:: [a] -> a

Warning: This is a partial function, it throws an error on empty lists. Use pattern matching, uncons or listToMaybe instead. Consider refactoring to use Data.List.NonEmpty.

Extract the last element of a list, which must be finite and non-empty. WARNING: This function is partial. Consider using unsnoc instead.

Examples

>>> last [1, 2, 3]
3

>>> last [1..]
* Hangs forever *

>>> last []
*** Exception: Prelude.last: empty list

Identical to head, namely that fails on an empty list. Useful to avoid the x-partial warning introduced in GHC 9.8.

headErr [] = error "Prelude.head: empty list"
headErr [1,2,3] = 1

Extract the first element of a list, which must be non-empty.

>>> head [1, 2, 3]
1

>>> head [1..]
1

>>> head []
*** Exception: Prelude.head: empty list

WARNING: This function is partial. You can use case-matching, uncons or listToMaybe instead.

Extract the last element of a list, which must be finite and non-empty.

>>> last [1, 2, 3]
3

>>> last [1..]
* Hangs forever *

>>> last []
*** Exception: Prelude.last: empty list

WARNING: This function is partial. You can use reverse with case-matching, uncons or listToMaybe instead.

Utility function to go from a singleton list to it's element. Wether or not the argument is a singleton list is only checked in debug builds.

Version of modes without a Data context, only to be used within cmdArgsQuote.

Version of enum without a Data context, only to be used within cmdArgsQuote.

Extract the first element of a list, which must be non-empty.

Examples

>>> head [1, 2, 3]
1

>>> head [1..]
1

>>> head []
*** Exception: Prelude.head: empty list

Extract the first element of a list, which must be non-empty.

>>> head [1, 2, 3]
1

>>> head [1..]
1

>>> head []
*** Exception: Prelude.head: empty list

Extract the first element of a list, which must be non-empty.

Extract the last element of a list, which must be finite and non-empty.

Unsafe head function with descriptive error message. This function is partial and will throw an error on empty lists. It should only be used when there's a strong invariant guaranteeing the list is non-empty. Why use this instead of a total function?

• Preserves existing type signatures and caller simplicity
• Makes invariant violations fail fast with clear error messages
• Avoids pushing complexity up the call chain for conditions that should never occur
• Used specifically in getGenerators where groupBy never produces empty groups

When to use:

• Internal functions with strong invariants
• Performance-critical code where the invariant is guaranteed
• When converting to total functions would complicate the entire call chain

When NOT to use:

• Public APIs where callers might pass invalid input
• When the input domain genuinely includes edge cases
• When safety is more important than performance

Extract the last element of a list, which must be finite and non-empty.

>>> last [1, 2, 3]
3

>>> last [1..]
* Hangs forever *

>>> last []
*** Exception: Prelude.last: empty list

An implementation of toList for Corecursive fixed-points of XNor.

An unsafe implementation of fromList for Steppable fixed-points of XNor.

O(1) First element.