defaults

[Ross Paterson]

On Wed, Nov 29, 2006 at 06:08:14PM +0000, Malcolm Wallace wrote:
> Unfortunately, I suspect that teaching is _the_ major use-case for
> defaulting.  Imagine, day one, lesson one, a student types
> 
>     Prelude> 1+2
> 
> into Hugs, and gets the response
> 
>     Unresolved overloading: Num a
> 
> Huh?  This is lesson one, and you already need to tell students about
> type classes and overloading, before you have even covered simple
> expressions fully?  I am certain this is the reason why defaulting was
> introduced.

Following a suggestion of Ralf Hinze, GHCi and Hugs already perform more
defaulting on interpreted expressions than on definitions in modules
(where they follow Haskell 98).  Thus "reverse []" just works, instead
of provoking

	ERROR: Cannot find "show" function for:
	*** expression  : reverse []
	*** of type     : [a]

See http://www.haskell.org/ghc/docs/latest/html/users_guide/ch03s04.html#extended-default-rules

So if defaulting were removed from the language it could still be done
at the interpreter prompt.