, + -- *Safe versions of standard functions. + head, + last, + tail, + init, + foldl1, + foldl1', + foldr1, + maximum, + minimum, + maximumBy, + minimumBy, + (!!), + -- * Generic wrapper for partial functions. + wrap, + -- * Exceptions for empty lists and negative indices. + -- |These are the only two exceptions that will be thrown. + EmptyListException(..), + NegativeIndexException(..), + )where + +import Prelude hiding (head, tail, init, last, foldl1, foldr1, maximum, minimum, (!!)) + +import Control.Monad.Catch +import qualified Data.List as L +import Data.List as LSafe hiding (head, last, tail, init, foldl1, foldl1', foldr1, maximum, minimum, maximumBy, minimumBy, (!!)) +import Data.Typeable + +-- |Signals that the list was empty or contained too few elements (in the case +-- or access by index). +data EmptyListException = EmptyListException deriving (Show, Read, Eq, Ord, Typeable) +-- |Singals that an element with a negative index was accessed. +data NegativeIndexException = NegativeIndexException deriving (Show, Read, Eq, Ord, Typeable) + +instance Exception EmptyListException +instance Exception NegativeIndexException + +-- |Takes a function that requires a non-empty list and wraps it in an instance +-- of 'MonadThrow'. For empty lists, an 'EmptyListException' is thrown. +wrap :: MonadThrow m => ([a] -> b) -> [a] -> m b +wrap _ [] = throwM EmptyListException +wrap f xs = return $ f xs + +-- |Extract the first element of a list. Empty lists throw an 'EmptyListException'. +head :: MonadThrow m => [a] -> m a +head = wrap L.head + +-- |Extract the last element of a list. Empty lists throw an 'EmptyListException'. +last :: MonadThrow m => [a] -> m a +last = wrap L.last + +-- |Extract the elements after the head of a list. +-- Empty lists throw an 'EmptyListException'. +tail :: MonadThrow m => [a] -> m [a] +tail = wrap L.tail + +-- |Return all the elements of a list except the last one. +-- Empty lists throw an 'EmptyListException'. +init :: MonadThrow m => [a] -> m [a] +init = wrap L.init + +-- |'foldl1' is a variant of 'foldl' that has no starting value, and thus must +-- be applied to non-empty lists. Empty lists throw an 'EmptyListException'. +foldl1 :: MonadThrow m => (a -> a -> a) -> [a] -> m a +foldl1 = wrap . L.foldl1 + +-- |A strict version of 'foldl1'. +foldl1' :: MonadThrow m => (a -> a -> a) -> [a] -> m a +foldl1' = wrap . L.foldl1' + +-- |'foldr1' is a variant of 'foldr' that has no starting value, and thus must +-- be applied to non-empty lists. Empty lists throw an 'EmptyListException'. +foldr1 :: MonadThrow m => (a -> a -> a) -> [a] -> m a +foldr1 = wrap . L.foldr1 + +-- |'maximum' returns the maximum value from a list, which must be non-empty, +-- finite, and of an ordered type. It is a special case of 'maximumBy', which +-- allows the programmer to supply their own comparison function. +-- Empty lists throw an 'EmptyListException'. +maximum :: (MonadThrow m, Ord a) => [a] -> m a +maximum = wrap L.maximum + +-- |'minimum' returns the maximum value from a list, which must be non-empty, +-- finite, and of an ordered type. It is a special case of 'minimumBy', which +-- allows the programmer to supply their own comparison function. +-- Empty lists throw an 'EmptyListException'. +minimum :: (MonadThrow m, Ord a) => [a] -> m a +minimum = wrap L.minimum + +-- |The 'maximumBy' function takes a comparison function and a list and returns +-- the greatest element of the list by the comparison function. The list must +-- be finite and non-empty. Empty lists throw an 'EmptyListException'. +maximumBy :: MonadThrow m => (a -> a -> Ordering) -> [a] -> m a +maximumBy = wrap . L.maximumBy + +-- |The 'minimumBy' function takes a comparison function and a list and returns +-- the least element of the list by the comparison function. The list must +-- be finite and non-empty. Empty lists throw an 'EmptyListException'. +minimumBy :: MonadThrow m => (a -> a -> Ordering) -> [a] -> m a +minimumBy = wrap . L.minimumBy + +-- |List index (subscript) operator, starting from 0. Indices larger than +-- @length xs - 1@ throw an 'EmptyListException', negative indices throw +-- an 'NegativeIndexException'. +(!!) :: (MonadThrow m, Integral n, Num n) => [a] -> n -> m a +(!!) [] _ = throwM EmptyListException +(!!) (x:xs) n | n == 0 = return x + | n < 0 = throwM NegativeIndexException + | otherwise = (!!) xs (n-1)
+ LICENSE.md view
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+ Setup.hs view
@@ -0,0 +1,2 @@+import Distribution.Simple +main = defaultMain
+ listsafe.cabal view
@@ -0,0 +1,22 @@+name: listsafe +version: 0.1.0.0 +synopsis: Safe wrappers for partial list functions, supporting MonadThrow. +description: Data.List includes a handful of partial functions that throw + uncatchable exceptions when given empty lists. This package + provides safe alternatives for such functions based on + MonadThrow which can provide a variety of failure + cases: Nothing, IOException, Left, etc. +homepage: https://github.com/ombocomp/listsafe +license: Apache-2.0 +license-file: LICENSE.md +author: Janos Tapolczai +maintainer: janos.tapolczai@gmail.com +category: Data, List, Safe +build-type: Simple +cabal-version: >=1.10 + +library + exposed-modules: Data.List.Safe + other-extensions: DeriveDataTypeable + build-depends: base >=4.7 && <5, exceptions >=0.4 && <0.7 + default-language: Haskell2010