[Haskell-cafe] Printing the empty list.

[Brent Yorgey]

On Fri, Jul 01, 2011 at 09:05:05AM +1000, Ivan Lazar Miljenovic wrote:
> On 1 July 2011 08:58, Joshua Ball <joshbball at gmail.com> wrote:
> > GHCi seems to be clever about some things:
> >
> > If I try to print the empty list in ghci, I encounter no problems:
> >
> > Prelude> []
> > []
> > Prelude> show []
> > "[]"
> > Prelude> print []
> > []
> >
> > Even though the type of the list is clearly unknown, it must be
> > picking SOME type. (why does it print [] instead of "")?
> 
> Type defaulting: if you don't specify a type, then ghci makes it
> [Integer].

In this case I'm pretty sure it is [()] since there is only a Show
constraint.  If there were a Num constraint it would pick
Integer.

  Prelude> sum []
  0
  Prelude> :t it
  it :: Integer

-Brent