#ifdef considered harmful (was: DData)

Alastair Reid alastair at reid-consulting-uk.ltd.uk
Tue Apr 20 12:54:04 EDT 2004

> I'm not clear on exactly what the FastInt type means.  I think I asked
> Alastair the same question recently - is FastInt an unlifted type?

Yes, it should be unlifted.

Being unboxed as well would be nice but I'd sacrifice that for any other 
benefits like polymorphism and overloading.

> Can I store it in a polymorphic data structure or pass
> it to a polymorphic function? 

That would be highly desirable.

I think it can be done using some kind of 'separate but equal' type system 
where we have two kinds of type variable: '*' for lazy types and '#' for 
strict types.  So, for example, there would be two versions of 'id':

  id_* :: forall a::*. a -> a
  id_# :: forall a::#. a -> a

My main question is how is a typechecker to infer kinds?  Does it apply a 
default rule of '*' unless otherwise noted?  What syntax does the programmer 
use to override that default?

This would be a bit tedious because not only would be need to write strict and 
lazy versions of '+' (say) but we'd also have to define two separate Num-like 
classes: one for types of kind * and one for types of kind #.  It would be 
really nice to have a single version of 'id' and a single definition of 'Num' 
but that would be hard to achieve because a piece of code like this:

  foo x = [bar (id (x + 1))]

has a very different evaluation order depending on whether x is strict or 
lazy.  I think we can implement it by passing round a dictionary that 
contains either 'seq' (for #) or '\ x y -> y' (for *) but that's not very 

> If FastInt is Int# in GHC, and we want it to be portable, then we have
> to introduce unlifted types & kinds in Hugs and nhc98 too.

Yes, I was proposing that Hugs and NHC be extended with unlifted types of some 

> Declaring 
> instances of classes for unlifted types isn't possible, so you can't
> have instane Num Int#, for example.  I think you need polymorphic kinds
> for that.

Yes, that was the conclusion I was slowly coming to above.

Alastair Reid

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