[Haskell-beginners] Question on (x:xs) form

[Kim-Ee Yeoh]

On Mon, Dec 23, 2013 at 9:57 PM, Angus Comber <anguscomber at gmail.com> wrote:

> Why are the brackets required?  And what do they signify?
>
> Eg reverse' x:xs = reverse' xs ++ [x] results in a parse error.
>

So a number of Haskell-specific themes are relevant here, especially if you
come from a Lisp background:

* types (but you knew that already!)
* pattern-matching & exhaustivity of matches
* operator infix notation
* associativity precedence
* special notation for lists

It's a good idea to watch out for all of the above when learning the
language. It helps to have some google-able names when the situation is
ripe for drilling deep into these topics.

To elaborate a bit,

Special List Notation: lists use (:) and [x]. But this syntax is baked
specially into the language definition! If you define your own list, it'll
typically look like
   data List a = Nil | Cons a (List a)   -- note the parens!

The alternative GADT-ish style may make this clearer:
   data List a  where
      Nil :: List a
      Cons :: a -> List a -> List a

Nil and Cons are data constructor *functions*, and because and only because
they are so, do they get to have Capitalized Names. <------- this is
another big WTF that trips up many, many learners. Especially with code
like:
   newtype X a = X a   -- whaaaa????? (It's ok, you'll get it soon enough.)

Back to reverse'. With our user-defined List a, reverse' will look like
   reverse' Nil =
   reverse' (Cons x xs) = ...  -- again note the parens!

(There is however, *NO* difference in the semantics. The code just looks
different but the compiler doesn't care.)

Note how the pattern-matching includes every single case of the sum-type;
here both of them: Nil and Cons. This is Generally A Good Thing!

-- Kim-Ee
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