deriving...

[Georg Martius]

On Tue, 19 Oct 2004 15:33:25 +0100, Simon Peyton-Jones <simonpj at microsoft.com> wrote:

>
> Thanks to those who responded to this thread about 'deriving'.  My current thoughts are:
>
> * I'd be happy to add the ability to separate a 'deriving' clause
>    from its data type declaration, if we can agree syntax
>    (see below).  It's fairly easy to do; it makes the language more
>    orthogonal; it's useful.
>
>    But in fact I think only Martin Sjögren has explicitly said that
>    the feature would be useful.... and every feature has a cost.
It would be quite useful indeed!
The scenarios I have experienced are quite complex, but they occur.

   Georg

>
>
> * I'm not at all keen on making '..deriving( Foo )' mean
>   $(derive 'Foo) or something like that.  Just make the TH
>   call yourself!
>
> * No one is arguing hard for instance declarations in hi-boot
>   files, so let's leave that for now.
>
>
> Re syntax, the obvious possibility (A) is to add
>
>   derive( pred1, .., predn )
>
> as a new top-level declaration.  E.g.
>
>   derive( Typeable (T a) )
>
> But that means adding 'derive' as a keyword.  Other possibilities:
>  deriving( Typeable (T a) )
> 	-- (B) Re-use 'deriving' keyword
>
> The trouble with (B) is that the thing inside the parens is different in this situation than in a data type declaration.
>
> Any other ideas?
>
> Simon