[Haskell-beginners] Re: Apparent bug

[Russ Abbott]

A few answers back, Matthew Coolbeth wrote,

(show v) where v is of type Test will evaluate to (showPrec v), and that
(showPrec v) will evaluate to (show v).  It should be clear that this is a
cyclic evaluation that will not terminate.


Presumably that was intentional. But why was that necessary?  It seems like
strange coding.  Furthermore, why isn't

instance Show Test

interpreted to mean

data Test = Test deriving (Show)

*
-- Russ *



On Tue, Nov 16, 2010 at 10:27 AM, Tobias Brandt
<tob.brandt at googlemail.com>wrote:

> On 16 November 2010 19:17, Edward Z. Yang <ezyang at mit.edu> wrote:
> > It seems to me that there is a possibility we could reify some
> information
> > that is traditionally specified in the documentation: that is, what
> > functions must be defined by a minimal instance, which could then give
> > GHC enough information to give meaningful warnings if not all functions
> > for a minimal instance are proviced.
>
> One could use a compiler pragma the defines possible sets of minimal
> definitions, e.g.
> {-# MINIMAL_DEF Num ((+),(*),abs,signum,fromInteger,(-)),
> ((+),(*),abs,signum,fromInteger,negate) #-}
>
> one could even add logical notation, like:
> {-# MINIMAL_DEF Num ((+), (*), abs, signum, fromInteger, (-) || negate) #-}
>
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