Re: [Haskell-beginners] safetail problem

13 Jul 2011

      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@dfki.de
To: orclev@gmail.com
CC: rwobben@hotmail.com; beginners@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 mailto:rwobben@hotmail.com> wrote:
With GHCI I get this message :
safetail.hs:1:16: parse error on input ','
Roelof
----------------------------------------
     > From: rwobben@hotmail.com mailto:rwobben@hotmail.com
     > To: beginners@haskell.org mailto:beginners@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@vidplace.com mailto:d@vidplace.com
     > > Date: Wed, 13 Jul 2011 08:10:38 -0400
     > > CC: beginners@haskell.org mailto:beginners@haskell.org
     > > To: rwobben@hotmail.com mailto:rwobben@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@vidplace.com mailto:d@vidplace.com
     > >
     > >
     > >
     > _______________________________________________
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     > Beginners@haskell.org mailto:Beginners@haskell.org
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