Type checking question

Adrian Hey ahey@iee.org
Mon, 5 Nov 2001 08:36:36 +0000

On Friday 02 November 2001  4:46 pm, Rijk-Jan van Haaften wrote:
> Therefore, you have to tell GHC the exact type of "textNew Nothing Nothing"
> with a type annotation, just like in the examples with show above:
> txt <- textNew (Nothing :: Maybe SomeDefaultAdjustmentClass) Nothing

Ah yes, I'd forgotten you could do that these days. This still lacks
elegance IMHO, but it's not quite as bad as what I proposed.

Thinking about this, it occurs to me that maybe this kind of ambiguity
could be safely ignored by the compiler if the following constraint
was enforced...

Functions can only make use of class methods if they have been given
a genuine class member in an argument. The compiler could check this
quite easily I think. Either the function does pattern matching on
the argument, or it passes the argument (and method table) on to
another function which is similarly constrained. Either way it's safe.
So in situations like this the compiler could use some suitable dummy
method table with confidence, knowing it would never be used.

Would this be possible? I suppose if the methods include functions
which don't take a class member as an argument (or constants) this
could cause problems.

Just an idea.

I don't think this would help with your show example though, which
as you observed, is ambiguous for a different reason.

Adrian Hey