[Haskell-beginners] Why is there no notion of a one-tuple (e.g., a '([])' as opposed to a '[]') in Haskell?

[C.M.Brown]

Hi Benjamin,

[] has the type [a] because there is no constraint on what the type of its
elements could be, hence it is polymorphic.

([]) is actually the parenthesised expression for [], in the same way any
expression e == (e).

Haskell does have a unit type:

Prelude> :t ()
() :: ()

I guess this is analogue of the empty tuple to the empty list. One element
tuples are not allowed - they are themselves expressions. The unit type is
like the Null construct in other languages.

Kind regards,
Chris.



On Wed, 24 Sep 2008, Benjamin L.Russell wrote:

> I'm having difficulty in understanding the following behavior:
>
> In GHCi:
>
> Prelude> :type []
> [] :: [a]
>
> but:
>
> Prelude> :type ([])
> ([]) :: [a]
>
> I.e., the types of both the empty-list '[]' and the one-tuple
> containing the empty-list '[]' are '[a]' (a list of a generic type
> variable).
>
> According to "Chapter 2. Types and Functions" (see
> http://book.realworldhaskell.org/read/types-and-functions.html) of
> Real World Haskell (beta) (see
> http://book.realworldhaskell.org/beta/),
>
> >Haskell doesn't have a notion of a one-element tuple.
>
> Why not?  It seems that a tuple is similar to a list, except that the
> elements need not be all of the same type, and that a tuple, unlike a
> list, cannot be extended.  Yet:
>
> Prelude> :type []
> [] :: [a]
>
> and
>
> Prelude> :type [[]]
> [[]] :: [[a]]
>
> so the types of the empty-list '[]' and the one-element list
> containing the empty-list '[[]]' are different.
>
> Forgive me if I am missing something, but something about this
> asymmetry bothers me....
>
> -- Benjamin L. Russell
>
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