reverse.txt

> To show the use of `map` and `andThen`, let me define a function
> `headButLast` that takes the `head` of a `List String` and removes its last
> character:
> 
> {lang=elm}
> ~~~~~~~~~~~~~~
> > headButLast ["Dawn"]
> Just "Daw" : Maybe.Maybe String
> ~~~~~~~~~~~~~~
> 
> Such a function must handle two
> unhappy cases:
> 
> 1. The list is empty:
> 
>    {lang=elm}
>    ~~~~~~~~~~~~~~
>    > headButLast []
>    Nothing : Maybe.Maybe String
>    ~~~~~~~~~~~~~~
> 
> 2. The list has a `head` but it's the empty string (and the empty
>    string doesn't have a last character):
> 
>    {lang=elm}
>    ~~~~~~~~~~~~~~
>    > headButLast [""]
>    Nothing : Maybe.Maybe String
>    ~~~~~~~~~~~~~~
>
> Here's the implementation:
> 
> {lang=elm}
> ~~~~~~~~~~~~~~~~~~
> headButLast : List String -> Maybe String
> headButLast list = 
>   list                               -- ["Dawn"]
>     |> List.head                     -- Just "Dawn"
>     |> Maybe.map String.reverse      -- Just "nwaD"
>     |> Maybe.andThen String.uncons   -- Just (n, "waD")
>     |> Maybe.map Tuple.second        -- Just "waD"
>     |> Maybe.map String.reverse      -- Just "Daw"
> ~~~~~~~~~~~~~~~~~~

What would be idiomatic PureScript?

headButLast1 list =
  String.reverse <$> _.tail <$> (String.uncons =<< String.reverse <$> head list)

headButLast2 list =
  head list <#> String.reverse >>= String.uncons <#> _.tail <#> String.reverse

headButLast3 list =
  head list
    <#> String.reverse
    >>= String.uncons
    <#> _.tail
    <#> String.reverse

Something else?