Data.List.Safe

Operations on lists. This module re-exports all safe functions of 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.

Safe versions of standard functions.

head :: MonadThrow m => [a] -> m a

Extract the first element of a list. Empty lists throw an EmptyListException.

last :: MonadThrow m => [a] -> m a

Extract the last element of a list. Empty lists throw an EmptyListException.

tail :: MonadThrow m => [a] -> m [a]

Extract the elements after the head of a list. Empty lists throw an EmptyListException.

init :: MonadThrow m => [a] -> m [a]

Return all the elements of a list except the last one. Empty lists throw an EmptyListException.

foldl1 :: MonadThrow m => (a -> a -> a) -> [a] -> m a

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

A strict version of foldl1.

foldr1 :: MonadThrow m => (a -> a -> a) -> [a] -> m a

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.

maximum :: (MonadThrow m, Ord a) => [a] -> m a

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.

minimum :: (MonadThrow m, Ord a) => [a] -> m a

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.

maximumBy :: MonadThrow m => (a -> a -> Ordering) -> [a] -> m a

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.

minimumBy :: MonadThrow m => (a -> a -> Ordering) -> [a] -> m a

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.

(!!) :: (MonadThrow m, Integral n, Num n) => [a] -> n -> m a

List index (subscript) operator, starting from 0. Indices larger than length xs - 1 throw an EmptyListException, negative indices throw an NegativeIndexException.

Generic wrapper for partial functions.

wrap :: MonadThrow m => ([a] -> b) -> [a] -> m b

Takes a function that requires a non-empty list and wraps it in an instance of MonadThrow. For empty lists, an EmptyListException is thrown.

Exceptions for empty lists and negative indices.

These are the only two exceptions that will be thrown.

data EmptyListException

Signals that the list was empty or contained too few elements (in the case or access by index).

Constructors

EmptyListException

Instances

Eq EmptyListException

Ord EmptyListException

Read EmptyListException

Show EmptyListException

Exception EmptyListException

Typeable * EmptyListException

data NegativeIndexException

Singals that an element with a negative index was accessed.

Constructors

NegativeIndexException

Instances

Eq NegativeIndexException

Ord NegativeIndexException

Read NegativeIndexException

Show NegativeIndexException

Exception NegativeIndexException

Typeable * NegativeIndexException