[Nix-dev] A standard function to handle deps

[Evgeny Egorochkin]

On Monday 21 June 2010 19:23:46 Marc Weber wrote:
> Hi Evgeny Egorochkin,
> 
> > I propose to add a standard function like this:
> > 
> > optionalInput =
> > 
> >   cond: elem: assert cond -> elem != null; if cond then [elem] else [];
> > 
> > This removes a good deal of asserts clutter and is easy enough to use so
> > that maintainers will actually specify asserts as a byproduct instead of
> > letting things fail at buildtime or silently not compile in the needed
> > functionality.
> 
> Just some more ways:
> 
> A)
>   buildInputs = nullGuard [ null ]; // guard fails if one element is null.
> Probably nullGuard does not exist yet.
> 
> B) remove the "? null" in the arg list. Assume that null is never
>   passed. Usually the option is passed. Then also remove the
>   assertion because you assume that null is never passed.
> 
> Usually you should keep code as is unless there is reason to change it.
> That's how I think about it.

I did indeed miss the fact that you can pass var=null and that asserts only 
for optional vars aren't enough.

So how about adding a "nullGuard" to stdenv.mkDerivation? It isn't going to 
affect a single working package but would help to catch errors early.

-- 
Evgeny