listToMaybe -package:singletons-base

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Basic usage:

>>> listToMaybe []
Nothing

>>> listToMaybe [9]
Just 9

>>> listToMaybe [1,2,3]
Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5]
[5]

>>> maybeToList $ listToMaybe []
[]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3]
[1]

Analogous to listToMaybe in Data.Maybe.

Convert between (the head of) a (singleton) list and Maybe (see listToMaybe). (invert maybeToList)

Converts a list to an Automaton in MaybeT, which outputs an element of the list at each step, throwing Nothing when the list ends.

Converts a list to an MSF in MaybeT, which outputs an element of the list at each step, throwing Nothing when the list ends.

Returns Nothing on an empty list or Just a where a is the first element of the slist.

Examples

Basic usage:

>>> slistToMaybe mempty
Nothing

>>> slistToMaybe (one 42)
Just 42

>>> slistToMaybe (cons 1 $ cons 2 $ one 3)
Just 1

Laws :

slistToMaybe . maybeToList ≡ id

Reverse is right only on singleton/empty lists

maybeToList . slistToMaybe {empty, singleton slist} ≡ {empty, singleton slist}