[Haskell-beginners] Picking apart getLine

[Angus Comber]

OK, so if the user enters "ABC\n" then the following happens:

x <- getChar  --A

  x <- getChar --B

     x <- getChar --B

       x <- getChar --\n

levels are different contexts? levels of recursion.

then when recursion ends get 'A' : 'B' : 'C' : []

So does the (x:xs) syntax mean this?

I thought of (x:xs) as 'A' : ['B','C']  But can the xs mean simply the
rest?  ie (x:xs) in this case x = 'A' and xs represents the rest 'B' : 'C'
: []  Is that maybe the way to think of it?

I suppose it does.

The confusing bit is how all the x's after the first one (ie after 'A') are
represented in returm (x:xs).  But I am now thinking that ['B','C'] is
actually the same as 'B' : 'C' : [] and all that the 'B' and 'C' and also
the last [] are the xs part of (x:xs)



On 7 January 2014 17:15, David McBride <toad3k at gmail.com> wrote:

> You have the idea.  The x is fetched with getChar, then it sits in
> that context until the return is executed.
>
> So the x is sitting there and getLine' is called.  It makes its own x
> via getChar, then maybe getLine' is called again.  Each getLine' sits
> there with its own version of x until finally the last getLine' get's
> a \n, and then returns a [].  Then the whole thing unwinds by
> prepending x to [], then x to [x], then another x to [x,x], until
> there are no more x's to return, and you have the whole string.
>
> Hopefully that paragraph makes sense.
>
> On Tue, Jan 7, 2014 at 11:54 AM, Angus Comber <anguscomber at gmail.com>
> wrote:
> > Before looking at getLine, I can understand this:
> >
> > getnumber :: IO Int
> > getnumber = do x <- getChar
> >                if isDigit x then
> >                      return (ord x - ord '0')
> >                else
> >                      return 0
> >
> > OK, it is not a very useful function but at least I understand it.
>  return
> > is required so that the function returns an IO Int.
> >
> > But I don't understand this:
> >
> > getLine' :: IO String
> > getLine'        = do x <- getChar
> >                      if x == '\n' then
> >                        return []
> >                      else
> >                        do
> >                          xs <- getLine'
> >                          return (x:xs)
> >
> >
> > I can understand what will happen if a user enters a newline (only).
>  return
> > [] brings the empty list into the monadic world.
> >
> > But what is happening if x is not a newline?
> >
> > xs <- getLine' will recursively call getChar and retrieve another
> character
> > from the input stream.  But it will do this BEFORE the return (x:xs) - so
> > what is happening to all the head elements in the list - the x element?
> >
> > It is difficult to picture in my mind how this is working.  I understand
> > recursion but this looks tricky.
> >
> > Can someone help me work this out?
> >
> >
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> >
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