[Template-haskell] Reification

[John O'Donnell]

Hi Alastair and Simon...

Alastair Reid wrote:

>>So I'm thinking that if the Type returned to you by TH contains free
>>type variables, that might simply signify types that might be later
>>refined.  Simple.
>>
>>But not very principled, because the type you see will depend on which
>>order the type checker happens to do its work in.
>>    
>>
>
>I've got some alarm bells ringing here.
>
>It seems to me that any application that used this extra information would be 
>very brittle - a small change in the code or, it seems, in the compiler could 
>change something from working to not working or from being fast to being slow 
>or <whatever benefit it is that the extra information gives you>.
>
The point is that my application (Hydra) shouldn't ever need the values 
of the type variables; what it needs is the top level function type.  An 
example might be something like

f :: Signal a => (a, [a]) -> a -> (a,[(a,a)])

Now, I believe it to be the case that at least this much of the type 
will always be known, and that's all I need.  Later on it might turn out 
that the type is really

f :: (Stream CMOS, [Stream CMOS]) -> Stream CMOS -> ...

and you're right, it would be brittle to rely on having that extra 
information -- but I don't need that extra information.

The problem is that conceivably the typechecker might only come up with 
something like

f :: b -> a -> (a, [(a,a)])

where later on it will be discovered that b is actually (a,[a]).  If 
this were to happen it might be a problem (but still wouldn't be 
disastrous).  However, I think there are going to be enough other 
constraints that such an over-general type would not arise at all.

If I understand things correctly, my TH code would at least know if 
there is a problem.  If it sees an unexpected free type like b in the 
above, it would *know* that it doesn't know enough about the type, and 
could emit an error message asking the user to provide a type declaration.

Does this sound reasonable?  I don't have time to be more clear - 
lectures coming up.

>Maybe I just don't understand John's application enough?
>[Sorry John, I missed your IFL talk when it was moved to after the end of 
>IFL.]
>  
>
Ah yes... that's what comes of trying to give a talk with laptop and 
beamer :-)

Cheers, John