[Haskell-beginners] safetail problem

[Roelof Wobben]

Oke, 

Then I made there a error.
I want to check if xs is a empty list.
So it must be if null xs then [] else xs

Roelof


> Date: Wed, 13 Jul 2011 18:28:40 +0200
> From: Christian.Maeder at dfki.de
> To: orclev at gmail.com
> CC: rwobben at hotmail.com; beginners at haskell.org
> Subject: Re: safetail problem
> 
> 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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> >      > http://www.haskell.org/mailman/listinfo/beginners
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> >
> >
 		 	   		  
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