[Haskell-beginners] GHCi bug?

[Antoine Latter]

On Mon, Nov 15, 2010 at 9:59 PM, Russ Abbott <russ.abbott at gmail.com> wrote:
> This looks like a GHCi bug.
> Load a file with these two lines into GHCi
>
> data Test = Test
> instance Show Test where  -- No body to the instance statement
>

A Show instance without a body isn't valid. Perhaps you wanted:

> data Test = Test
>  deriving Show

If you take a look at the source for the Show class:
http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/src/GHC-Show.html#Show

you can see that the function 'show' has a default implmentation of
'show' in terms of 'showsPrec', whicle 'showsPrec' has a default
implementation in terms of 'show', so you end up with an infinite
loop.

The idea is that you can pick which one to implement - but you do need
to pick :-)

Antoine