How lazy is DData.Seq?

[Wolfgang Jeltsch]

Am Samstag, 8. Mai 2004 18:37 schrieben Sie:
> --- Wolfgang Jeltsch <wolfgang at jeltsch.net> wrote:
> > Hello,
> >
> > I would suspect that in DData.Seq the result of
> >     head (singleton 'a' `append` undefined)
> > is 'a' but instead it is _|_.  Why is this the case?
>
> I have the invariant that sublists are not empty.
> There is, for example:
>
> append as None = as

Is this necessary/desirable?  I have an application where I think that it can 
be important for efficiency that the above expression evaluates to 'a'.  So I 
would like it to not result in _|_.

> Cheers,
> JP.

Wolfgang