[Haskell-beginners] safetail problem

[Christian Maeder]

Am 13.07.2011 17:40, schrieb Kyle Murphy:
> Your new code declares that you're expecting a [x,xs] which isn't a
> valid type. What you're expecting is a [x]. Furthermore you then try to
> pattern match against (x,xs) which is a tuple even though your function
> just declared it's expecting a list. What you want is:
>
> safetail :: [x] -> [x]
> safetail (x:xs) = if null[xs] then [] else xs

Here I miss an equation:

   safetail [] = ...

Also "if null[xs] then [] else xs" happens to correctly simplify to "xs" 
because "null[xs]" is never True, because "[xs]" is a singleton list (of 
lists).

C.

>
> (x:xs) is a pattern match using one of the two list constructors (:)
> which has type:
>
> (:) :: a -> [a] -> [a]
>
> This means that (:) takes two arguments, anything, and a list of
> anything, and returns a new list of anything (in this case with the
> first argument pre-pended to the second argument).
>
> The other list constructor ([]) is a zero argument constructor of type:
>
> [] :: [a]
>
> That is, it can be used to construct an empty list of any type.
>
> With these two constructors it's possible to understand how lists
> working in Haskell.
> [1,2,3,4] is syntactic sugar (that is, it's replaced by the compiler for
> you and exists solely for convenience of writing) for 1:2:3:4:[].
> Following the types you have:
> [] -> Constructs an empty list of type [a], we don't know what "a" is yet.
> 4:[] -> Takes an Int (4) and a list [Int] ([]) and constructs a new list
> of type [Int]. The previous type of [a] is forced to be [Int] at this point.
> 3:[4] -> Takes an Int (3) and a list ([4]) and returns a new list ([3,4]).
> 2:[3,4] -> Takes an Int (2) and a list ([3,4]) and returns a new list
> ([2,3,4]).
> 1:[2,3,4] -> Takes an Int (1) and a list ([2,3,4]) and returns a new
> list ([1,2,3,4]).
>
> N.B. I'm not an expert on Haskell, so the following might be wrong in
> some of the details, but is the way I understand lists to function in
> Haskell.
>
> Don't be confused at this point with the empty list constructor [], and
> the list type ([]), which even though they use the same name are
> actually two different things.
> To be clear, the data declaration of List actually looks similar to the
> following:
>
> data ([]) a = [] | (:) a (([]) a)
>
> which could also be written as:
>
> data [a] = [] | a:[a]
>
> The (([]) a) on the left of the equal sign is the Type. The [] on the
> right of the equal sign is the constructor.
>
> -R. Kyle Murphy
> --
> Curiosity was framed, Ignorance killed the cat.
>
>
> On Wed, Jul 13, 2011 at 10:58, Roelof Wobben <rwobben at hotmail.com
> <mailto:rwobben at hotmail.com>> wrote:
>
>
>     With GHCI I get this message :
>
>
>
>     safetail.hs:1:16: parse error on input ','
>
>
>
>     Roelof
>
>
>
>     ----------------------------------------
>      > From: rwobben at hotmail.com <mailto:rwobben at hotmail.com>
>      > To: beginners at haskell.org <mailto:beginners at haskell.org>
>      > Date: Wed, 13 Jul 2011 14:54:01 +0000
>      > Subject: Re: [Haskell-beginners] safetail problem
>      >
>      >
>      > Hello,
>      >
>      >
>      >
>      > I have GHC installed so I will try this one with ghci.
>      >
>      >
>      >
>      > Roelof
>      >
>      >
>      >
>      > ----------------------------------------
>      > > Subject: Re: [Haskell-beginners] safetail problem
>      > > From: d at vidplace.com <mailto:d at vidplace.com>
>      > > Date: Wed, 13 Jul 2011 08:10:38 -0400
>      > > CC: beginners at haskell.org <mailto:beginners at haskell.org>
>      > > To: rwobben at hotmail.com <mailto:rwobben at hotmail.com>
>      > >
>      > > On Jul 13, 2011, at 3:07 AM, Roelof Wobben wrote:
>      > >
>      > > > changed it to this : http://codepad.org/ROV4ASAB
>      > >
>      > >
>      > > Hi, Roelof.
>      > >
>      > > Have you installed GHC? You will find the error messages to be
>     more helpful when you use ghci to execute your code.
>      > >
>      > > I don't know Programming in Haskell by Graham Hutton. I am sure
>     that it very good, but maybe not the best for you. You might also
>     try this tutorial.
>      > >
>      > > > http://learnyouahaskell.com/chapters
>      > >
>      > > ____________________
>      > > David Place
>      > > Owner, Panpipes Ho! LLC
>      > > http://panpipesho.com
>      > > d at vidplace.com <mailto:d at vidplace.com>
>      > >
>      > >
>      > >
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