[Haskell-beginners] safetail problem

[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

(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> wrote:

>
> With GHCI I get this message :
>
>
>
> safetail.hs:1:16: parse error on input ','
>
>
>
> Roelof
>
>
>
> ----------------------------------------
> > From: rwobben at hotmail.com
> > To: 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
> > > Date: Wed, 13 Jul 2011 08:10:38 -0400
> > > CC: beginners at haskell.org
> > > To: 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
> > >
> > >
> > >
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://www.haskell.org/mailman/listinfo/beginners
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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